Chloe Mitchell

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Background

Casting and introduction

The Young and the Restless called Hendrickson for a meet-and-greet audition. "Chloe wasn't even on the page when I came in for a chemistry audition," said Hendrickson. Impressed with her acting abilities, The Young and the Restless cast her in a recurring role as stylista Chloe. When the character was introduced, she was only intended to be integrated into the storyline for ten episodes. Hendrickson's interpretation of the character "grabbed the attention" of co-head writers Maria Arena Bell and Hogan Sheffer. They offered her a contract and created a story for her to be revealed as a member of one of the show's core families; Esther Valentine as her mother, and Katherine Chancellor as her godmother. In addition, she was given "a killer wardrobe" and love interests Cane Ashby and Billy Abbott. panasonic lumix lx1

Characterization and portrayal lumix lx1

In an August 2008 interview with Soaps in Depth, before the reveal of Chloe's complete family background, Hendrickson talked about the transformation of Chloe from a day player to a "compelling, integral" part of The Young and the Restless canvas. She said, "In the back of my mind, when I first came [to The Young and the Restless] knowing it was only a show-by-show basis, I was going to give it my all and hope they'd fall in love with me and ask me to stay, I will not deny that was my plan." When asked about her transition from her good-girl demeanor as Maggie Stone on All My Children to now portraying Chloe, she said, "It's so nice to step out of that. When I started on All My Children, I played Frankie, and she was a little troublemaker, and I had so much fun with it!" Of Chloe, Hendrickson said, "When I found out that she was going to be this little fashionista, I said, 'Sign me up right away!' Fashion is probably my second most favorite thing in the world." canon gadget bag

Chloe's personality called for her to be portrayed with a ruthless ambition and abrasive personality, but Hendrickson was humored by certain scenes. "You don't really find soaps with a lot of humor, but I think the Y&R writers have done a brilliant job with Chloe. Some of the lines I get, I just laugh out loud at home," she said. "I hope the humor is coming through." Hendrickson felt Chloe "always has an agenda" but is also "just really lonely and needs attention". When asked about Chloe's past, Hendrickson stated, "I don't think she was necessarily born a vixen. She's masking a lot. I think she's like this little wounded bird trying to put on this big front, but deep down she's a softie."

On November 16, 2008, TV Guide asked Hendrickson if she "ever [thought she would] be starring on daytime's biggest and best soap in a breakout role so soon after leaving All My Children". Hendrickson replied, "No, this has been a lovely surprise. I used to always watch Y&R when I was on AMC. I always liked the look and feel of the show, so I've always been a big fan." Hendrickson said, "To see how the role evolved from those initial scenes has been astonishing. My top soap choices were Y&R first, and General Hospital, second. So I'm very happy. Life is good." Asked about working with Daniel Goddard (Cane), she said, "[Goddard's] the hardest-working person I have come across. He carries a week's worth of scripts with him at all times. He's always studying his lines, talking about the motivation of his character, and his method." TV Guide credited Hendrickson with having chemistry with everyone on the show, and especially Michael Graziadei (Daniel). "I love working with Graz. He's so much fun to work with," she stated. "What a talented kid! Whenever you're in a scene with Graz, you never feel like you're working off the page. We met a few years ago through friends of friends." Regarding Billy Miller (Billy), she said, "[Miller's] my new favorite boy to work with. I can't believe how much fun we're having together. I think we have a lot of chemistry."

Hendrickson said she did not know Chloe was Esther's long-lost daughter, Kate, and that Chloe was having Billy's baby, not Cane's, from the beginning. "They didn't tell me anything until I had to know. I've dealt with that for years on AMC; that's how soaps work," she stated. She said as a new addition to the series she was not going impose on certain storylines. "I just told Y&R, give me what you want, and I'll act it out to the best of my ability," said Hendrickson. "As an actor, I created an initial backstory for Chloe, so I could explain her behavior at first. When they finally told me her real backstory, I was so happy because I could finally make Chloe three-dimensional."

Chloe's initial relationship with her mother, Esther (Kate Linder), was detailed as hostile. "Chloe is so mean to Esther," said Hendrickson. "You should see [Linder's] face when we tape our scenes, because she takes it so personally! I have to remind [Linder] that I love her." Despite the character's hostility in this regard, Hendrickson felt it was realistic. "Chloe and Esther remind me of a lot of mother-and-daughter relationships. It actually makes a lot of sense. Chloe is very young and immature as well, hurt and ashamed about her upbringing. She's not going to let her mother forget it easily, either," said Hendrickson.

Hendrickson said she infuses her real-life personality into Chloe. "Our writers come up with the best zingers and lines, so it's easy to be inspired on Y&R. In fact, this is the only comedy I've been blessed to do on daytime that is actually working!" she stated. "Comedy and soaps don't necessarily go hand in hand as you know. I try not to ad lib. I get scared I want to keep my job." Hendrickson added, "I have writer friends in soaps who say, 'when I see my lines changed, I cringe!' Sometimes if the line doesn't work, then you have to collaborate and make it resonate, but on this show that's a rarely a problem."

On January 8, 2009, Hendrickson did an interview with Soap Opera Digest. She said she was excited when she found out Chloe was going to have family ties. "I knew it would give me, as an actor, a chance to finally explain to viewers why Chloe is the way she is," she said. Hendrickson said Chloe is "not just this bitch who likes to destroy people's lives" and that the character's turn in story "gives [her] the opportunity to show people that Chloe is damaged goods". "She was missing a mother and a family, and the love that every child needs and deserves. She wasn't raised by her [parents]. She never had anyone tell her wrong from right," said Hendrickson. Digest asked if Hendrickson had anything to do with Chloe's development. "I was the one who wanted to make her edgy and funny. Most of the people I know who are in the fashion business are a little bitchy," she said. "I wanted to make Chloe interesting, instead of just playing her like some girl who was putting on fancy clothes. There was no conflict there," Hendrickson continued. She added, "The writers definitely gave me that in the writing, and I tweaked it a little. They seemed to enjoy it. Each day, Chloe kept getting funnier and bitchier."

Storylines

Kate is the only daughter of Esther Valentine and Tiny, a plumber. She is named for her godmother and Esther's employer Katherine Chancellor. As a child, she is rarely seen onscreen and attends boarding school courtesy of her godmother. At some point during her absence, she officially changed her name to Chloe Mitchell without telling her mother.

Chloe is hired by Jabot Cosmetics in 2008 to help coach the winning model of the Fresh Face of Jabot contest, which throws her into the orbit of Lily Winters (Christel Khalil) and her boyfriend Cane Ashby (Daniel Goddard). After she wins the competition, Chloe begins working with Lily. Chloe began to push Lily to the best of her ability, in which began an antagonistic relationship between them. At one point Lily begins quits, but then Chloe pressures her into staying in. Despite everything, Lily lets Chloe move in with her and Devon (Bryton) when she needs a place to stay.

Upon learning that she is pregnant, Chloe begins an elaborate scheme to trick Cane into thinking he is the father. At Indigo, she succeeds in getting him drunk, and offers to take him home, only to have taken both their clothes off, and making Cane believe they had sex. This ultimately damages Cane and Lily's plans to wed. Lily tells Cane to marry Chloe for the baby's sake, and he eventually does, seeking a stable home for the child be believes to be his. After she causes trouble for Lily, Jill fires Chloe as the Fresh Face coordinator. Seeking new employment, she visits Restless Style where Phyllis Newman (Michelle Stafford) and Nicholas Newman (Joshua Morrow) hire her to be the fashion editor. She also meets Amber Moore (Adrienne Frantz), who befriends her. In late August 2008 it is revealed that Chloe is actually the long-absent daughter of Esther Valentine (Kate Linder). It is also revealed that Chloe had affair with Cane's half-brother Billy Abbott (Billy Miller). Chloe and Billy agree to pretend they do not know each other. He begins to question whether there is a chance he could be the father of her baby. His suspicions are confirmed after he finds out the baby's due date is earlier than Chloe claimed.

Chloe is jealous as she watches Lily move on from Cane to Billy. She becomes dissatisfied with her loveless marriage to Cane and longs for a life with Billy. When she learns that Billy is going up to the Abbott cabin with Lily for Valentine's Day she follows them up and overhears him confessing that he is the father of Chloe's baby to Lily. Chloe gives birth in the cabin with Billy and Lily's help.

Cane names the baby Cordelia Katherine Valentine Ashby. When Chloe tells him the baby's real father is Billy, Cane reunites with Lily and decides to pursue custody of Cordelia, believing he would make a better parent. To retain custody of his daughter, Billy reluctantly enters into a loveless marriage with Chloe. Right after the ceremony, the real love of Billy's life, Mackenzie Browning (Clementine Ford), shows up. Billy tries to reunite with Mac, but she continually rejects him. Chloe tries to change Billy and hopes they can have a real family together, but then she finds out that he has been having sex with Sharon Newman, both before and after they married. When Chloe discovers this, she moves out. After he and Chloe separated, Billy begins to purse a relationship with Mac. Chloe and Billy are both stunned when it is revealed that Cane is not the biological son of Jill, and that Phillip Chancellor III (Thom Bierdz) is actually alive. This also proves that Cane knew the whole time that he was not the father of Cordelia. He tells Chloe, Billy and Lily that the did not want to have two DNA samples on file, so he thought the only choice could be Billy, and that he would step up and be a father. While a still married Billy pursues a relationship with Mac, Billy's nephew Chance Chancellor (Phillip Chancellor IV) reemerges on the scene, all grown up and now a young army veteran. Chance becomes smitten with Chloe, blushing in her presence; he admits to her that he is a virgin and has little experience with women. They share an impromptu kiss in a playful moment. Chloe, still in love with Billy, tries to set obvious good-guy Chance up with do-gooder Mackenzie. The pair realize they are being set-up, and Chance alludes to his crush on Chloe when he cryptically tells Mac he is already interested in someone else. Chance takes Jill's side (which Chloe likewise does) when Billy rails against her. He later shows distaste over how his uncle treats his wife and child, to which Chloe admits Billy continuously breaks her heart. Chance caresses Chloe's face, and moves in as if to kiss her, just to reveal he has found an eyelash. Chloe makes a wish, but coyly refuses to share it. Chance is shown grinning after she exists, and is caught in his happiness by grandmother Jill, who realizes his mind is preoccupied with something or someone in particular. Mac finally confesses to Chloe that she and Billy are together. A hurt Chloe leans on Chance as he comforts her. Billy and Chloe agree to divorce and get shared custody of Delia. Chance asks Chloe out on a date and she accepts. After a kiss, Chance tells her nothing will ever happen as long as she is hooked up on Billy. Later, Billy takes a little long to sign divorce papers, and Chloe, thinking they are getting back together, decides to use Chance to make Billy jealous. Chloe is warned by her mother not to use such a nice guy to make billy jealous yet she chooses to ignore her. Chloe and Chance grow increasingly close when Chance defends her from his mother, Nina, and her ex, Billy. When Chloe begins to see where her heart truly is she finally signs the divorce papers. Eager to tell Chance the good news Chloe is disappointed and hurt when Chance treats her news like it's nothing. After realizing Chloe really does care Chance forgives her. While trying to stop a robbery at crimson lights off duty cop Chance is stabbed. Chloe is terrified for Chance. Luckily Chance pulls through. After professing her feelings while he sleeps Chance awakens to hear the tail end of her speech. Chloe has still not had sex with virgin Chance although she has almost said "I love everything about you," but she changed the word "love" to "like". Both hope for their first time together to be special and seem to be on the way to a bright future together.

Reception

Chloe's introduction as a vixen, as well as a core character, was generally well-received by critics and viewers. Hendrickson was praised for her "adventurous and comedic performance" as the character, which has been credited as being "reminiscent of the divas of yesteryear, like As the World Turns' Lisa Miller, and Another World's Rachel Cory". In 2008, TV Guide named her one of soap opera's best new characters. In 2009, Soap Opera Digest said of the character, "What started as a 10-episode arc as a bitchy stylist transformed into a meaty contract character and an even meatier storyline."

References

^ a b "Who's Who in Genoa City: Chloe Mitchell". Soapcentral.com. http://www.soapcentral.com/yr/whoswho/chloe.php. Retrieved 2009-07-07. 

^ The Young and the Restless recap (February 6, 2008) - Soapcentral.com

^ The Young and the Restless recap (August 29, 2008) - Soapcentral.com

^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac Branco, Nelson (2008-11-05). "Queen Elizabeth". TV Guide. http://tvguide.sympatico.msn.ca/Queen+Elizabeth/Soaps/Features/Articles/081105_elizabeth_hendrickson_NB.htm?isfa=1. Retrieved 2008-11-16. 

^ a b c d e f g h Di Lauro, Janet (2009-01-08). "Y&R's Elizabeth Hendrickson: "Mean Girl"". Soap Opera Digest. http://www.soapoperadigest.com/features/young-and-restless/interviews/elizabeth_hendrickson_mean_girl. Retrieved 2009-07-12. 

^ a b c d e f g "In Her Own Fashion". Soaps In Depth. 2008-08-04. p. Pages 52-55. http://www.elizabeth-hendrickson.net/SID080408.html. Retrieved 2009-07-12. 

^ http://www.soaps.com/youngandrestless/update/3769/A_Bumpy_Night

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The Young and the Restless

Current characters

Ashley Abbott  Billy Abbott  Jack Abbott  Jill Foster Abbott  Cane Ashby  Michael Baldwin  Gloria Bardwell  Mackenzie Browning  Chance Chancellor  Katherine Chancellor  Kevin Fisher  Devon Hamilton  Jana Hawkes  J.T. Hellstrom  Tucker McCall  Chloe Mitchell  Amber Moore  Adam Newman  Nicholas Newman  Nikki Newman  Phyllis Newman  Sharon Newman  Victor Newman  Victoria Newman  Daniel Romalotti  Esther Valentine  Patty Williams  Paul Williams  Lily Winters  Malcolm Winters  Neil Winters

Recurring characters

Fenmore Baldwin  Lauren Fenmore Baldwin  Jeffrey Bardwell  Abby Carlton  Ryder Callahan  Phillip Chancellor III  Traci Abbott Connolly  Eden Gerick  Reed Hellstrom  Noah Newman  Brock Reynolds  Daisy Sanders  Deacon Sharpe  Heather Stevens  Rafe Torres  Nina Webster 

Notable past characters

John Abbott  Christine Blair  Brad Carlton  Colleen Carlton  Sheila Carter  Phillip Chancellor II  Matt Clark  Tom Fisher  Scott Grainger, Sr.  Raul Guittierez  Nathan Hastings  Brittany Hodges  Cole Howard  Diane Jenkins  Mamie Johnson  Veronica Landers  Leanna Love  Bobby Marsino  Mari Jo Mason  Ryan McNeil  Tricia McNeil  Dina Abbott Mergeron  Cassie Newman  Sabrina Costelana Newman  Brock Reynolds  Danny Romalotti  Rex Sterling  Grace Turner  Megan Dennison Viscardi  Larry Warton  Isabella Braa Williams  Mary Williams  Hope Wilson  Drucilla Winters

Popular couples

Victor and Nikki  Nick and Sharon  J.T. and Colleen  Daniel and Lily

Related articles

Genoa City  Nadia's Theme  Storylines  Children  Minor characters  Cast members  Crew  The Bold and the Beautiful

Crew

Executive producers: Paul Rauch and Maria Arena Bell  Head writers: Maria Arena Bell, Hogan Sheffer and Scott Hamner  Created by: William J. Bell and Lee Phillip Bell.

Categories: The Young and the Restless characters | Fictional people in fashion

Silicon dioxide

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Crystal structure

In the vast majority of silicates, the Si atom shows tetrahedral coordination, with 4 oxygen atoms surrounding a central Si atom. The most common example is seen in the quartz crystalline form of silica SiO2. In each of the most thermodynamically stable crystalline forms of silica, on average, only 2 out of 4 of each the vertices (or oxygen atoms) of the SiO4 tetrahedra are shared with others, yielding the net chemical formula: SiO2.

Tetrahedral structural unit of silica (SiO2), the basic building block of the most ideal glass former. hematite beads

For example, in the unit cell of alpha-quartz, the central tetrahedron shares all 4 of its corner O atoms, the 2 face-centered tetrahedra share 2 of their corner O atoms, and the 4 edge-centered terahedra share just 1 of their O atoms with other SiO4 tetrahedra. This leaves a net average of 12 out of 24 (or 1 out of 2) total vertices for that portion of the 7 SiO4 tetrahedra which are considered to be a part of the unit cell for silica (see 3-D Unit Cell). pewter charm

SiO2 has a number of distinct crystalline forms in addition to amorphous forms. With the exception of stishovite and fibrous silica, all of the crystalline forms involve tetrahedral SiO4 units linked together by shared vertices in different arrangements. Silicon-oxygen bond lengths vary between the different crystal forms, for example in -quartz the bond length is 161 pm, whereas in -tridymite it is in the range 154171 pm. The Si-O-Si angle also varies between a low value of 140 in -tridymite, up to 180 in -tridymite. In -quartz the Si-O-Si angle is 144. filigree jewelry

The amorphous structure of glassy silica (SiO2) in two-dimensions. No long-range order is present, however there is local ordering with respect to the tetrahedral arrangement of oxygen (O) atoms around the silicon (Si) atoms.

Fibrous silica has a structure similar to that of SiS2 with chains of edge-sharing SiO4 tetrahedra. Stishovite, the highest pressure form, in contrast has a rutile like structure where silicon is 6 coordinate. The density of stishovite is 4.287 g/cm3, which compares to -quartz, the densest of the low pressure forms, which has a density of 2.648 g/cm3. The difference in density can be ascribed to the increase in coordination as the six shortest Si-O bond lengths in stishovite (four Si-O bond lengths of 176 pm and two others of 181 pm) are greater than the Si-O bond length (161 pm) in -quartz. The change in the coordination increases the ionicity of the Si-O bond. But more important is the observation that any deviations from these standard parameters constitute microstructural differences or variations which represent an approach to an amorphous, vitreous or glassy solid.

Note that the only stable form under normal conditions is -quartz and this is the form in which crystalline silicon dioxide is usually encountered. In nature impurities in crystalline -quartz can give rise to colours (see list).

Note also that both high temperature minerals, cristobalite and tridymite, have both a lower density and index of refraction than quartz. Since the composition is identical, the reason for the discrepancies must be in the increased spacing in the high temperature minerals. As is common with many substances, the higher the temperature the farther apart the atoms due to the increased vibration energy.

The high pressure minerals, stishovite and coesite, on the other hand, have a higher density and index of refraction when compared to quartz. This is probably due to the intense compression of the atoms that must occur during their formation, resulting in a more condensed structure.

Faujasite silica is another form of crystalline silica. It is obtained by dealumination of a low-sodium, ultra-stable Y zeolite with a combined acid and thermal treatment. The resulting product contains over 99% silica, has high crystallinity and high surface area (over 800 m2/g). Faujasite-silica has very high thermal and acid stability. For example, it maintains a high degree of long-range molecular order (or crystallinity) even after boiling in concentrated hydrochloric acid.

Crystalline forms of SiO2

Form

Crystal symmetry

Pearson symbol, group, No

Notes

Structure

-quartz

rhombohedral (trigonal)

hP9, P3121 No.152

Helical chains making individual single crystals optically active; -quartz converts to -quartz at 573 C

-quartz

hexagonal

hP18, P6222, No.180

closely related to -quartz (with an Si-O-Si angle of 155) and optically active; -quartz converts to -tridymite at 870 C

-tridymite

orthorhombic

oS24, C2221, No.20

metastable form under normal pressure

-tridymite

hexagonal

hP12, P63/mmc, No. 194

closely related to -tridymite; -tridymite converts to -cristobalite at 1470 C

-cristobalite

tetragonal

tP12, P41212, No. 92

metastable form under normal pressure

-cristobalite

cubic

cF104, Fd3m, No.227

closely related to -cristobalite; melts at 1705 C

keatite

tetragonal

tP36, P41212, No. 92

Si5O10, Si4O14, Si8O16 rings; synthesised from amorphous silica and alkali at high pressure

coesite

monoclinic

mS48, C2/c, No.15

Si4O8 and Si8O16 rings; high pressure form (higher than keatite)

stishovite

tetragonal

tP6, P42/mnm, No.136

rutile like with 6-fold coordinated Si; high pressure form (higher than coesite) and the densest of the polymorphs

melanophlogite

cubic

cP*, P4232, No.208

Si5O10, Si6O12 rings; mineral always found with hydrocarbons in interstitial spaces-a clathrasil

fibrous

orthorhombic

oI12, Ibam, No.72

like SiS2 consisting of edge sharing chains

faujasite

cubic

cF576, Fd3m, No.227

sodalite cages connected by hexagonal prisms; 12-membered ring pore opening; faujasite structure.

Molten silica exhibits several peculiar physical characteristics that are similar to the ones observed in liquid water: negative temperature expansion, density maximum and a heat capacity minimum. When molecular silicon monoxide, SiO, is condensed in an argon matrix cooled with helium along with oxygen atoms generated by microwave discharge, molecular SiO2 is produced which has a linear structure. Dimeric silicon dioxide, (SiO2)2 has been prepared by reacting O2 with matrix isolated dimeric silicon monoxide, (Si2O2). In dimeric silicon dioxide there are two oxygen atoms bridging between the silicon atoms with an Si-O-Si angle of 94 and bond length of 164.6 pm and the terminal Si-O bond length is 150.2 pm. The Si-O bond length is 148.3 pm which compares with the length of 161 pm in -quartz. The bond energy is estimated at 621.7 kJ/mol.

Quartz glass

Main article: glass

When silicon dioxide SiO2 is cooled rapidly enough, it does not crystallize but solidifies as a glass. The glass transition temperature of pure SiO2 is about 1600 K.

Chemistry

Manufactured silica fume at maximum surface area of 380 m2/g

Silicon dioxide is formed when silicon is exposed to oxygen (or air). A very thin layer (approximately 1 nm or 10 ) of so-called 'native oxide' is formed on the surface when silicon is exposed to air under ambient conditions. Higher temperatures and alternative environments are used to grow well-controlled layers of silicon dioxide on silicon, for example at temperatures between 600 and 1200 C, using the so-called "dry" or "wet" oxidation with O2 or H2O, respectively. The thickness of the layer of silicon replaced by the dioxide is 44% of the thickness of the silicon dioxide layer produced.

Alternative methods used to deposit a layer of SiO2 include:

Low temperature oxidation (400450 C) of silane

SiH4 + 2 O2 SiO2 + 2 H2O

Decomposition of tetraethyl orthosilicate (TEOS) at 680730 C

Si(OC2H5)4 SiO2 + H2O + 2 C2H4

Plasma enhanced chemical vapor deposition using TEOS at about 400 C

Si(OC2H5)4 + 12 O2 SiO2 + 10 H2O + 8 CO2

Polymerization of tetraethyl orthosilicate (TEOS) at below 100 C using amino acid as catalyst.

Pyrogenic silica (sometimes called fumed silica or silica fume), which is a very fine particulate form of silicon dioxide, is prepared by burning SiCl4 in an oxygen rich hydrocarbon flame to produce a "smoke" of SiO2:

SiCl4 + 2 H2 + O2 SiO2 + 4 HCl

Amorphous silica, silica gel, is produced by the acidification of solutions of sodium silicate to produce a gelatinous precipitate that is then washed and then dehydrated to produce colorless microporous silica.

Quartz exhibits a maximum solubility in water at temperatures about 340 C. This property is used to grow single crystals of quartz in a hydrothermal process where natural quartz is dissolved in superheated water in a pressure vessel which is cooler at the top. Crystals of 0.51 kg can be grown over a period of 12 months. These crystals are a source of very pure quartz for use in electronic applications.

Fluorine reacts with silicon dioxide to form SiF4 and O2 whereas the other halogen gases (Cl2, Br2, I2) react much less readily.

Silicon dioxide is attacked by hydrofluoric acid (HF) to produce "hexafluorosilicic acid":

SiO2 + 6 HF H2SiF6 + 2 H2O

HF is used to remove or pattern silicon dioxide in the semiconductor industry.

Silicon dioxide dissolves in hot concentrated alkali or fused hydroxide:

SiO2 + 2 NaOH Na2SiO3 + H2O

Silicon dioxide reacts with basic metal oxides (e.g. sodium oxide, potassium oxide, lead(II) oxide, zinc oxide or mixtures of oxides forming silicates and glasses as the Si-O-Si bonds in silica are broken successively). As an example the reaction of sodium oxide and SiO2 can produce sodium orthosilicate, sodium silicate and glasses, depending on the proportions of reactants:

2 Na2O + SiO2 Na4SiO4

Na2O + SiO2 Na2SiO3

(0.250.8)Na2O + SiO2 glass

Examples of such glasses have commercial significance e.g. soda lime glass, borosilicate glass, lead glass. In these glasses, silica is termed the network former or lattice former.

With silicon at high temperatures gaseous SiO is produced:

SiO2 + Si 2 SiO (gas)

Sol-gel

Bundle of optical fibers composed of high purity silica.

The sol-gel process is a wet chemical technique used for the fabrication of both glassy and ceramic materials. In this process, the sol (or solution) evolves gradually towards the formation of a gel-like network containing both a liquid phase and a solid phase. The basic structure or morphology of the solid phase can range anywhere from discrete colloidal particles to continuous chain-like polymer networks.

The term olloid is specific to the size of the individual particles, which are larger than atoms but small enough not to settle to the bottom of a container immediately. If the particles are large enough, then their dynamic behavior would be governed by forces of gravity and sedimentation. But if they are small enough to be colloids, then they may remain suspended in a liquid medium indefinitely. This critical size range (or particle diameter) typically ranges from tens of angstroms to a few microns.

1) In basic solutions (pH > 7), the particles may grow to sufficient size to become colloids, which are affected both by sedimentation and forces of gravity. Particles like these may become highly ordered in a manner similar to those seen in precious opal.

2) Under acidic conditions (pH < 7), a more open continuous network of chain-like polymers is formed. Polymers like this can be useful due to their viscosity, which allows them to be drawn or spun from solution into fibers, or drawn as thin films into surface coatings. Such glass fiber is useful for guided lightwave transmission, with ceramic fiber providing excellent thermal insulation.

Silica fiber mesh for thermal insulation.

In either case, the sol evolves towards the formation of a 2-phase gel. In the case of the colloid, the number of particles in an extremely dilute suspension may be so low that a significant amount of solvent may need to be removed initially for the gel-like properties to be recognized. This can be accomplished in any number of ways. The simplest method is to allow time for sedimentation to occur, and then pour off the remaining liquid. A variable speed centrifuge can also be used to accelerate the process of liquid removal.

Removal of the remaining liquid (solvent) phase requires a drying process, which is typically accompanied by a significant amount of shrinkage and densification. Since the remaining water will most likely reside within microstructural pores, the rate at which the solvent can be removed is ultimately determined by the distribution of pore space in the gel. Subsequent thermal treatment (or low temperature sintering at 500 600 C) may be performed in order to obtain a higher density product. With regard to methods of application:

1) The sol can be deposited on a substrate to form a film using dip-coating or spin-coating;

2) It can be cast into a suitable container with the desired shape;

3) It can be used to synthesize fine high-purity powders.

The sol-gel approach is a cheap and low-temperature technique that maintains a high degree of chemical purity. Thus it allows for total control of the product chemical composition. It can be used in ceramics manufacturing processes, as an investment casting material, or as a means of producing thin films or coatings.

Sol-gel derived components have diverse applications in optics, electronics, energy, space, physical and chemical sensors, biosensors, controlled drug release in medicine, and chemical separation on a cellular level. Ceramic powders of a wide range of chemical composition can be formed by such techniques. The generation of particles uniform in size and shape was investigated extensively by Egon Matijevic and his co-workers. In the case of chromium, aluminum and titanium salts, spherical particles were formed whereas particles of crystallographic symmetry resulted from solutions of copper and iron salts.

In 1956, Kolbe described the formation of spherical silica particles in basic solution. The mechanisms of precipitationnd the chemical conditions that bias the structure toward linear or branched structuresre the most critical issues faced in the chemistry laboratory by sol-gel scientists. Again, these are the factors which will ultimately determine the form of the microstructure over a range of length scales in the green or unfired body. Factors, such as chemical acidity which lead to the formation of linear polymers (as opposed to particles), are ideal for the formation of spinnable solutions such as those used for the formation of thin films and coatings as well as optical quality fiber.

Biomaterials

Sand from Pismo Beach, California including quartz, shell and rock fragments.

Silicification is quite common in the biological world and occurs in bacteria, single-celled organisms, plants, and animals (invertebrates and vertebrates). Crystalline minerals formed in the this environment often show exceptional physical properties (e.g. strength, hardness, fracture toughness) and tend to form hierarchical structures that exhibit microstructural order over a range of length or spatial scales. The minerals are crystallized from an environment that is undersaturated with respect to silicon, and under conditions of neutral pH and low temperature (040 C). Formation of the mineral may occur either within or outside of the cell wall of an organism, and specific biochemical reactions for mineral deposition exist that include lipids, proteins and carbohydrates.

Health effects

Quartz sand (silica) as main raw material for commercial glass production

Inhaling finely divided crystalline silica dust in very small quantities (OSHA allows 0.1 mg/m3) over time can lead to silicosis, bronchitis or (much more rarely) cancer, as the dust becomes lodged in the lungs and continuously irritates them, reducing lung capacities (silica does not dissolve over time). This effect can be an occupational hazard for people working with sandblasting equipment, products that contain powdered crystalline silica and so on. Children, asthmatics of any age, allergy sufferers and the elderly (all of whom have reduced lung capacity) can be affected in much shorter periods of time. Amorphous silica, such as fumed silica is not associated with development of silicosis. Laws restricting silica exposure with respect to the silicosis hazard specify that the silica is both crystalline and dust-forming.

In respects other than inhalation, pure silicon dioxide is inert and harmless. Because some silicas take on water, extended exposure may cause local drying of the skin or other tissue. Pure silicon dioxide produces no fumes and is insoluble in vivo. (in the body) It is indigestible, with zero nutritional value and zero toxicity.[citation needed] When silica is ingested orally, it passes unchanged through the gastrointestinal (GI) tract, exiting in the feces, leaving no trace behind.[citation needed] Small pieces of silicon dioxide are equally harmless[citation needed], as long as they are not large enough to mechanically obstruct the GI tract, or jagged enough to lacerate its lining.

A study which followed subjects for 15 years found that higher levels of silica in water appeared to decrease the risk of dementia. The study found that for every 10 milligram-per-day intake of silica in drinking water, the risk of dementia dropped by 11%.

See also

Sol-gel

Crystal structure

Physics of glass

Glass transition

Amorphous carbonia

Fused silica

Silicon carbide

Mesoporous silica

Diatomaceous earth

Thermal oxidation

References

^ Iler, R.K. (1979). The Chemistry of Silica. Plenum Press. ISBN 047102404X. 

^ Lynn Townsend White, Jr. (1961). "Eilmer of Malmesbury, an Eleventh Century Aviator: A Case Study of Technological Innovation, Its Context and Tradition". Technology and Culture 2 (2): 97111. doi:10.2307/3101411. 

^ Lidong Wang, D Major, P Paga, D Zhang, M G Norton, D N McIlroy (2006). "High yield synthesis and lithography of silica-based nanospring mats". Nanotechnology 17: S298303. doi:10.1088/0957-4484/17/11/S12. 

^ Kihara, K. (1990). "An X-ray study of the temperature dependence of the quartz structure". European Journal of Mineralogy 2: 63. 

^ a b c d e f g Holleman, A. F.; Wiberg, E. (2001), Inorganic Chemistry, San Diego: Academic Press, ISBN 0-12-352651-5 

^ a b c d e f g Greenwood, Norman N.; Earnshaw, A. (1984), Chemistry of the Elements, Oxford: Pergamon, pp. 39399, ISBN 0-08-022057-6 

^ Wells A.F. (1984). Structural Inorganic Chemistry. Oxford Science Publications. ISBN 0-19-855370-6. 

^ Kirfel, A.; Krane, H. G.; Blaha, P.; Schwarz, K.; Lippmann, T. (2001). "Electron-density distribution in stishovite, SiO2: a new high-energy synchrotron-radiation study". Acta Crystallographica A 57: 663. doi:10.1107/S0108767301010698. 

^ a b J. Scherzer (1978). "Dealuminated faujasite-type structures with SiO2/Al2O3 ratios over 100". Journal of Catalysis 54: 285. doi:10.1016/0021-9517(78)90051-9. 

^ Lager G.A., Jorgensen J.D., Rotella F.J. (1982). "Crystal structure and thermal expansion of a-quartz SiO2 at low temperature". Journal of Applied Physics 53: 67516756. doi:10.1063/1.330062. 

^ Wright A.F., Lehmann M.S. (1981). "The Structure of Quartz at 25 and 590 C Determined by Neutron Diffraction". Journal of Solid State Chemistry 36: 371380. doi:10.1016/0022-4596(81)90449-7. 

^ a b Kuniaki Kihara, Matsumoto T., Imamura M. (1986). "Structural change of orthorhombic-I tridymite with temperature: A study based on second-order thermal-vibrational parameters". Zeitschrift fur Kristallographie 177: 2738. 

^ Downs R.T., Palmer D.C. (1994). "The pressure behavior of a cristobalite". American Mineralogist 79: 914. http://www.geo.arizona.edu/xtal/group/pdf/AM79_9.pdf. 

^ Wright A.F., Leadbetter A.J. (1975). "The structures of the b-cristobalite phases of SiO2 and AlPO4". Philosophical Magazine 31: 13911401. doi:10.1080/00318087508228690. 

^ Shropshire J., Keat P.P., Vaughan P.A. (1959). "The crystal structure of keatite, a new form of silica". Zeitschrift fur Kristallographie 112: 409413. 

^ Levien L., Prewitt C.T. (1981). "High-pressure crystal structure and compressibility of coesite". American Mineralogist 66: 324333. http://www.minsocam.org/ammin/AM66/AM66_324.pdf. 

^ Smyth J.R., Swope R.J., Pawley A.R. (1995). "H in rutile-type compounds: II. Crystal chemistry of Al substitution in H-bearing stishovite". American Mineralogist 80: 454456. http://rruff.geo.arizona.edu/doclib/am/vol80/AM80_454.pdf. 

^ Skinner B.J., Appleman D.E. (1963). "Melanophlogite, a cubic polymorph of silica". American Mineralogist 48: 854867. http://www.minsocam.org/ammin/AM48/AM48_854.pdf. 

^ Rosemarie Szostak (1998). Molecular sieves: Principles of Synthesis and Identification. Springer. ISBN 0751404802. http://books.google.com/books?id=lteintjA2-MC&printsec=frontcover. 

^ Weiss A. (1954). "Zur Kenntnis der faserigen Siliciumdioxyd-Modifikation". Zeitschrift fuer Anorganische und Allgemeine Chemie 276: 95112. 

^ Hriljac J.A., Eddy M.M., Cheetham A.K., Donohue J.A., Ray G.J. (1993). "Powder Neutron Diffraction and 29Si MAS NMR Studies of Siliceous Zeolite-Y". Journal of Solid State Chemistry 106: 6672. doi:10.1006/jssc.1993.1265. 

^ Shell, Scott M.; Pablo G. Debenedetti, Athanassios Z. Panagiotopoulos (2002). "Molecular structural order and anomalies in liquid silica". Phys. Rev. E 66: 011202. doi:10.1103/PhysRevE.66.011202. http://www.engr.ucsb.edu/~shell/papers/2002_PRE_silica.pdf. 

^ Peter Jutzi, Ulrich Schubert (2003). Silicon chemistry: from the atom to extended systems. Wiley-VCH. ISBN 3527306471. http://books.google.com/books?id=iRNDUz0F0rwC&pg=PP1. 

^ a b Sunggyu Lee (2006). Encyclopedia of chemical processing. CRC Press. ISBN 0824755634. 

^ Robert Doering, Yoshio Nishi (2007). Handbook of Semiconductor Manufacturing Technology. CRC Press. ISBN 1574446754. http://books.google.com/books?id=Qi98H-iTgLEC&printsec=frontcover. 

^ A.B.D. Nandiyanto; S.-G Kim; F. Iskandar; and K. Okuyama (2009). "Synthesis of Silica Nanoparticles with Nanometer-Size Controllable Mesopores and Outer Diameters". Microporous and Mesoporous Materials 120 (3): 447453. doi:10.1016/j.micromeso.2008.12.019. 

^ Fournier R.O., Rowe J.J. (1977). "The solubility of amorphous silica in water at high temperatures and high pressures". American Mineralogist 62: 10521056. http://www.minsocam.org/ammin/AM62/AM62_1052.pdf. 

^ Brinker, C.J.; G.W. Scherer (1990). Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing. Academic Press. ISBN 0121349705. 

^ L.L.Hench, J.K.West; West, Jon K. (1990). "The Sol-Gel Process". Chem. Rev. 90: 3372. doi:10.1021/cr00099a003. 

^ a b Matijevic, Egon. (1986). "Monodispersed colloids: art and science". Langmuir 2: 12. doi:10.1021/la00067a002. 

^ Brinker, C. J.; Mukherjee, S. P. (1981). "Conversion of monolithic gels to glasses in a multicomponent silicate glass system". Journal of Materials Science 16: 1980. doi:10.1007/BF00540646. 

^ Klein, L. (1994). Sol-Gel Optics: Processing and Applications. Springer Verlag. ISBN 0792394240. http://books.google.com/books?id=wH11Y4UuJNQC&printsec=frontcover. 

^ Dislich, Helmut (1971). "New Routes to Multicomponent Oxide Glasses". Angewandte Chemie International Edition in English 10: 363. doi:10.1002/anie.197103631. 

^ Brinker, C. J.; Mukherjee, S. P. (1981). "Conversion of monolithic gels to glasses in a multicomponent silicate glass system". Journal of Materials Science 16: 1980. doi:10.1007/BF00540646. 

^ Sakka, S; Kamiya, K (1980). "Glasses from metal alcoholates". Journal of Non-Crystalline Solids 42: 403. doi:10.1016/0022-3093(80)90040-X. 

^ Yoldas, B. E. (1979). "Monolithic glass formation by chemical polymerization". Journal of Materials Science 14: 1843. doi:10.1007/BF00551023. 

^ Prochazka, S.; Klug, F. J. (1983). "Infrared-Transparent Mullite Ceramic". Journal of the American Ceramic Society 66: 874. doi:10.1111/j.1151-2916.1983.tb11004.x. 

^ Ikesue, Akio; Kinoshita, Toshiyuki; Kamata, Kiichiro; Yoshida, Kunio (1995). "Fabrication and Optical Properties of High-Performance Polycrystalline Nd:YAG Ceramics for Solid-State Lasers". Journal of the American Ceramic Society 78: 1033. doi:10.1111/j.1151-2916.1995.tb08433.x. 

^ Ikesue, A (2002). "Polycrystalline Nd:YAG ceramics lasers". Optical Materials 19: 183. doi:10.1016/S0925-3467(01)00217-8. 

^ "Toxicological Overview of Amorphous Silica in Working Environment". http://www.degussa-nano.de/nano/MCMSbase/Pages/ProvideResource.aspx?respath=/NR/rdonlyres/1E02FAD4-E5D6-4CE2-8E5C-E19F4DAC7838/0/Toxicological_Overview_Amorphous_Silica_in_Working_Environment.pdf. 

^ Rondeau, V; Jacqmin-Gadda, H; Commenges, D; Helmer, C; Dartigues, Jf (2009). "Aluminum and silica in drinking water and the risk of Alzheimer's disease or cognitive decline: findings from 15-year follow-up of the PAQUID cohort.". American journal of epidemiology 169 (4): 48996. doi:10.1093/aje/kwn348. PMID 19064650. 

External links

Tridymite, International Chemical Safety Card 0807

Quartz, International Chemical Safety Card 0808

Cristobalite, International Chemical Safety Card 0809

amorphous, NIOSH Pocket Guide to Chemical Hazards

crystalline, as respirable dust,NIOSH Pocket Guide to Chemical Hazards

Formation of silicon oxide layers in the semiconductor industry. LPCVD and PECVD method in comparison. Stress prevention.

Quartz SiO2 piezoelectric properties

Silica (SiO2) and Water

Media related to Silicon dioxide at Wikimedia Commons

 "Silica". Encyclopdia Britannica (11th ed.). 1911. 

v  d  e

  Silicon compounds

SiBr4  SiC  SiCl4  SiF4  SiI4  SiO  SiO2  SiS2  Si3N4

v  d  e

Silica minerals

Crystalline

Coesite  Cristobalite  Moganite  Quartz  Stishovite  Tridymite

Cryptocrystalline

Chalcedony  Chert  Flint  Jasper

Amorphous

Fulgurite  Lechatelierite  Opal

Miscellaneous

Quartzite  Tiger's eye

Notable varieties

Chalcedony

Agate  Carnelian  Chrome chalcedony  Chrysoprase  Heliotrope  Moss agate  Lake Superior agate  Onyx

Jasper / Chert

Mozarkite  Orbicular jasper

Opal

Fiorite  Geyserite

Quartz

Amethyst  Ametrine  Citrine  Milky quartz  Rose quartz  Smoky quartz  Shocked quartz

Categories: Silicon compounds | Oxides | Ceramic materials | Refractory materials | IARC Group 1 carcinogens | Common oxide glass components | ExcipientsHidden categories: Chemboxes which contain changes to watched fields | All articles with unsourced statements | Articles with unsourced statements from January 2010

Roof

China Product


Parts of a roof

There are two parts to a roof, its supporting structure and its outer skin, or uppermost weatherproof layer. In a minority of buildings, the outer layer is also a self-supporting structure.

The roof structure is generally supported upon walls, although some building styles, for example, geodesic and A-frame, blur the distinction between wall and roof. teak mat

Support bath mat non slip

Main article: Roof construction spa bath mat

The roof of a library, Sweden.

Tree-like supporting pillars of roof (Sagrada Famlia, Barcelona).

The supporting structure of a roof usually comprises beams that are long and of strong, fairly rigid material such as timber, and since the mid 19th century, cast iron or steel. In countries that use bamboo extensively, the flexibility of the material causes a distinctive curving line to the roof, characteristic of Oriental architecture.

Timber lends itself to a great variety of roof shapes. The timber structure can fulfil an aesthetic as well as practical function, when left exposed to view.

Stone lintels have been used to support roofs since prehistoric times, but cannot bridge large distances. The stone arch came into extensive use in the ancient Roman period and in variant forms could be used to span spaces up to 140 feet across. The stone arch or vault, with or without ribs, dominated the roof structures of major architectural works for about 2,000 years, only giving way to iron beams with the Industrial Revolution and the designing of such buildings as Paxton's Crystal Palace, completed 1851.

With continual improvements in steel girders, these became the major structural support for large roofs, and eventually for ordinary houses as well. Another form of girder is the reinforced concrete beam, in which metal rods are encased in concrete, giving it greater strength under tension.

Outer layer

This part of the roof shows great variation dependent upon availability of material. In simple vernacular architecture, roofing material is often vegetation, such as thatches, the most durable being sea grass with a life of perhaps 40 years. In many Asian countries bamboo is used both for the supporting structure and the outer layer where split bammboo stems are laid turned alternately and overlapped. In areas with an abundance of timber, wooden shingles are used, while in some countries the bark of certain trees can be peeled off in thick, heavy sheets and used for roofing.

The 20th century saw the manufacture of composition shingles which can last from a thin 20-year shingle to the thickest which are limited lifetime shingles, the cost depending on the thickness and durability of the shingle. When a layer of shingles wears out, they are usually stripped, along with the underlay and roofing nails, allowing a new layer to be installed. An alternative method is to install another layer directly over the worn layer. While this method is faster, it does not allow the roof sheathing to be inspected and water damage, often associated with worn shingles, to be repaired. Having multiple layers of old shingles under a new layer causes roofing nails to be located further from the sheathing, weakening their hold. The greatest concern with this method is that the weight of the extra material could exceed the dead load capacity of the roof structure and cause collapse.

Slate is an ideal, and durable material, while in the Swiss Alps roofs are made from huge slabs of stone, several inches thick. The slate roof is often considered the best type of roofing. A slate roof may last 75 to 150 years, and even longer. However, slate roofs are often expensive to install in the USA, for example, a slate roof may have the same cost as the rest of the house. Often, the first part of a slate roof to fail is the fixing nails; they corrode, allowing the slates to slip. In the UK, this condition is known as "nail sickness". Because of this problem, fixing nails made of stainless steel or copper are recommended, and even these must be protected from the weather.

Roofs made of cut turf (modern ones known as Green roofs, traditional ones as sod roofs) have good insulating properties and are increasingly encouraged as a way of "greening" the Earth. Adobe roofs are roofs of clay, mixed with binding material such as straw or animal hair, and plastered on lathes to form a flat or gently sloped roof, usually in areas of low rainfall.

In areas where clay is plentiful, roofs of baked tiles have been the major form of roof. The casting and firing of roof tiles is an industry that is often associated with brickworks. While the shape and colour of tiles was once regionally distinctive, now tiles of many shapes and colours are produced commercially, to suit the taste and pocketbook of the purchaser.

Sheet metal in the form of copper and lead has also been used for many hundreds of years. Both are expensive but durable, the vast copper roof of Chartres Cathedral, oxidised to a pale green colour, having been in place for hundreds of years. Lead, which is sometimes used for church roofs, was most commonly used as flashing in valleys and around chimneys on domestic roofs, particularly those of slate. Copper was used for the same purpose.

In the 19th century, iron, electroplated with zinc to improve its resistance to rust, became a light-weight, easily-transported, waterproofing material. While its insulating properties were poor, its low cost and easy application made it the most accessible commercial roofing, world wide. Since then, many types of metal roofing have been developed. Steel shingle or standing-seam roofs last about 50 years or more depending on both the method of installation and the moisture barrier (underlayment) used and are between the cost of shingle roofs and slate roofs. In the 20th century a large number of roofing materials were developed, including roofs based on bitumen (already used in previous centuries), on rubber and on a range of synthetics such as thermoplastic and on fibreglass.

Outer layer

Cameroon, a wattle and daub house, roofed with banana leaves.

Japan, rice straw thatch.

England, slate.

Hungary, terracotta tiles.

Namibia, metal roof.

Insulation

Some roofing materials, particularly those of natural fibrous material, such as thatch, have excellent insulating properties. For those that do not, extra insulation is often installed under the outer layer. In developed countries, the majority of dwellings have a ceiling installed under the structural member of the roof. The purpose is to insulate against heat and cold, noise, dirt and often from the droppings and lice of birds who frequently choose roofs as nesting places.

Other forms of insulation are felt or plastic sheeting, sometimes with a reflective surface, installed directly below the tiles or other material; synthetic foam batting laid above the ceiling and recycled paper products and other such materials that can be inserted or sprayed into roof cavities.

So called Cool roofs are becoming increasingly popular, and in some cases are mandated by local codes. Cool roofs are defined as roofs with both high reflectivity and high emissivity.

Drainage

The primary job of most roofs is to keep out water. The large area of a roof repels a lot of water, which must be directed in some suitable way, so that it does not cause damage or inconvenience.

Flat roof of adobe dwellings generally have a very slight slope. In a Middle Eastern country, where the roof may be used for recreation, it is often walled, and drainage holes must be provided to stop water from pooling and seeping through the porous roofing material.

Similar problems, although on a very much larger scale, confront the builders of modern commercial properties which often have flat roofs. Because of the very large nature of such roofs, it is essential that the outer skin is of a highly impermiable material. Most industrial and commercial structures have conventional roofs of low pitch.

In general, the pitch of the roof is proportional to the amount of precipitation. Houses in areas of low rainfall frequently have roofs of low pitch while those in areas of high rainfall and snow, have steep roofs. The longhouses of Papua New Guinea, for example, being roof-dominated architecture, the high roofs sweeping almost to the ground. The high steeply-pitched roofs of Germany and Holland are typical in regions of snowfall. In parts of the North America such as Buffalo USA or Montreal Canada, there is a required minimum slope of 6 inches in 12 inches, a pitch of 30 degrees.

There are regional building styles which contradict this trend, the stone roofs of the Alpine chalets being usually of gentler incline. These buildings tend to accumulate a large amount of snow on them, which is seen as a factor in their insulation. The pitch of the roof is in part determined by the roofing material available, a pitch of 3/12 or greater slope generally being covered with asphalt shingles, wood shake, corrugated steel, slate or tile.

The water repelled by the roof during a rainstorm is potentially damaging to the building that the roof protects. If it runs down the walls, it may seep into the mortar or through panels. If it lies around the foundations it may cause seepage to the interior, rising damp or dry rot. For this reason most buildings have a system in place to protect the walls of a building from most of the roof water. Overhanging eaves are commonly employed for this purpose. Most modern roofs and many old ones have systems of valleys, gutters, waterspouts, waterheads and drainpipes to remove the water from the vicinity of the building. In many parts of the world, roofwater is collected and stored for domestic use.

Areas prone to heavy snow benefit from a metal roof because their smooth surfaces shed the weight of snow more easily and resist the force of wind better than a wood shingle or a concrete tile roof.

See also: Trade hall roof collapse in Katowice, Poland and Bad Reichenhall ice rink roof collapse

Insulation, drainage and solar roofing

Snow on the roof of houses in Poland.

The flat roofs of the Middle East, Israel.

Steeply pitched, gabled roofs in Northern Europe.

The overhanging eaves of China.

Green roof with solar panels, Findhorn.

Solar roofs

Newer systems include solar shingles which generate electricity as well as cover the roof. There are also solar systems available that generate hot water or hot air and which can also act as a roof covering. More complex systems may carry out all of these functions: generate electricity, recover thermal energy, and also act as a roof covering.

Solar systems can be integrated with roofs by:

integration in the covering of pitched roofs, e.g. solar shingles.

mounting on an existing roof, e.g. solar panel on a tile roof.

integration in a flat roof membrane using heat welding, e.g. PVC.

mounting on a flat roof with a construction and additional weight to prevent uplift from wind.

Roof shapes

Arched Roof

Barrel-arched

Catenary

Conical

Cut Roof

Domical

Flat

Helm Roof - Rhenish helm - a pyramidal roof with gable ends -- Speyer Cathedral

Outshot

Pyramidal

Ridged

Pitched or gabled

Asian traditional style

Crow-stepped gable (also called corbie step) gable

Dutch gable a hybrid of hipped and gable

Shaped gable

Salt-box

Saddleback a gabled roof atop a tower

Hip roof includes a sketch of a Dutch gable (Australian terminology)

Half-hipped

Mansard with the pitch divided into a shallow slope above a steeper slope

Gambrel as a mansard, but on only two sides of the roof

Bell-cast as a mansard, but with the shallow slope below the steeper slope

pavilion

Skillion roof single-sloped, lean to, or shed roof

cat-slide

Lean-to

Saw-tooth

Roof shapes

Flat roof, Western Australia.

Mansard roof on a county jail, Mount Gilead, Ohio.

Temple roof Chang Mai, Thailand with a decorated gable end and ceramic tile covering.

Commercially available roofing materials

The weather proofing material is the topmost or outermost layer, exposed to the weather. Many materials have been used as weather proofing material:

Thatch is roofing made of plant material, in overlapping layers.

Wheat Straw, widely used in England, France and other parts of Europe.

Seagrass, used in coastal areas where there are esturies such as Scotland. Has a longer life than straw. Claimed to have a life in exccess of 60 years.

Shingles, Wood shingles longer than 16" are called shakes in North America. Shingles is the generic term for a roofing material that is in many overlapping sections, regardless of the nature of the material.

Redcedar. Life expectancy, up to 30 years. However, young growth redcedar has a short life expectancy. High cost. Should be allowed to breathe, usually installed on lath strip for this purpose. The lath may rest on a roof deck with underlayment or be fastened directly to the rafters.

Hardwood. Very durable roofing found in Colonial Australian architecture, its use now limited to restorations.

Slate. Higher cost with a life expectancy of 50 to 200 years depending on the thickness and type of slate used. Being a heavy material, the supporting structure must be rated to support the weight load.

Ceramic tile. High cost, life of 20-100 years.

Imbrex and tegula, style dating back to ancient Greece and Rome.

Metal shakes or shingles. Long life. High cost, suitable for roofs of 4/12 pitch or greater. Because of the flexibility of metal, they can be manufactured to lock together, giving durability and reducing assembly time.

Mechanically seamed metal. Long life. High cost, suitable for roofs of low pitch such as 0.5/12 to 3/12 pitch.

Concrete, usually reinforced with fibres of some sort. Not suitable in climates that experience many freeze/thaw cycles during a year which will cause this type of material to form cracks and fail.

Asphalt shingle, made of bitumen embedded in an organic or fiberglass mat, usually covered with colored, man-made ceramic grit. Cheaper and lighter than slate or tiles. Life expectancies vary from 20 to 50 years depending on the product. Sun is the enemy of asphalt shingles so longer life can be expected in cloudier locations or at higher latitudes.

Asbestos shingles. Lifespans vary. Fireproof. Rarely used anymore because of health concerns. Abatement costs can be high when the old roof needs to be replaced and is subject to additional state and local environmental regulation and oversight.

Membrane. membrane roofing is in large sheets, generally fused in some way at the joints to form a continuous surface.

Thermosetting plastic (e.g. EPDM rubber). Synthetic rubber sheets adhered together with contact adhesive or tape. Primary application is big box store with large open areas and little vertical protrusions.

Thermoplastic (e.g. PVC, TPO, CSPE). Plastic sheets welded together with hot air creating one continuous sheet membrane. Can be rewelded with the exception of CSPE. Lends itself well to both big box and small roof application because of its hot air weldability.

Modified bitumen heat welded, asphalt adhered or installed with adhesive. Asphalt is mixed with polymers such as APP or SBS, then applied to fiberglass and/or polyester mat, seams sealed by locally melting the asphalt with heat, hot mopping of asphalt, or adhesive. Lends itself well to all applications.

Built-Up Roof Multiple plies of asphalt saturated organic felt or coated fiberglass felts. Plies of felt are adhered with hot asphalt, coal tar pitch or adhesive.

Sprayed-in-Place Polyurethane Foam (SPUF) Foam sprayed in-place on the roof, then coated with a wide variety of coatings, or in some instances, covered with gravel.

Fabric.

Polyester.

PTFE, (synthetic fluoropolymer) embedded in fibreglass.

Metal roofing. Generally a relatively inexpensive building material.

Galvanised steel frequently manufactured with wavy corrugations to resist lateral flexing and fitted with exposed fasteners. Widely used for low cost and durability. Sheds are normally roofed with this material. Known as Gal iron or Corro, it was the most extensively used roofing material of 20th century Australia, now replaced in popularity by steel roofing coated with an alloy of zinc and aluminium, claimed to have up to four times the life of galvanized steel.

Standing-seam metal with concealed fasteners.

Mechanically seamed metal with concealed fasteners contains sealant in seams for use on very low sloped roofs.

Flat-seam metal with soldered seams.

Reed thatch on the island of Sylt

Wooden shingles

A church roof under repair with terracotta tiles

Imbrex and tegula tiles, with a newly tiled roof in the foreground

Bitumen, USA

Corrugated iron, Australia

Sheet metal roof

PVC roof

Gallery of significant roofs

Imbrex and tegula tiles on the dome of Florence Cathedral.

The marble dome of the Taj Mahal.

The hip roofs and dormers of Chateau Chenonceau.

The polychrome tiles of the Hospice of Beaune, France.

The copper roof of Speyer Cathedral, Germany. photo Wolfi.

The lead roof of King's College Chapel, England.

The glass roof of the Grand Palais, Paris.

The glazed ceramic tiles of the Sydney Opera House.

See also

Wikimedia Commons has media related to: Roofs

Bituminous waterproofing

Building construction

Building envelope

Green roof

Metal roof

Metal Roofing Alliance

Roof crush

Roof garden

Roofer

Roofing felt

Solar panel

Tensile architecture

Tented roof

Thin-shell structure

Tile

History

List of Greco-Roman roofs

References

^ Thatching specifications

^ Fleming, Honour, & Pevsner, A Dictionary of Architecture

^ Thatching Information

^ a b c Robert Roskind (2000). Building Your Own House. Ten Speed Press. p. 353. http://books.google.com/books?id=Q3QCV5wzmJoC. Retrieved 2009-03-14. 

^ Hometips Wooden shingle roofing, with good diagrams

^ a b Taunton Press Staff (1997). Roofing. Taunton Press. p. 11. http://books.google.com/books?id=VFTkJrl3WEEC&pg=PA11&dq=. Retrieved 2009-03-14. 

^ a b Steven Bolt (1996). Roofing the right way. McGraw-Hill Professional. p. 7. ISBN 0070066507. http://books.google.com/books?id=iDjyitMjGHkC&pg=PA8&dq=. Retrieved 2009-03-14. 

^ HomeTips: Metal shingle roofing

^ Asbestos and Your Health, Victorian Government

^ Asbestos Diseases Advisory Service

^ Ken Watson, Executive Director, National Association of Steel Framed Housing. Steel Framed Housing. p. 2. http://www.innovatek.co.nz/pdfs/Steel_Framed_Housing.pdf. Retrieved accessdate=2009-03-14. 

Further reading

Francis Ching; Building Construction Illustrated, Visual Dictionary of Architecture, Architecture: Form, Space, and Order.

External links

Roof innovations and patents

Categories: Roofs | Structural engineering | Structural system | Tensile architecture

Pressure measurement

China Product


Absolute, gauge and differential pressures - zero reference

Although pressure is an absolute quantity, everyday pressure measurements, such as for tire pressure, are usually made relative to ambient air pressure. In other cases measurements are made relative to a vacuum or to some other ad hoc reference. When distinguishing between these zero references, the following terms are used:

Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure. commercial steam cleaner

Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. upholstery steam cleaner

Differential pressure is the difference in pressure between two points. surface steam cleaner

The zero reference in use is usually implied by context, and these words are only added when clarification is needed. Tire pressure and blood pressure are gauge pressures by convention, while atmospheric pressures, deep vacuum pressures, and altimeter pressures must be absolute. Differential pressures are commonly used in industrial process systems. Differential pressure gauges have two inlet ports, each connected to one of the volumes whose pressure is to be monitored. In effect, such a gauge performs the mathematical operation of subtraction through mechanical means, obviating the need for an operator or control system to watch two separate gauges and determine the difference in readings. Moderate vacuum pressures are often ambiguous, as they may represent absolute pressure or gauge pressure without a negative sign. Thus a vacuum of 26 inHg gauge is equivalent to an absolute pressure of 30 inHg (typical atmospheric pressure) 26 inHg = 4 inHg.

Atmospheric pressure is typically about 100 kPa at sea level, but is variable with altitude and weather. If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid will vary as atmospheric pressure changes. For example, when a car drives up a mountain, the tire pressure goes up. Some standard values of atmospheric pressure such as 101.325 kPa or 100 kPa have been defined, and some instruments use one of these standard values as a constant zero reference instead of the actual variable ambient air pressure. This impairs the accuracy of these instruments, especially when used at high altitudes.

Use of the atmosphere as reference is usually signified by a (g) after the pressure unit e.g. 30 psi g, which means that the pressure measured is the total pressure minus atmospheric pressure. There are two types of gauge reference pressure: vented gauge (vg) and sealed gauge (sg).

A vented gauge pressure transmitter for example allows the outside air pressure to be exposed to the negative side of the pressure sensing diaphragm, via a vented cable or a hole on the side of the device, so that it always measures the pressure referred to ambient barometric pressure. Thus a vented gauge reference pressure sensor should always read zero pressure when the process pressure connection is held open to the air.

A sealed gauge reference is very similar except that atmospheric pressure is sealed on the negative side of the diaphragm. This is usually adopted on high pressure ranges such as hydraulics where atmospheric pressure changes will have a negligible effect on the accuracy of the reading, so venting is not necessary. This also allows some manufacturers to provide secondary pressure containment as an extra precaution for pressure equipment safety if the burst pressure of the primary pressure sensing diaphragm is exceeded.

There is another way of creating a sealed gauge reference and this is to seal a high vacuum on the reverse side of the sensing diaphragm. Then the output signal is offset so the pressure sensor reads close to zero when measuring atmospheric pressure.

A sealed gauge reference pressure transducer will never read exactly zero because atmospheric pressure is always changing and the reference in this case is fixed at 1 bar.

An absolute pressure measurement is one that is referred to absolute vacuum. The best example of an absolute referenced pressure is atmospheric or barometric pressure.

To produce an absolute pressure sensor the manufacturer will seal a high vacuum behind the sensing diaphragm. If the process pressure connection of an absolute pressure transmitter is open to the air, it will read the actual barometric pressure.

Units

Pressure Units

 

pascal

(Pa)

bar

(bar)

technical atmosphere

(at)

atmosphere

(atm)

torr

(Torr)

pound-force per

square inch

(psi)

1 Pa

1 N/m2

105

1.0197105

9.8692106

7.5006103

145.04106

1 bar

100,000

106 dyn/cm2

1.0197

0.98692

750.06

14.5037744

1 at

98,066.5

0.980665

1 kgf/cm2

0.96784

735.56

14.223

1 atm

101,325

1.01325

1.0332

1 atm

760

14.696

1 torr

133.322

1.3332103

1.3595103

1.3158103

1 Torr;  1 mmHg

19.337103

1 psi

6.894103

68.948103

70.307103

68.046103

51.715

1 lbf/in2

Example reading:  1 Pa = 1 N/m2  = 105 bar  = 10.197106 at  = 9.8692106 atm, etc.

The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (Nm2 or kgm1s2). This special name for the unit was added in 1971; before that, pressure in SI was expressed in units such as N/m. When indicated, the zero reference is stated in parenthesis following the unit, for example 101 kPa (abs). The pound per square inch (psi) is still in widespread use in the US and Canada, notably for cars. A letter is often appended to the psi unit to indicate the measurement's zero reference; psia for absolute, psig for gauge, psid for differential, although this practice is discouraged by the NIST .

Because pressure was once commonly measured by its ability to displace a column of liquid in a manometer, pressures are often expressed as a depth of a particular fluid (e.g. inches of water). The most common choices are mercury (Hg) and water; water is nontoxic and readily available, while mercury's density allows for a shorter column (and so a smaller manometer) to measure a given pressure.

Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column does not define pressure precisely. When 'millimetres of mercury' or 'inches of mercury' are quoted today, these units are not based on a physical column of mercury; rather, they have been given precise definitions that can be expressed in terms of SI units. The water-based units usually assume one of the older definitions of the kilogram as the weight of a litre of water.

Although no longer favoured by measurement experts, these manometric units are still encountered in many fields. Blood pressure is measured in millimetres of mercury in most of the world, and lung pressures in centimeters of water are still common. Natural gas pipeline pressures are measured in inches of water, expressed as '"WC' ('Water Column'). Scuba divers often use a manometric rule of thumb: the pressure exerted by ten metres depth of water is approximately equal to one atmosphere. In vacuum systems, the units torr, micrometre of mercury (micron), and inch of mercury (inHg) are most commonly used. Torr and micron usually indicates an absolute pressure, while inHg usually indicates a gauge pressure.

Atmospheric pressures are usually stated using kilopascal (kPa), or atmospheres (atm), except in American meteorology where the hectopascal (hPa) and millibar (mbar) are preferred. In American and Canadian engineering, stress is often measured in kip. Note that stress is not a true pressure since it is not scalar. In the cgs system the unit of pressure was the barye (ba), equal to 1 dyncm2. In the mts system, the unit of pressure was the pieze, equal to 1 sthene per square metre.

Many other hybrid units are used such as mmHg/cm or grams-force/cm (sometimes as kg/cm and g/mol2 without properly identifying the force units). Using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as a unit of force is forbidden in SI; the unit of force in SI is the newton (N).

Static and Dynamic pressure

Static pressure is uniform in all directions, so pressure measurements are independent of direction in an immovable (static) fluid. Flow, however, applies additional pressure on surfaces perpendicular to the flow direction, while having little impact on surfaces parallel to the flow direction. This directional component of pressure in a moving (dynamic) fluid is called dynamic pressure. An instrument facing the flow direction measures the sum of the static and dynamic pressures; this measurement is called the total pressure or stagnation pressure. Since dynamic pressure is referenced to static pressure, it is neither gauge nor absolute; it is a differential pressure.

While static gauge pressure is of primary importance to determining net loads on pipe walls, dynamic pressure is used to measure flow rates and airspeed. Dynamic pressure can be measured by taking the differential pressure between instruments parallel and perpendicular to the flow. Pitot-static tubes, for example perform this measurement on airplanes to determine airspeed. The presence of the measuring instrument inevitably acts to divert flow and create turbulence, so its shape is critical to accuracy and the calibration curves are often non-linear.

Applications

Altimeter

Barometer

MAP sensor

Pitot tube

Sphygmomanometer

Instruments

Many instruments have been invented to measure pressure, with different advantages and disadvantages. Pressure range, sensitivity, dynamic response and cost all vary by several orders of magnitude from one instrument design to the next. The oldest type is the liquid column (a vertical tube filled with mercury) manometer invented by Evangelista Torricelli in 1643. The U-Tube was invented by Christian Huygens in 1661.

Hydrostatic

Hydrostatic gauges (such as the mercury column manometer) compare pressure to the hydrostatic force per unit area at the base of a column of fluid. Hydrostatic gauge measurements are independent of the type of gas being measured, and can be designed to have a very linear calibration. They have poor dynamic response.

Piston

Piston-type gauges counterbalance the pressure of a fluid with a solid weight or a spring. Another name for piston gauge is deadweight tester. For example, dead-weight testers used for calibration or tire-pressure gauges.

Liquid column

The difference in fluid height in a liquid column manometer is proportional to the pressure difference.

Liquid column gauges consist of a vertical column of liquid in a tube whose ends are exposed to different pressures. The column will rise or fall until its weight is in equilibrium with the pressure differential between the two ends of the tube. A very simple version is a U-shaped tube half-full of liquid, one side of which is connected to the region of interest while the reference pressure (which might be the atmospheric pressure or a vacuum) is applied to the other. The difference in liquid level represents the applied pressure. The pressure exerted by a column of fluid of height h and density is given by the hydrostatic pressure equation, P = hg. Therefore the pressure difference between the applied pressure Pa and the reference pressure P0 in a U-tube manometer can be found by solving Pa P0 = hg. If the fluid being measured is significantly dense, hydrostatic corrections may have to be made for the height between the moving surface of the manometer working fluid and the location where the pressure measurement is desired.

Although any fluid can be used, mercury is preferred for its high density (13.534 g/cm3) and low vapour pressure. For low pressure differences well above the vapour pressure of water, water is commonly used (and "inches of water" is a common pressure unit). Liquid-column pressure gauges are independent of the type of gas being measured and have a highly linear calibration. They have poor dynamic response. When measuring vacuum, the working liquid may evaporate and contaminate the vacuum if its vapor pressure is too high. When measuring liquid pressure, a loop filled with gas or a light fluid must isolate the liquids to prevent them from mixing. Simple hydrostatic gauges can measure pressures ranging from a few Torr (a few 100 Pa) to a few atmospheres. (Approximately 1,000,000 Pa)

A single-limb liquid-column manometer has a larger reservoir instead of one side of the U-tube and has a scale beside the narrower column. The column may be inclined to further amplify the liquid movement. Based on the use and structure following type of manometers are used

Simple Manometer

Micromanometer

Differential manometer

Inverted differential manometer

A McLeod gauge, drained of mercury

McLeod gauge

A McLeod gauge isolates a sample of gas and compresses it in a modified mercury manometer until the pressure is a few mmHg. The gas must be well-behaved during its compression (it must not condense, for example). The technique is slow and unsuited to continual monitoring, but is capable of good accuracy.

Useful range: above 10-4 torr (roughly 10-2 Pa) as high as 106 Torr (0.1 mPa),

0.1 mPa is the lowest direct measurement of pressure that is possible with current technology. Other vacuum gauges can measure lower pressures, but only indirectly by measurement of other pressure-controlled properties. These indirect measurements must be calibrated to SI units via a direct measurement, most commonly a McLeod gauge.

Aneroid

Aneroid gauges are based on a metallic pressure sensing element which flexes elastically under the effect of a pressure difference across the element. "Aneroid" means "without fluid," and the term originally distinguished these gauges from the hydrostatic gauges described above. However, aneroid gauges can be used to measure the pressure of a liquid as well as a gas, and they are not the only type of gauge that can operate without fluid. For this reason, they are often called mechanical gauges in modern language. Aneroid gauges are not dependent on the type of gas being measured, unlike thermal and ionization gauges, and are less likely to contaminate the system than hydrostatic gauges. The pressure sensing element may be a Bourdon tube, a diaphragm, a capsule, or a set of bellows, which will change shape in response to the pressure of the region in question. The deflection of the pressure sensing element may be read by a linkage connected to a needle, or it may be read by a secondary transducer. The most common secondary transducers in modern vacuum gauges measure a change in capacitance due to the mechanical deflection. Gauges that rely on a change in capacitances are often referred to as Baratron gauges.

Bourdon

Membrane-type manometer

A Bourdon gauge uses a coiled tube, which, as it expands due to pressure increase causes a rotation of an arm connected to the tube. In 1849 the Bourdon tube pressure gauge was patented in France by Eugene Bourdon.

The pressure sensing element is a closed coiled tube connected to the chamber or pipe in which pressure is to be sensed. As the gauge pressure increases the tube will tend to uncoil, while a reduced gauge pressure will cause the tube to coil more tightly. This motion is transferred through a linkage to a gear train connected to an indicating needle. The needle is presented in front of a card face inscribed with the pressure indications associated with particular needle deflections. In a barometer, the Bourdon tube is sealed at both ends and the absolute pressure of the ambient atmosphere is sensed. Differential Bourdon gauges use two Bourdon tubes and a mechanical linkage that compares the readings.

In the following illustrations the transparent cover face of the pictured combination pressure and vacuum gauge has been removed and the mechanism removed from the case. This particular gauge is a combination vacuum and pressure gauge used for automotive diagnosis:

Indicator side with card and dial

Mechanical side with Bourdon tube

the left side of the face, used for measuring manifold vacuum, is calibrated in centimetres of mercury on its inner scale and inches of mercury on its outer scale.

the right portion of the face is used to measure fuel pump pressure and is calibrated in fractions of 1 kgf/cm on its inner scale and pounds per square inch on its outer scale.

Mechanical details

Mechanical details

Stationary parts:

A: Receiver block. This joins the inlet pipe to the fixed end of the Bourdon tube (1) and secures the chassis plate (B). The two holes receive screws that secure the case.

B: Chassis plate. The face card is attached to this. It contains bearing holes for the axles.

C: Secondary chassis plate. It supports the outer ends of the axles.

D: Posts to join and space the two chassis plates.

Moving Parts:

Stationary end of Bourdon tube. This communicates with the inlet pipe through the receiver block.

Moving end of Bourdon tube. This end is sealed.

Pivot and pivot pin.

Link joining pivot pin to lever (5) with pins to allow joint rotation.

Lever. This an extension of the sector gear (7).

Sector gear axle pin.

Sector gear.

Indicator needle axle. This has a spur gear that engages the sector gear (7) and extends through the face to drive the indicator needle. Due to the short distance between the lever arm link boss and the pivot pin and the difference between the effective radius of the sector gear and that of the spur gear, any motion of the Bourdon tube is greatly amplified. A small motion of the tube results in a large motion of the indicator needle.

Hair spring to preload the gear train to eliminate gear lash and hysteresis.

Diaphragm

A pile of pressure capsules with corrugated diaphragms in an aneroid barograph.

A second type of aneroid gauge uses the deflection of a flexible membrane that separates regions of different pressure. The amount of deflection is repeatable for known pressures so the pressure can be determined by using calibration. The deformation of a thin diaphragm is dependent on the difference in pressure between its two faces. The reference face can be open to atmosphere to measure gauge pressure, open to a second port to measure differential pressure, or can be sealed against a vacuum or other fixed reference pressure to measure absolute pressure. The deformation can be measured using mechanical, optical or capacitive techniques. Ceramic and metallic diaphragms are used.

Useful range: above 10-2 Torr (roughly 1 Pa)

For absolute measurements, welded pressure capsules with diaphragms on either side are often used.

Shape:

Flat

corrugated

flattened tube

capsule

Bellows

In gauges intended to sense small pressures or pressure differences, or require that an absolute pressure be measured, the gear train and needle may be driven by an enclosed and sealed bellows chamber, called an aneroid, which means "without liquid". (Early barometers used a column of liquid such as water or the liquid metal mercury suspended by a vacuum.) This bellows configuration is used in aneroid barometers (barometers with an indicating needle and dial card), altimeters, altitude recording barographs, and the altitude telemetry instruments used in weather balloon radiosondes. These devices use the sealed chamber as a reference pressure and are driven by the external pressure. Other sensitive aircraft instruments such as air speed indicators and rate of climb indicators (variometers) have connections both to the internal part of the aneroid chamber and to an external enclosing chamber.

Electronic pressure sensors

Main article: Pressure sensor

Piezoresistive Strain Gage

Uses the piezoresistive effect of bonded or formed strain gauges to detect strain due to applied pressure.

Capacitive

Uses a diaphragm and pressure cavity to create a variable capacitor to detect strain due to applied pressure.

Magnetic

Measures the displacement of a diaphragm by means of changes in inductance (reluctance), LVDT, Hall Effect, or by eddy current principal.

Piezoelectric

Uses the piezoelectric effect in certain materials such as quartz to measure the strain upon the sensing mechanism due to pressure.

Optical

Uses the physical change of an optical fiber to detect strain due applied pressure.

Potentiometric

Uses the motion of a wiper along a resistive mechanism to detect the strain caused by applied pressure.

Resonant

Uses the changes in resonant frequency in a sensing mechanism to measure stress, or changes in gas density, caused by applied pressure.

Thermal conductivity

Generally, as a real gas increases in density -which may indicate an increase in pressure- its ability to conduct heat increases. In this type of gauge, a wire filament is heated by running current through it. A thermocouple or Resistance Temperature Detector (RTD) can then be used to measure the temperature of the filament. This temperature is dependent on the rate at which the filament loses heat to the surrounding gas, and therefore on the thermal conductivity. A common variant is the Pirani gauge which uses a single platinum filament as both the heated element and RTD. These gauges are accurate from 10 Torr to 103 Torr, but they are sensitive to the chemical composition of the gases being measured.

Two wire

One wire coil is used as a heater, and the other is used to measure nearby temperature due to convection.

Pirani (one wire)

A Pirani gauge consists of a metal wire open to the pressure being measured. The wire is heated by a current flowing through it and cooled by the gas surrounding it. If the gas pressure is reduced, the cooling effect will decrease, hence the equilibrium temperature of the wire will increase. The resistance of the wire is a function of its temperature: by measuring the voltage across the wire and the current flowing through it, the resistance (and so the gas pressure) can be determined. This type of gauge was invented by Marcello Pirani.

Thermocouple gauges and thermistor gauges work in a similar manner, except a thermocouple or thermistor is used to measure the temperature of the wire.

Useful range: 10-3 - 10 Torr (roughly 10-1 - 1000 Pa)

Ionization gauge

Ionization gauges are the most sensitive gauges for very low pressures (also referred to as hard or high vacuum). They sense pressure indirectly by measuring the electrical ions produced when the gas is bombarded with electrons. Fewer ions will be produced by lower density gases. The calibration of an ion gauge is unstable and dependent on the nature of the gases being measured, which is not always known. They can be calibrated against a McLeod gauge which is much more stable and independent of gas chemistry.

Thermionic emission generate electrons, which collide with gas atoms and generate positive ions. The ions are attracted to a suitably biased electrode known as the collector. The current in the collector is proportional to the rate of ionization, which is a function of the pressure in the system. Hence, measuring the collector current gives the gas pressure. There are several sub-types of ionization gauge.

Useful range: 10-10 - 10-3 torr (roughly 10-8 - 10-1 Pa)

Most ion gauges come in two types: hot cathode and cold cathode, a third type exists which is more sensitive and expensive known as a spinning rotor gauge, but is not discussed here. In the hot cathode version an electrically heated filament produces an electron beam. The electrons travel through the gauge and ionize gas molecules around them. The resulting ions are collected at a negative electrode. The current depends on the number of ions, which depends on the pressure in the gauge. Hot cathode gauges are accurate from 103 Torr to 1010 Torr. The principle behind cold cathode version is the same, except that electrons are produced in a discharge created by a high voltage electrical discharge. Cold Cathode gauges are accurate from 102 Torr to 109 Torr. Ionization gauge calibration is very sensitive to construction geometry, chemical composition of gases being measured, corrosion and surface deposits. Their calibration can be invalidated by activation at atmospheric pressure or low vacuum. The composition of gases at high vacuums will usually be unpredictable, so a mass spectrometer must be used in conjunction with the ionization gauge for accurate measurement.

Hot cathode

Bayard-Alpert hot cathode ionization gauge

A hot cathode ionization gauge is mainly composed of three electrodes all acting as a triode, where the cathode is the filament. The three electrodes are a collector or plate, a filament, and a grid. The collector current is measured in picoamps by an electrometer. The filament voltage to ground is usually at a potential of 30 volts while the grid voltage at 180210 volts DC, unless there is an optional electron bombardment feature, by heating the grid which may have a high potential of approximately 565 volts. The most common ion gauge is the hot cathode Bayard-Alpert gauge, with a small ion collector inside the grid. A glass envelope with an opening to the vacuum can surround the electrodes, but usually the Nude Gauge is inserted in the vacuum chamber directly, the pins being fed through a ceramic plate in the wall of the chamber. Hot cathode gauges can be damaged or lose their calibration if they are exposed to atmospheric pressure or even low vacuum while hot. The measurements of a hot cathode ionization gauge are always logarithmic.

Electrons emitted from the filament move several times in back and forth movements around the grid before finally entering the grid. During these movements, some electrons collide with a gaseous molecule to form a pair of an ion and an electron (Electron ionization). The number of these ions is proportional to the gaseous molecule density multiplied by the electron current emitted from the filament, and these ions pour into the collector to form an ion current. Since the gaseous molecule density is proportional to the pressure, the pressure is estimated by measuring the ion current.

The low pressure sensitivity of hot cathode gauges is limited by the photoelectric effect. Electrons hitting the grid produce x-rays that produce photoelectric noise in the ion collector. This limits the range of older hot cathode gauges to 108 Torr and the Bayard-Alpert to about 1010 Torr. Additional wires at cathode potential in the line of sight between the ion collector and the grid prevent this effect. In the extraction type the ions are not attracted by a wire, but by an open cone. As the ions cannot decide which part of the cone to hit, they pass through the hole and form an ion beam. This ion beam can be passed on to a

Faraday cup

Microchannel plate detector with Faraday cup

Quadrupole mass analyzer with Faraday cup

Quadrupole mass analyzer with Microchannel plate detector Faraday cup

ion lens and acceleration voltage and directed at a target to form a sputter gun. In this case a valve lets gas into the grid-cage.

See also: Electron ionization

Cold cathode

There are two subtypes of cold cathode ionization gauges: the Penning gauge (invented by Frans Michel Penning), and the Inverted magnetron, also called a Redhead gauge. The major difference between the two is the position of the anode with respect to the cathode. Neither has a filament, and each may require a DC potential of about 4 kV for operation. Inverted magnetrons can measure down to 1x1012 Torr.

Such gauges cannot operate if the ions generated by the cathode recombine before reaching the anodes. If the mean-free path of the gas within the gauge is smaller than the gauge's dimensions, then the electrode current will essentially vanish. A practical upper-bound to the detectable pressure is, for a Penning gauge, of the order of 103 Torr.

Similarly, cold cathode gauges may be reluctant to start at very low pressures, in that the near-absence of a gas makes it difficult to establish an electrode current - particularly in Penning gauges which use an axially symmetric magnetic field to create path lengths for ions which are of the order of metres. In ambient air suitable ion-pairs are ubiquitously formed by cosmic radiation; in a Penning gauge design features are used to ease the set-up of a discharge path. For example, the electrode of a Penning gauge is usually finely tapered to facilitate the field emission of electrons.

Maintenance cycles of cold cathode gauges is generally measured in years, depending on the gas type and pressure that they are operated in. Using a cold cathode gauge in gases with substantial organic components, such as pump oil fractions, can result in the growth of delicate carbon films and shards within the gauge which eventually either short-circuit the electrodes of the gauge, or impede the generation of a discharge path.

Calibration

Pressure gauges are either direct- or indirect-reading. Hydrostatic and elastic gauges measure pressure are directly influenced by force exerted on the surface by incident particle flux, and are called direct reading gauges. Thermal and ionization gauges read pressure indirectly by measuring a gas property that changes in a predictable manner with gas density. Indirect measurements are susceptible to more errors than direct measurements.

Dead weight tester

McLeod

mass spec + ionization

Dynamic transients

When fluid flows are not in equilibrium, local pressures may be higher or lower than the average pressure in a medium. These disturbances propagate from their source as longitudinal pressure variations along the path of propagation. This is also called sound. Sound pressure is the instantaneous local pressure deviation from the average pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water. The effective sound pressure is the root mean square of the instantaneous sound pressure over a given interval of time. Sound pressures are normally small and are often expressed in units of microbar.

frequency response of pressure sensors

resonance

History

Further information: Timeline of temperature and pressure measurement technology

European (CEN) Standard

EN 472 : Pressure gauge - Vocabulary.

EN 837-1 : Pressure gauges. Bourdon tube pressure gauges. Dimensions, metrology, requirements and testing.

EN 837-2 : Pressure gauges. Selection and installation recommendations for pressure gauges.

EN 837-3 : Pressure gauges. Diaphragm and capsule pressure gauges. Dimensions, metrology, requirements and testing..

See also

Force gauge

Piezometer

Vacuum engineering

External links

Home Made Manometer

Manometer

References

^ NIST

^ [Was: "fluidengineering.co.nr/Manometer.htm". At 1/2010 that took me to bad link. Types of fluid Manometers]

^ Techniques of high vacuum

^ Beckwith, Thomas G.; Roy D. Marangoni and John H. Lienhard V (1993). "Measurement of Low Pressures". Mechanical Measurements (Fifth ed.). Reading, MA: Addison-Wesley. pp. 591595. ISBN 0-201-56947-7. 

^ Product brochure from Schoonover, Inc

^ VG Scienta

^ Robert M. Besanon, ed (1990). "Vacuum Techniques" (3rd edition ed.). Van Nostrand Reinhold, New York. pp. 12781284. ISBN 0-442-00522-9. 

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