Sensor Technology - Humidity Sensor

8/3/2019 Sensor Technology - Humidity Sensor

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C H A P T E R 12H um idity SensorsJohn Fontes, Se nior Ap plicat ions Engineer,Ho neyw ell Sensing and C ontrol

12.1 Humidi tyHumid i ty i s def ined as the water vapor con ten t in a i r o r o ther gases . Humid i ty i su s u a l l y m ea s u red i n te rm s o f ab s o l u te h u m i d i t y ( t h e ra t io o f t h e m as s o f wa t e r v ap o rto the vo lume of a i r o r gas ) , dew po in t ( the tempera tu re and p ressure a t which a gasbeg ins to con dens e in to a l iqu id ) , and re la t ive hum id i ty , o r RH ( the ra t io o f the mois -tu re con ten t o f a i r compared to the sa tu ra ted mois tu re l eve l a t the same tempera tu re o rp ressure) .T h e rm a l co n d u c t i v it y h u m i d i t y s en s o rs , a l s o k n o wn a s ab s o lu t e h u m i d i t y s en s or s , a r ecap ab l e o f m eas u r i n g ab s o l u te h u m i d i t y u s in g a s y s t em t h a t em p l o y s t wo t h e rm i s t o r sin a b r idge connect ion , even a t h igh tempera tu res o r in po l lu ted env i ronments .S ince the ear ly 1960s, ch i l l ed mir ro rs have been u sed to meas ure dew po in t , bu t thed ev e l o p m en t o f t h in f i lm cap ac i t iv e s en s o r s n o w a l lo ws m eas u rem en t o f d ew p o i n t s a tt emp era tu res as low as -4 0~ a t fa r l ess cos t and wi th g rea te r accuracy .R e l a t i ve h u m i d i t y was o n ce d e t e rm i n ed b y m eas u r i n g t h e ch an g e i n m o i s t u re ab s orp -t ion in s i lk , human ha i r , and la te r , ny lon and syn the t ics . Mechan ica l methods fo rm ea s u r i n g R H were i n t ro d u ced i n t h e 1 9 40 s . R ecen tl y , p o l y m er -b as ed r e s i st iv e an dcapac i t ive sensors have been developed .12.2 Senso r Types and TechnologiesR ecen t d ev e l o p m en t s i n s em i co n d u c t o r t e ch n o l o g y h av e m ad e p o s s i b l e h u m i d i t y s en -sors tha t a re h igh ly accura te , durab le , and cos t e f fec t ive . The mos t common humid i tysensors a re capac i t ive , res i st ive , and therma l conduct iv i ty . The fo l lowing sec t ionsd iscuss how each sensor type i s cons t ruc ted and used to measure humid i ty .

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Chapter 12C a p a c i t i v e R H S e n s o r sCapaci t ive RH sensors are used widely in indust r ial , commercial , and weather telem-e t ry app lica tions . They dom inate bo th a tmospher ic and process measurem ents and arethe on ly types o f fu ll - range RH mea sur ing dev ices capab le o f opera t ing accura te lydown to 0% RH. Because of thei r low temperature effect , they are often used overwide t em pera tu re ranges wi thou t ac tive t empera tu re com pensa t ion .In a capaci t ive RH sensor, change in dielect r ic constant i s almost di rect ly proport ionalto relat ive humidi ty in the environment . Typical change in capaci tance is 0 .2-0.5 pFfor 1% RH change . Bu lk capac i tance i s be tween 100 and 500 pF a t 50% RH a t 25~These sensors have low temperature coeff icient and can funct ion at h igh temperaturesup to 200~ The y are able to ful ly recove r from cond ensat ion and res is t chem icalvapors . Respo nse t ime ranges f rom 30 to 60 seconds fo r a 63% RH s tep change .Thermoset polymer-based capacit ive RH sensors di rect ly detect changes in rela-t ive saturat ion as a cha nge in sensor capac i tance wi th fas t response, h igh l ineari ty ,low hysteres is , and ex cel lent long-term s tabi li ty . Relat ive saturat ion is the sam e asambient relat ive humidi ty when the sensor is at ambient temperature. Because this i sa lmos t a lways the case , sensor capac i tance change i s then a measure o f RH change .These sensors use an indust r ial ly proven thermoset polymer, three layer capaci tanceconstruct ion, p lat inum elect rodes and except for high temperature vers ions , on-chipsi l icon integrated vol tage output s ignal condi t ioning. (See Figure 12.2.1 .)

DIRT, DU ST & O IL DOESNOT AFFECT SENSOR

THERMOSET.:.:.:PO LYM ER "" ":~PORO US PLATINUMTHERMOSET POLYMERPLATINUM LAYER

O O

SUBSTRATE (SILICON )

Figure 12.2. 1 Th is re la t ive hu m id i ty sensor has three- layer capac i tancecons t ruc t ion and cons is ts o f the rmose t po lymer , p la t inum e lec t rodes ,and a s il icon ch ip w i th in teg ra ted vo ltage o u tp u t s igna l cond i ti on ing .

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Humidity SensorsIn opera t ion , w ater vapor in the ac tive capac i to r ' s d ie lec tr ic l ayer equ i l ib ra tes w i ththe su rround ing gas . The porous p la t inum layer sh ie lds the d ie lec t r ic response f romexterna l in f luences whi le the p ro tec t ive po lymer over lay p rov ides mechan ica l p ro -tec t ion fo r the p la t inum layer f rom con tam inan ts such as d ir t, dus t and o i ls . A hea vycon taminan t l ayer o f d i r t wi l l s low down the sensor ' s response t ime because i t wi l lt ake longer fo r w ater vapo r to equ i l ib ra te in the sensor.T h e rm o s e t p o l y m er -b as ed cap ac it iv e s en so r s , a s o p p o s ed t o t h e rm o p l a s t i c -b as edcapac i t ive sensors , a l low h igher op era t ing tem pera tu res and p rov ide be t te r res is t iv ityaga ins t chemica l l iqu ids and vapors such as i sopropy l , benzene , to luene , fo rmalde-h y d es , oi ls , co m m o n c l ean i n g ag en t s, an d am m o n i a v ap o r in co n cen tra t io n s co m m o nto ch icken coops and p ig barns . In add i t ion , thermoset po lymer RH sensors p rov idethe longes t opera t ing l i fe in e thy lene ox ide-based (ETO) s te r i l i za t ion p rocesses .Thermoset th in f i lm po lymer capac i t ive sensors have been shown to have an a lmos tidea l response to RH, as opposed to abso lu te mois tu re ( i . e . , water vapor p ressure) .Th is response i s due to the d r iv ing fo rce f ree ene rgy fo r absorp t ion , G:

G = R T Ln (P/Po)w h e r e

G = d r iv ing fo rceR = gas cons tan tP - par t i a l water vapor p ressureP0 - sa tu ra t ion water vapor p ressure

P/Po i s the same as ambien t RH when the sensor i s a t ambien t t empera tu re . The re la -t ive sa tu ra t ion leve l d r iv ing sensor respo nse i s 100% a t the sensor t empera tu re T .Research has a l so demons t ra ted tha t the RH sensor ca l ib ra t ion in a i r app l ies to re la -t ive sa tu ra t ion measurement in o i l to wi th in 0 .3% (a resu l t which can be ex tended too ther chemica l ly compat ib le l iqu ids ) .R e s is tive H um i d i t y S e ns orsR es i s ti v e h u m i d i t y s en so r s m eas u re t h e im p ed an ce ch an g e , wh i ch u s u a l l y h as aninverse expone n t ia l re la t ionsh ip to humid i ty . (F igure 12 .2 .2 . ) Typ ica lly , the impe d-ance cha nge o f a m ediu m such as a conduct ive p o lymer , sa lt , o r t rea ted subs t ra te ism eas u red .T h e f i rs t m as s -p ro d u ced h u m i d i t y s en s o r was th e Du n m o re t y p e . P ro d u ced i n 1 9 40 , iti s st il l wide ly used in p rec i s ion a i r cond i t ion ing con t ro l s and fo r moni to r ing t ransmis -s ion l ines , an tennas , and wavegu ides used in t e lecommunica t ions .

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C h a p t e r 1 2

\0= 1000

1 i i I i i i i i i i i i i i i i i i i I i i i i i i i i

2O 30 4O SO 6O 70 8O 90RH (% )

Figure 12.2.2 Relationship o f impeda nce chang e to h um idi tySource : Rove t i, Denes. "Choos ing a Hu m id i ty Sensor : A Rev iew o f Th ree Techno log ies . "Sensors On l ine . Ju ly 2001 . h t tp :/ /ww w.senso rsrnag , co rn /a r t i c les /0701/54/ma in .sh tmlThe la tes t res i s tive hum id i ty sensors use cera mic coa t ing to p rov ide p ro tec t ion inenv i ronments where condensa t ion occurs . These sensors a re cons t ruc ted wi th nob leme ta l e lec t rodes dep os i ted by a pho to res i s t p rocess , and a subs t ra te su r face coa tedwi th a conduct ive po lymer /ceramic b inder mix tu re . The sensor i s p ro tec ted in a p las -t ic housing.I n t e rch an g eab i li t y i s b e t te r t han 3 % R H o v e r t h e 1 5 % -9 5 % R H ran g e , wh i le p re -c i s ion is conf i rmed to +2% RH . The recove ry t ime fo r resi s tive sensors f rom fu l lconde nsa t ion to 30% is a few m inu tes . Vol tage ou tpu t i s d i rec t ly p ropor t iona l to theambien t re la t ive humid i ty when a s igna l cond i t ioner i s used . For mos t res i s t ive sen-sors, response t ime i s f rom 10 to 30 seconds fo r a 63% s tep change , w hi le impe danc erange var ies f rom 1 k f~ to 100 M~ .Therma l C onductivi ty H um idity SensorsThermal conduct iv i ty humid i ty sensors (a l so known as abso lu te humid i ty sensors )measure abso lu te humid i ty by ca lcu la t ing the d i f fe rence be tween the thermal conduc-t iv i ty of dry air and air containing water vapor.These sensors a re cons t ruc ted us ing two negat ive tempera tu re coeff ic ien t (NTC)thermis to r e leme nts in a D C b r idge c i rcu it . O ne o f the e lements i s sea led in d ryn i t rogen , whi le the o ther i s expose d to the env i ronme nt . (See F igure 12 .2 .3 . ) Thed i f fe rence in the res i s tance be tw een the two therm is to rs i s d i rec t ly p ropor t iona l toabso lu te humid i ty .

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H u m i d i t y S e n s o r s

V ~ S E N S OR I; " " : ~ t le m e o e r e s ~ /AM BIE NT AIR ', ', of lhennis tors is directly .L..'>"THERM ISTOR : , proportional to alx ;du te .~i, , humick y.I !+ : ,!I !!I !! !! I

DR Y N I TR OGE N :E A L E D ' ,T H E R M I S T O R : ,! !

I . . , , . , 4

Figure 12.2.3: Thermal cond uct iv i ty (or abso lu te) h um id i ty sensors .Source: Rovet i , Denes . "Choos ing a Hu m id i ty Sensor : A Re v iew o f T hree Techno log ies . "S ens o rs O n l ine . J u l y 2001 . h t t p :/ /w w w .s ens o rs ma g , c o rn /a r t ic l es /07 01 /5 4 /m a i n . s h tm l12.3 Select ing and Specifying Humidity SensorsThe fo l lowing sec t ions address what d i f fe ren t ia tes each sensor f rom ano ther , inc lud-ing tempera tu re , accuracy , and in te rchangeab i l i ty . The advan tages and d i sadvan tagesof each sensor type a re a l so iden ti f ied .Selecting H um idity SensorsI m p o r t an t co n s i d e ra t io n s wh e n s e l ec ti n g a h u m i d i t y s en s o r i nc l u de :

9 A c c u r a c y9 I n t e rch an g eab i l i t y9 Repeatab i l i ty9 Stabi l i ty9 C o n d en s a t i o n r eco v e ry9 C o n t am i n an t r e s i s tan ce9 S i z e an d p ack ag i n g9 Co s t e f fec t iveness9 Co s t to rep lace sensor9 Cal ib ra t ion9 Co m plex i ty and re l iab i l i ty o f s igna l cond i t ion ing and da ta acqu is i t ion c i rcu i t ry

In genera l , env i ronmenta l cond i t ions fo r the g iven app l ica t ion wi l l d ic ta te the cho iceof sensor.

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Chap ter 12Selecting C apaci t ive RH SensorsApplicat ions for capaci t ive RH sensors are wide ranging, including

9 Au tomot ive onboard dev ices such as windsh ie ld defoggers9 Co mp uter p r in te rs9 M edical devices such as vent i lators and incubatorsm Appliances such as microwave ovens , refr igerators , and clothes dryers9 H V A Cm Recorders and da ta loggers9 Lea k de tec t ion Weather s tat ionsm Indus t ri a l and food process ing equ ipm ent Envi ronmenta l t es t chambers

Taking advan tage o f cu t t ing-edge p r inc ip les in sem iconductor des ign , man y ca-paci t ive sensors have minimal long-term dri f t and hysteres is . Incorporat ing acomplementary meta l ox ide semiconductor (CMOS) t imer pu l ses the sensor p roduc-ing near- l inear vol tage output . (See Figure 12.3.1 .)

2 5 0

m m m m m ~ ~ ~ i m~ ~ ~ m m m m m m mm n m m m m m m m munnmmmnm unnm m mmmm mmmmnmm

0 10 20 30 40 50 60 70 80 90 100R H ( % )

F i g u re 1 2 . 3 . 1 V o l ta g e o u t p u t w i t h a C M OS t im e r5ource : Rove t i , Denes. Cho os ing a Hum id i ty Sensor : A Rev iew o f Th ree Techno log ies . "Sensors Onf ine. July 2001 . ht tp: //wvv~.se nsorsm ag, orn/ar t ic les/0701/54/m ain.sh ml

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Hu m idity SensorsT y p ica l u n ce rt a i n ty i s + 2 % R H f ro m 5 % t o 9 5 % R H u s i n g t wo -p o i n t ca li b ra t io n . T h ecapac i t ive e f fec t o f the connect ing cab le re la tive to the smal l capac i tance changes o fthe sensor l imi t s the d i s tance the sens ing e lement can be loca ted f rom the s igna l con-di t ioning circui t ry to a pract ical range of less than 10 feet .Laser t r imm ing re duces var iance _+2%, imp rov ing d i rec t f i eld in te rchangeab i l i ty .C o m p u t e r r eca l i b ra t i o n p ro g ram s a re a ls o cap ab l e o f co m p en s a t i n g fo r s en so r cap ac i-tance f rom 100 to 500 pECapaci t ive RH sensors a re no t l inear be low a few percen t RH, which i s why manys en so r s i n co rp o ra t e a d ew-p o i n t m ea s u rem en t s y s t em t h a t em p l o y s m i c ro p ro ces s o r-based c i rcu i t ry to s to re ca l ib ra t ion da ta . Th is deve lopment has reduced the cos t o fhygrometers and t ransmi t te rs in HVAC and weather t e lemet ry app l ica t ions .Advantages

9 Ne ar- l inear vo l tage ou tpu t9 W i d e R H ran g e an d co n d en s a t i o n t o l eran ce9 In te rchang eab le , i f l aser t r imm ed9 S tab le over long- te rm use

Disadvantages9 D is tance f rom sens ing e lem ent to s igna l cond i t ion ing c i rcu it ry l imi ted

Select ing Resist ive Humidity SensorsRes is t ive sensors a re smal l , low-cos t humid i ty sensors tha t p rov ide long- te rm s tab i l -i ty and are h igh ly in te rchangeab le . They are su i tab le fo r many indus t r ia l , commerc ia l ,and res iden t ia l app l ica t ions , espec ia l ly co n t ro l and d i sp lay p roducts .Res i s t ive sensors respond non l inear ly to humid i ty changes , bu t they may be l inear izedby ana log o r d ig i ta l methods . Typ ica l var iab le res i s tance ranges f rom a few k i lohmsto 100 MV. Nominal exc i ta t ion f requency i s f rom 30 Hz to 10 kHz .RH sensors a re h igh ly in te rchange ab le (wi th in _+2% RH). E lec t ron ic s igna l cond i t ion-ing c i rcu i try c an be ca l ib ra ted a t a f ixed RH po in t , e l imina t ing the nee d fo r humid i tyca l ib ra t ion s tandards . Accuracy can be tes ted in an RH ca l ib ra t ion chamber o r by acomputer -based sys tem referenced to a s tandard ized env i ronment . Res i s t ive sensorsh av e a n o m i n a l o p e ra t in g t em p era t u re o f -4 0 ~ t o 1 0 0~

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Chapter 12Life ex pec tan cy i s less than f ive years in res iden t ia l and com me rc ia l app l ica t ions , bu texposure to con taminan ts may cause p remature fa i lu re . Res i s t ive sensors a l so tend tosh i f t va lues dur ing exposure to condensa t ion when water -so lub le coa t ings a re used .Advantages

9 N o ca l ib ra t ion standards , so h igh ly in te rchan geab le and f ield rep lace ab le9 L o n g - t e rm s t ab il it y9 Us ab l e f ro m rem o t e l o ca t io n s9 Sm al l s i ze9 L o w co s t

Disadvantages9 E x p o s u re t o ch em i ca l v ap o r s an d co n t am i n an t s m ay cau s e p rem a t u re f a i lu re9 Values m ay sh i f t wh en wa ter -so lub le coa t ings a re used

Selecting Thermal Conductiv i ty Humidity SensorsT h erm a l co n d u c t i v i t y h u m i d i t y s en s o r s a re co m m o n l y u s ed i n ap p li an ces , in c l u d i n gclo thes d ryers and microwave ovens . They are used in many indus t r ia l app l ica t ionsi n c l u d in g wo o d -d ry i n g k i l n s , d ry i n g m ach i n e ry , p h a rm aceu t i ca l p ro d u c t i o n , co o k i n g ,and food dehydra t ion .C o n s t ru c t ed wi t h g l a s s, s em i co n d u c t o r m a t e r ia l , h i g h - t em p era t u re p l a s ti c s , an d a l u m i -num, thermal conduct iv i ty sensors a re very durab le and res i s tan t to chemica l vapors .They p rov ide be t te r reso lu t ion than capac i t ive and res i s t ive sensors in t empera tu resg rea t e r t h an 2 0 0 ~ T y p i ca l accu racy i s + 3 g/m 3, wh i ch co n v e r ts t o ap p ro x i m a te ly+_5% RH at 40~ and _+0.5% RH at 100~Advantages

9 Very durab le9 W o rk we l l i n co r ro s iv e an d h i g h - t em p era t u re en v i ro n m en t s u p t o 5 7 5 ~9 Be t te r reso lu t ion than capac i t ive and res i s t ive sensors

Disadvantages9 Res pond s to any gas wi th therm al p roper t ies d i ffe ren t than d ry n i t rogen , wh ich

m ay a f f ec t m eas u rem en t .

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Hum idity Sensors1 2 .4 A p p l i c a b l e S t a n d a r d sStandards Bodies

American National Standards Insti tute (ANSI): http: / /www.ansi.orgA private, non-profit organization responsible for ad ministering the U.S. vol-untary s tandardization and conformity assessment system.

American Society of Testing and Materials (ASTM): http: / /www.astm.orgOne of the largest voluntary s tandards developm ent organizations in the world.Develops and publishes voluntary consensus s tandards for materials , products ,systems, and services.

Canadian Standards Association (CSA): http: / /www.csa.caNot-for-profit me mb ership-based association serving business, industry, gov-ernment, and con sumers in Can ada and around the world. Develops s tandardsfor enhanc ing public safety and health, advancing the q uali ty of l ife, helpingto preserve the environment, and facilitating trade.

Instrumentation, Sy stems, and Au toma tion Society (ISA): http: / /ww w.isa.orgHelps advance the theory, design, manufacture, and use o f sensors , instru-ments , com puters , and systems for measurem ent and control in a variety ofapplications.

International Electrotechnical Commission (IEC): http: / /www.iec.chPrepares and publishes international standards for all electrical, electronic, andrelated technologies .

International O rganization for Standardization (ISO)" http: / /ww w.iso.ch/ iso/en/ISOOnline .openerpageA n etwork of national s tandards insti tutes from 146 countries working in part-nership w ith international organizations, g overnments , industry, business andconsumer representatives .

Japane se Standards Association (JSA): http: / /ww w.jsa.or .jp/default_english.aspObjective is " to educate the public regarding the s tandardization and unif ica-t ion of industrial s tandards, and thereby to contribute to the imp rovem ent oftechno logy and the enha ncem ent of production eff iciency."

National In sti tute of Standards and Techno logy (NIST): http: / /www.nist .govFou nded in 1901, NIST is a non-regulatory federal agency w ithin the U.S.Commerce Department 's Technology Administration. I ts mission is to developand promote m easureme nt, s tandards, and techno logy to enhance productivity,facil i tate trade, and improve the quali ty o f l ife.

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Chapter 12Industry Organizat ions

American Society for Quali ty (ASQ): ht tp: / /www.asq.org/Purpose is to improve workplace and communit ies by advancing learning,quali ty improvement, and knowledge exchange. Advises the U.S. Congress,governm ent agencies, state legislatures, and other groups and individuals onquality-related topics.

International Measurement Confederat ion (IMEKO): ht tp: / /www.mit . tut . f i / imeko/Non-governmental federat ion of 36 member organizat ions. Promotes inter-national interchange of scientific and technical information in the field ofmeasurement and instrumentat ion and the international cooperat ion amongscientists and engineers f rom research and industry.

National Conference of Standards Laborator ies International (NCSL Internation-al):ht tp: / /www.ncsl i .org/A p rofessional associat ion for individuals involved in all aspects o f measure-ment science.

Underwriter ' s Laborator ies (UL): ht tp: / /www.ul .comAn independent , not-for-profit product-safety test ing and cer t if ication organi-zation.

12.5 In ter fac ing and D esign In form at ionTemperature and Humidity EffectsThe output o f all absorption-based hum idity sensors (capacit ive, bulk resistive,conductive f ilm, etc. ) is affected by both temp erature and % RH. Beca use o f this , tem-perature co mp ensation is used in applications that cal l for ei ther higher accuracy orwider operat ing temperature ranges.W hen cal ibrating a humidity sensor to com pensate for temperature, i t is best to mak ethe temperature m easurem ent as close as possible to the humidity sensor 's act ivearea~i .e . , within the same moisture microenvironment. This is especial ly true whencombining RH and temperature as a method for measuring dew point . Industr ialgrade hum idity and dew point instruments incorporate a 1000-ohm plat inum R TDon the back o f the ceramic sensor substrate for unm atched tem perature com pensa-t ion meas urem ent integri ty . No on-chip signal condit ioning is provided in these hightemperature sensors.

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Humidity SensorsVol tage OutputA h u m i d i t y s e n s o r w i t h a r e l a t i v e h u m i d i t y i n t e g r a t e d c i r c u i t ( R H I C ) h a s a l i n e a rvo l tage ou tp u t tha t i s a f unc t ion of Vsupply, % R H and te mp era ture . The ou tp u t i s " r a t io -met r ic , " so a s the supply vo l tage r i s es , the ou tpu t vo l tage r i s es in the same propor t ion .A s u rf a c e p l o t o f t h e s e n s o r b e h a v i o r f o r t e m p e r a t u r e s b e t w e e n 0 ~ a n d 8 5 ~ i ss h o w n i n F i g u r e 1 2 .5 . 1.

F igu re 12 .5 . 1 Sur face p l o t o f sens or behav io r .T h i s s u r f a c e p l o t is w e l l a p p r o x i m a t e d b y a c o m b i n a t i o n o f t w o e q u a t io n s "

1 . A "B es t F i t L in e a t 25~ ' ' o r a s imi la r, s ensor - spec i f i c equa t ion a t 25~ Thes e n s o r i n d e p e n d e n t " t y p i c a l " B e s t F i t L i n e a t 2 5 ~ ( b o l d l in e i n g r a p h ) is"

Vou J Vsupply ( 0 . 0 0 6 2 ( % R H ) + 0 . 1 6 )A s e n s o r - s p e c i f i c e q u a t i o n c a n b e o b t a i n e d f r o m a n R H s e n s o r p r i n t o u t . T h ep r i n t o u t e q u a t i o n assumes VsupplyJ 5 V D C a n d i s i n c l u d e d o r a v a i l a b l e a s a n o p -t ion on the sensor s .

2 . A s e n s o r - i n d e p e n d e n t e q u a t i o n , w h i c h c o r re c t s t h e % R H r e a d i n g ( f r o m t h eB e s t F i t L i n e E q u a t i o n ) f o r t e m p e r a t u r e , T :

T r u e R H - ( % R H ) / ( 1 . 0 5 4 6 - . 0 0 2 1 6 T ); T - ~O r T r ue R H - ( % R H ) / ( 1 .0 9 3 - . 0 0 1 2 T ); T - ~

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Chapter 12The previous equat ions match the typical surface plot (Best Fi t Line at 25~ or theactual surface plot (sensor-specif ic equat ion at 25~ to within the fol lowing to lerances:

+ 1 % fo r T > 2 0 ~_+2% for 10~ 176+ 5 % fo r T < 1 0 ~

D ew poin t ins t rum ents m ay ac coun t d i rec t ly fo r a sensor-spec if ic vers ion o f the su r-face p lo t v ia a look-up tab le .Note: Con ver t the obser ved outpu t vo ltage to % RH values v ia the f i rs t equat ion be foreapply ing the second equat ion .C o n d e n s a t i o n a n d W e t t i n gCo n de n sa t io n occurs w henev er the su rface tempera tu re o f the sensor ' s ac t ive a readrops be low the ambien t dew po in t o f the su rround ing gas . Condensa t ion fo rms onthe sensor (o r any su rface) even i f the su rface tempera tu re on ly momentar i ly d ropsbelow the ambien t dew po in t . Smal l t empera tu re f luc tua t ions near the sensor can un-k n o w i n g l y cau s e co n d en s a t i o n t o fo rm wh en o p e ra t in g a t h u m i d i t y l ev el s ab o v e 9 5 % .W hi le q u ick to condense , water i s s low to evapora te in h igh hum id i ty cond i t ions ( i. e. ,when the su rface tempera tu re o f the sensor i s on ly s l igh t ly above the ambien t dewpoin t . ) Because o f th i s , a sensor ' s recovery per iod f rom e i ther condensa t ion o r wet -t ing is muc h long er than i ts norma l t ime response . D ur ing recovery , the sensor ou tpu tsa cons tan t 100% RH s igna l regard less o f the ambien t RH.When an app l ica t ion ca l l s fo r con t inuous moni to r ing o f RH a t humid i ty leve l s o f 90%and above , t ake s teps to avo id in te rmi t ten t conde nsa t ion . Som e s t ra teg ies a re :

1. M ain ta in a good a i r mix ing to min imiz e loca l t empe ra tu re f luc tua tions .2 . So m e sensors use a s in tered s tainless s teel f i lter to protec t the sensor from

sp lash ing . A hydrophobic coa t ing fu r ther suppresses condensa t ion and wet t ingin rap id ly sa tu ra t ing and de-sa tu ra t ing o r sp lash-prone env i ronments .

3 . H eat the RH senso r so that the act ive area is hot ter than the local dew point .Th is can be done th rough an ex terna l hea ter o r by se l f hea t ing o f the CMOSRH ch ip by op era t ing i t a t a h igher supp ly vo l tage .

No te : H e a t in g an R H se n so r abo v e am b ie n t t e m pe ra tu re c han ge s i ts c a lib ra tio n an dma kes i t sens i t ive to therma l d is turbanc es such as a ir flow.

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Hu m idity SensorsIntegrated Signal ConditioningAl l R H s en s o rs q u i ck l y r eco v e r f ro m co n d en s a t i o n o r we t t i n g wi t h n o s h if t in ca l i-b ra t ion . How ever , a f te r 24-hour o r longe r exposures to e ither h igh (>95 %) R H orco n t i n u o u s co n d en s a t i o n , an u p ward s h if t o f 2 % t o 3 % R H m a y o ccu r. T h i s s h if t i sr ep ea t ab l e an d can b e r ev e r s ed b y p lac i n g t h e s en s o r in a lo w (1 0 % ) R H en v i ro n m en tfo r a 10-ho ur per iod .S i l i con in tegra ted humid i ty sensors incorpora te s igna l cond i t ion ing c i rcu i t ry on-ch ipwi th the sens ing capac i to r . These RHIC humid i ty sensors a re l aser t r immed so tha t a tVsupplyJ 5 V~ the ou tpu t vo l tage typ ica l ly spans 0 .8 V to 3 .9 V fo r the 0% RH to 100%RH rang e a t 25~ (Sensor-spec i f ic ca l ib ra t ion da ta p r in tou ts and Be s t F i t L ines a t25~ are e i ther inc luded o r ava ilab le as an op t ion on these sensors . )R H I C -b as ed s en s o rs a re f ac t o ry ca l ib ra ted , m i c ro -p o w er d ev i ce s w i t h e i t h e r in d i vi d -ua l ca l ib ra t ion and /o r good un i t - to -un i t in te rchangeab i l i ty . These fea tu res he lp OEMmanufac tu rers avo id in -house humid i ty ca l ib ra t ion cos t s and ex tend ba t te ry l i fe inp o r t ab l e i n s tru m en t s . I m p ro v ed accu racy can b e o b t a i n ed b y t u n i n g s y s t em e l ec t ro n -ics to accoun t fo r an ind iv idual se nsor ' s B es t F i t L ine a t 25~Interchangeability def ines the range o f vo l tages fo r any popu la t ion o f sensors a t anR H p o i nt . An i n t e rch an g eab i l i t y o f _+ 5% @ 0 % R H i s co m p ared t o th e b as e l i n e o u t p u tfo r the RH IC ch ip , which i s 0 .8 V to 3 .9 V (0 to 100% RH ) w i th an exc i ta t ion vo l tageo f 5 V D C .I f you take the base l ine s lope , 0 .031 V /% RH t imes _+ 5% RH you ge t _+ 0 .155 V. Th ism ean s tha t the outpu t vol ta ge fo r th is dev ice is 0 .8 V _+ 0 .155 V or a range of 0 .645 Vto 0 .955 V. When exposed to an RH of 0%, the ou tpu t o f the en t i re popu la t ion o f sen-sors wil l fal l within th is range.I n t e rch an g eab i l i t y i n c rease s w i t h i n c reas i n g R H s i n ce t h e R H I C d i e is a c t iv e l yt r i m m ed o n l y a t 0 % R H. T r i m m i n g a t o t h e r R H v a l u es is i m p rac ti ca l .I n t e rch an g eab i l i t y l e ts y o u l o wer d es i g n co s t b y av o i d in g ca l i b ra t in g y o u r s y s t em toeach i n d i v i d u a l s en so r . T h e R HI C s en s o r k eep s i ts i n t e rch an g eab i l it y an d accu racyadvan tages a t h igher humid i ty .Accuracy i s based on the spec i f ic ca l ib ra t ion curve fo r any ind iv idual sensor andequals _+2% RH. For ex amp le , i f a spec if ic sensor has an o u tpu t vo l tage o f .850 V a t0 % R H (5 V DC s u p p l y a s s u m ed ) , t h en th i s s en s o r sh o u l d a l way s o u t p u t t h is v o l t ag e_+0.062 V *** o r a range o f 0 .788 V to 0 .912 V. A ccu racy equals in te rchan geab i l i ty_+2% wh en y ou d on ' t ca l ib ra te you r sys te m to each sensor . I f you c a l ib ra te to eachR H I C s en so r , th en t o t a l a ccu racy can b e _ + 1 -2 % R H.

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Chapter 12R e f e r e n c e s a n d R e s o u r c e s

Christ ian, Stephan. "New generat ion of humidity sensors." Sensor Review 22(2002):300-2.

Honeywell web si te , humidity sensor information:http : / /content .honeywell .com/sens ing/prodinfo/humiditym oisture/#technical

M easurements S c ience Conference (MSC): h t tp : //www .msc-conf .com/Quelch, D . "H um idity Sensors for Industr ial Applicat ions." International C onfer-

ence on S ensors and T ransducers, Vol. 1. Tavistock, UK: Trident Exhibit ions,2001 .

Rit tersma, ZM. "Recent Achievements in Miniatur ized Humidity Sensors: A Re-view o f Transduction Techniques." Sensors and Actuators 96 (2002 ):196-210.

Roveti , D.K. "C hoosing a Hu midity S ensor: A Review of Three Technologies."Sensors 18 (2001):54-8.

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Sensor Technology - Humidity Sensor