Publisher’s version / Version de l'éditeur:
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la
première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
Building Research Note, 1980-01
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.
https://nrc-publications.canada.ca/eng/copyright
NRC Publications Archive Record / Notice des Archives des publications du CNRC : https://nrc-publications.canada.ca/eng/view/object/?id=d5c3f82a-394f-4caa-b404-afbdbdb707d7 https://publications-cnrc.canada.ca/fra/voir/objet/?id=d5c3f82a-394f-4caa-b404-afbdbdb707d7
NRC Publications Archive
Archives des publications du CNRC
This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.
For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.
https://doi.org/10.4224/40000560
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
Calibration of a humidity sensor using a cellulose crystallite strip
Hedlin, C. P.; Nicholson, R. G.
Ser
TH1 B9 2
National Research Conseil national
CALI BRATION
OF
A 1IUMIL)ITY SENSOR USlNG A CELLULOSE CRYSTALLlTE STRIPC.P.
Hedlin and R.G. Nicholson*
INTRODUCTION
'l'he Central Control and Monitoring System at the University of Saskatchewan, Saskatoon, provides the central control for heating and air-conditioning of the University buildings. This requires measurement of humidity. A device being considered for this work was calibrated at the Prairie Regional Station at the outset and after 18 months of use to assess its sensitivity to humidity change, rate of response, and sta- bility of calibration.
This hygrometer, "My-Cal model HS-3552-B" employs a cellulose crystallite strip which expands or contracts with the risc and fall of relative humidity. The active, hygroscopic material gains or loses moisture and changes dimension. This bends a stainless stcel strip and activates a "temperature-coefficient-matched pair of piezo resistive silicone strain gauges." A current transmitter is used to convert input resistance changes of the strain gauge system to a current output. Nomi- nally, the relationship is linear with a 4 mA current at 0% K.11. and 20 nu4 at 100%
R.H.
The manufacturer states, in part:
- the sensor can operate in the range of - 4 0 " ~ to + 2 5 0 ' ~ ~ and from 0 to 3000 psi,
-
the full range accuracy is given as 6% R.H.,- the sensor was designed to be resistant to contaminants; it can be cleaned with regular detergents or aliphatic hydrocarbon paint thinner,
- if washed with alcohol, other polar compounds or aromatic solvents, the sensor should be rinsed in water and dried scveral times before it is placed back in operation,
- liquid water absorbed by the cellulose structure will slow the
response to decreasing humidities until the liquid water evaporates.
C A L I BIIA'I' I O N
'rl~e scnsor was calibrated at the Prairie Regional Station of the Ilivision of Building Research, Saskatoon, in March 1977, using the Atmos- phcric Producer (1,2). Observations were made at three temperatures, 23.0, 4.4 and -12.1°C, in that order. At each level, the sensor was put through a humidity cycle. Finally, the calibration temperature was returned to 23.0°C and several points were checked on the desorption leg
of
the hysteresis loop. A single check was made on its response rate.In September 1978, after 18 months use in the Central Control and Moni- toring System of the University of Saskatchewan, it was recalibrated.
RESULTS
The c a l i b r a t i o n r e s u l t s a r e g i v e n i n F i g u r e 1 , a l o n g w i t h t h e m a n u f a c t u r e r ' s c a l i b r a t i o n v a l u e s . The v a l u e s f o r e a c h t e m p e r a t u r e a r e shown s e p a r a t e l y t o a v o i d t h e c o n f u s i o n c a u s e d by t h e i r o v e r l a p p i n g . For t h e same r e a s o n , t h e f i r s t and second c a l @ r a t i o n s a r e shown s e p a r a - t e l y . The s o r p t i o n p r o c e s s a t s u b f r e e z i n g t e m p e r a t u r e s i s o f t e n more a c c u r a t e l y r e p r e s e n t e d by b a s i n g r e l a t i v e h u m i d i t y on vapour p r e s s u r e o v e r subcooled w a t e r , r a t h e r t h a n t h e vapour p r e s s u r e o v e r i c e . A t -12.1°C, t h e vapour p r e s s u r e o f i c e i s 0.89 o f t h a t o v e r s u b c o o l e d w a t e r . Both b a s e s a r e u s e d i n F i g u r e l c .
For each new h u m i d i t y l e v e l , t h e s e t t i n g o f t h e Atmospheric Producer was a d j u s t e d ; from one h a l f t o one hour e l a p s e d b e f o r e a r e a d i n g was t a k e n . An hour was u s u a l l y a l l o w e d a t t h e lower t e m p e r a t u r e s where s e n s o r r e s p o n s e i s s l o w e r .
The s e n s i t i v i t y o f t h e s e n s o r d e c r e a s e s w i t h t e m p e r a t u r e . A t
23.0°C, i t a v e r a g e d 0.165 mA/% R . H . , a t 4 . 4 ' ~ i t was 0 . 1 5 mA/% f R . I i . , and a t -12.1°C it was 0.134 mA/% R . H . when r e f e r r e d t o s u b c o o l e d w a t e r and 0.119 mA/% R . H . when r e f e r r e d t o i c e .
A t 23.0°C, t h e maximum h y s t e r e s i s e f f e c t i s a b o u t 2% R . H . , a t 4.4OC
i t i s 4% R . H . , and a t - 1 2 . l U C i t i s roughly 7 % . (The l a s t i s based on a s i n g l e o b s e r v a t i o n on t h e lower chord o f t h e l o o p . ) These v a l u e s might have been reduced i f l o n g e r t i m e s had been a l l o w e d f o r t h e s e n s o r t o e q u i l i b r a t e .
The second c a l i b r a t i o n showed t h a t a s h i f t had t a k e n p l a c e , t h e r e l a t i v e h u m i d i t y b e i n g a b o u t 5% less f o r a g i v e n s e n s o r o u t p u t t h a n it
was when f i r s t c a l i b r a t e d .
A s i n g l e check on t h e r e s p o n s e r a t e o f t h e s e n s o r was made a t room w m p e r a t u r e . I t was t a k e n from t h e Atmospheric P r o d u c e r a t 90.6% R.ti. and 23.0uC, p l a c e d o v e r a s a t u r a t e d s o l u t i o n o f sodium c h l o r i d e (75% R.11.) and k e p t t h e r e u n t i l i t gave a s t e a d y r e a d i n g . I t was t h e n w i t h - drawn and q u i c k l y r e t u r n e d t o t h e c a l i b r a t i o n chamber o f t h e Atmospheric P r o d u c e r . ?'he s e n s o r o u t p u t was t h e n r e a d a t i n t e r v a l s f o r about h a l f an hour. 'rhe r e s p o n s e c u r v e shows t h a t i t accomplished 6 3 h f t h e changc i n a p p r o x i m a t e l y t h r e e m i n u t e s ( F i g u r e 2 ) .
CONCLUSION
1 . The o u t p u t o f t h e hygrometer system i s n e a r l y l i n e a r f o r t h e r e l a t i v e humidity r a n g e s c o v e r e d i n t h e s e t e s t s .
2. With t h e e q u i l i b r i u m t i m e s o f a h a l f t o one h o u r , h y s t e r e s i s ranged from about 2 % R . H . a t 23.0°C t o a b o u t 7% a t -12.1°C.
3 . 'l'he s e n s i t i v i t y of t h e system d e c r e a s e s w i t h f a l l i n g t e m p e r a t u r e , a v e r a g i n g a p p r o x i m a t e l y 0.165 mA/% R . H . a t 2 3 . 0 ° c , 0.154 mA/% K . H . a t 4 . 4 ' ~ , and 0 . 1 3 4 mA/% R . H . a t -12.1°C.
4 . A s i n g l e check on t h e r e s p o n s e r a t e a t 23.0°C i n d i c a t e d a 63'0 r e s p o n s e t i m e o f about 3 m i n u t e s o v e r t h e r e l a t i v e h u m i d i t y r a n g c o f 75 t o 90.8%
5 . A f t e r 18 months o f u s e , t h e s e n s o r c a l i b r a t i o n had s h i f t e d s u b s t a n - t i a l i , y ; t h e r e l a t i v e h u m i d i t y r e q u i r e d t o produce a g i v e n v o l t a g e o u t p u t was about 5% l e s s t h e second t i m e t h a n i t w s t h e f i r s t t i m e . 'Ihis c h e c k , on a s i n g l e s e n s o r , p r o v i d e s some i n f o r m a t i o n a b o u t t h e s t a b i l i t y and performance o f t h i s t y p e o f s e n s o r b u t n o t s u f f i c i e n t t o draw g e n e r a l c o n c l u s i o n s a b o u t t h e i r long-term b e h a v i o u r .
REFERENCES
1 . l l e d l i n , C . P . , Use and C a l i b r a t i o n of Dunmore-type E l e c t r i c a l Humidity S e n s o r s . T e c h n i c a l Note No. 421, D i v i s i o n o f B u i l d i n g R e s e a r c h , N a t i o n a l Research C o u ~ l c i l o f Canada. J u n e 1964 [ r c v i s e d J u l y 1 9 6 9 ) .
2. T i l l , C . E . and G . O . Handegord. Proposed Humidity S t a n d a r d . ' l ' r a n s .
@
23.0°C+ CHECKS AFTER LOW
-
TEMP. CALIBRATION- - C A L I B R A T I O N-
- - - 0 INCREASING HUM I D l f Y -0 DECREASING H U M I D I T Y 4 0 20 40 60 80 1000 20 40 60 80 100 0 20 40 60 80 100RELATIVE HUMIDITY,
O/oFigure 1. Calibration curves for Hy-Cal humidity sensor Ser. $59605 at 23.0, 1.4 and -12.1°C, curves are given for relative humidities calculated on the basis of vapour pressure over supercooled water and ice. First calibration (a,b,c) blarch 1977, second set (d ,e,f) September 1978.
1 I I I I I I E Q U l L l 8 R l U M L E V E L --
