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The electrical heating effect in Dunmore sensors
Hedlin, C. P.; Handegord, G. O.; Nicholson, R. G.
C A N A D A S er TH1 B92 no.
61
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9 THE E L E C T N C A L HEATING E F F E C TIN
D U i W O R E SENSORS C. P. H e d i n , G. 0. Handegord andR.
G.
N i c h o l s o n33
J'
.?
3
D I V I S P O N O F BUILDING R E S E A R C H N A T I O N A L R E S E A R C H C O U N C l L O T T A W A-
C A N A D ATHE ELECTRICAL HEATING E F F E C T
IN
DUNMORE SENSORSby
C.
P. Wedlin, G. 0. Handegord and R, G. NicholsonHEATING D U E TO MEASURING CURRENT
IN
DUNMORE SENSORSHumidity sensors of t h e D u n m o r e t y p e require that a voltage b e
applied to a thin hygroscopic film t o determine the electrical resistance.
This procedure results in an increase in film temperature through
electric heating which, if not ~ e c o g n i z e d , m a y lead to e r r o r s which a r e significant in pr w i s e rnea sur ement applications.
The
temperature rise of t h e sensor depends on the measuring v o l t a g e and current, and the rate of heat dissipation t o the surroundings.In an attempt to evaluate these factors, s e r i e s of experiments w e r e
conducted using two different measuring circuits and c o m m e r c i a l l y
available sensors with and without protective casings shielding the
active surface.
MEASURING INSTRUMENTS
F i g u r e 1 shows a simplified electrical circuit diagram which
adequately describes the operation of the measuring instruments f o r
t h e purposes of t h i s investigation. The c i r c u i t includes a 910 K r e s i s t o r
a n d auxiliary components i n s e r i e s with the humidity sensor. The e l e c
-
trical resistances of t h e auxiliary components have been d i s r e g a r d e d , asthey a r e small compared to those of the resistor and t h e sensor. It is
supposed that the total voltage d r o p occurs across the sensor (E ) and
S
the fixed r e s i s t o r <E ).
R
Two measuring instruments were used, one applying approxi-
m a t e l y 3 2 v o l t s t o t h e s y s t e m and the other 1 1 0 v o l t s . The amounts of
heating for each instrument have been calculated f o r a range of sensor
r e s i s t a n c e (R ) and a r e plotted in F i g u r e 2 . The heating effect is small
at l o w rresistaxces, increases to a peak, and falls t o a l a w value in the
high resistance region. Further, it is evident that t h e heating caused b y the 32 -volt instrument is much smaller than that of the higher v o l t a g e
EUMIDITY
SENSORS
Two effects are involved in the sensor heating-humidity
indication relationship. There i s a character is t i c decrease in the
resistance due to the properties of the film and an increase in r e s i s t - ance due to the localized d r o p in relative h w i d i t y resulting f r o m the
increase in t e m p e x a h r e at the film surface. The l a t t e r e f f e c t is the
mare pronounced and the final result is an indication of relative humidity
lower than that of t h e ambient space. T h e net result is illustrated in
F i g u r e 3, w h e r e the change from t. to t . C A t at constant v a p o r pressure
1 1
cosr esponds t o a decrease in the !'localrt relative h m i d i t y (at film
t e m p e r a t u r e ) f r o m
8 .
to fl a n d a c h a n g ein
sensor resistance from1 fi
Ri to Rf.
T h e
apparent relative humidity,pa,
is the value observedwhen t h e r i s e in sensor temperature is disregarded, In this report,
the 'kcr ror1' is taken t o be the difference between the correct relative
humidity,
li,
andthe
apparent r e l a t i v e humidity.RESULTS
Experiments were c a r r i e d o u t in an atmosphere- producer
(1) and i n a two-pr essure s y s t e m (2) t o find t h e e r r o r that would exist if the h e a t i n g were ignored. Both of &ese humidity producers are used
regularly in calibrating sensors of this type. A sensor was allowed to come to equilibrium in the conditioned space and then connected to t h e
measuring instrument, Examples of the ensuing change in sensor
resistance with time are shown in F i g u r e 4 f o r a sensor located in t h e
t w o - p r e s s u ~ e systeril, f o r the t w o measuring instruments. The time
required for equilibrium to be reached was found in a number of t e s t s
to b e about 1 0 rnin.
h the f i r s t s e t of experiments, a pair of 4 0 gauge copper constantan thermocouples w e r e mounted inside a calibrated s e n s o r of tubular form,
and u s e d in a four-junction thermopile, the other two junctions being located in the surrounding air.
In
this way, A t could b e measured. Thesensor w a s placed in the two-pressure system and experiments c a r r i e d
out a t a series of relative humidities: and hence diff ex ent sensor resistances
at 7 0 " F. Table I contains t h e results. All the variables w e r e defined
e a r l i e r except
Q
C'
which is the local or f i l m relative humidity calculatedu s i n g t h e known vapor pressure and the saturation pressure at t . f A t .
U s i n g values in Table
I,
A t has been plotted against sensor resistance in Figure 5 and follows the s a m e pattern as would b epredicted on the basis of t h e calculated sensor heating values s h a m
in Figure 2. Also, thevalues of (Ia r e i n m o s t c a s e s in good a g r e e -
f
rnent with t h e values (J C calculated with the measured sensor temp-
e x atur e. This supports the view that t h e change i n sensor resistance is
p r i m a r i l y due to the heating effect of the ins truhnent current and that o t h e r effects (if any) caused by application of a v o l t a g e to the sensor
either take p l a c e s o r a p i d l y that they a r e undetected or are relatively
insignificant.
The conditions employed
-
a high voltage instrument, lowventilation rate, and low pressure (approximately 113 atm)
-
w e r ep u r p o s e l y selected t o g i v e a large, easily m e a s u r e d heating effect, A second group of t e s t s made at about 5 0 per cent
R.
H.,
using t h eatmosphere producer, w h e r e a m u c h higher air flaw rate exists, showed
an e f f e c t only about one-third as large as those in Table I. A l s o tests
made with a 32-volt measuring instrument g a v e a much s m a l l e r effect.
A s anticipated, t h e s e amounted to only 1 0 t o 20 per cent of those of
the I 1 0 -volt instrument.
A f i n a l series of experiments w a s c a r r i e d out t o determine the
effect of a i r movement on the e r r o r . A sensor was mounted on t h e end
of an a r m
which
could be rotated at the d e s i r e d rate. The electricalleads were connected through
a
pair of r o t a r y contacts t a the electricalhygrometer controller. The r o t a t i n g a r m was mounted inside an
e n c l o s u r e 24 in. i n diameter and 14 in. deep, in which the temperature
and humidity w e r e closely controlled. As is shown in Table 11, the
a i r
-
sensor velocity was v a r i e d between 4 0 and 65 0 f . p. m. For one s e r i e s
of o b s e r v a t i o n s the protective casing w a s on the s e n s o r ; f o r another series
it w a s removed, leaving the active surface f u l l y exposed to the air. In
o r d e r t o check the accuracy of measuring t h r o u g h the rotary contacts, a
fixed r e s i s t o r was used in place of the sensor and the r e s i s t a n c e was
m e a s u r e d at several rates of rotation. T h e difference between the measured and actual resistance w a s found t o be negligible.
T h e t e s t s in t h e s e experiments w e r e made at relative humidities b e t w e e n 2 5 and 7 0 per cent.
It
should b e mentioned that the error inrelative humidity f o r a g i v e n cha-nge in sensor t e m p e r a t u r e is proportional
to the relative humidity itself. F o r example, a t 7 0 F , a change of 1F" r e s u l t s in an e r r o r of 0.84 per cent R.H. at 2 5 p e r c e n t R . H . , but 3 . 3 5 p e r cent a t 100 per cent R . H.
"Cold sensort' results, that is, results obtained b e f o r e a s e n s o r heating had t i m e to affect the measured values, w e r e obtained b y taking
a reading immediately after connecting the sensor to the measuring
instrument.
The
values obtained by the two measuring instruments w e r e n e a r l y the same in a l l of the c a s e s in which they w e r e compared. Thus, though in some cases the resistance increases quite rapidly due to s e n s o rheating, it i s possible to obtain a c c u r a t e results c o ~ r e s p a n d ~ g to the cold
condition of the sensor. Because of t h e wide variation of the "'hotTr sensor
results, and because of the added trouble of measuring the sensor temper-
ature, it appears preferable to take measurements at the cold condition
when possible.
CONCLUSIONS
The self heating of Dunrnor e -typ e humidity sensors when used with
s o m e i n s t r u m e n t s can cause significant e r r o r
in
the measured results.The effect i n c r e a s e s a s t h e measuring instrument voltage
is
increasedand a l s o as the rate of air flaw decreases; it may cause a r i s e in sensor temperature of a s much a s
IF".
Experiments w e r e carried out with a variety of air-blow conditions. The maximum errors wexe found to occurwhen a h i g h voltage measuring instrument was u s e d with a sensor having
a p r o t e c t i v e casing and with low r a t e of a i r - f l o w .
This
represents anextreme s e t of conditions. W i t h a l o w v o l t a g e instrurnen;t and a higher
r a t e of air-flow, the heating effect d o e s not exceed a few tenths per c e n t
relative humidity and can generally b e d i s r e g a r d e d .
In general, if p r e c i s e results are required, it is necessary to
c o n s i d e r that this e f f e c t may exist and t o avoid it by making the rneasure-
rnent at the c o l d condition of the sensor or, alternatively, to a p p l y a
suitable c o r r e c t i o n based on m e a s u r e d sensor temperature. REFERENCES
1. Till, C.
E.
and 6 . 0. Handegord. P r o p o s ed humidity standard.Transactions ASHRP,E
6 6 ,
1 9 6 0 , p. 288-306.2 . Hedlin,
C.
P. A device f o r calibrating humidity sensors. M a t e r i a l s-
TABLE I
E F F E C T O F SENSOR HEATING ON INDICATED R E L A T I V E HUMIDITY
AND SENSOR T E M P E R A T U R E
R . 1 ( m e g . ) Rf(rneg. )
8 .
1
g
a?*
A t"F
9
C&
Obtained b y a temperature c o r r e c t i o n of approximately
- 0 . 17 per cent R.H. / F a to
9
.
P E R C E N T A G E RELATIVE HUMIDITY ERROR
F O R
5-MIN. TESTS FOR A RANGE O F SENSOR-AIR VELOCITIESAMBIENT R E L A T I V E HUMIDITY 66 P E R C E N T Sensor-Air V e l o c i t y f . p. m. 40 7 5 125 220 4 5 0 1650 Protective casing in p l a c e 1- 5 1 - 3 1 . 2 1 . 0 0.
