Polymer-based capacitive humidity sensor: characteristics and experimental results

Sensors and Actuators,

4 (1983)

97 - 104

97

POLYMER-BASED CAPACITIVE HUMIDITY CHARACTERISTICS AND EXPERIMENTAL

G DELAPIERRE,

LETI -

H GRANGE,

Comm#ssanat

a’l%nergxe

B CHAMBAZ

Atomrque

SENSOR: RESULTS*

and L DESTANNES

85 X - 38041

Grenoble

Ckdex

(France)

Abstract We present the mam charactenstlcs at room temperature, high temperature and after aging, of a polymer-based capacltlve hygrometer. Partrcular emphasis E lrud on the hysteresis phenomenon which lunlts the range of apphcatlon of a first generation of transducers, now commercially avzulable Some charactenstics of new sensors m the development stage are also aven, showmg that further progress can still be antlclpated.

1. Introduction

Among all the different types of hygrometers that can be used to measure atmosphenc humidity, those based on the measurement of impedance vanatlon are probably the sunplest and the most suitable for electronic systems. However, psychrometnc and dew-pomt measurements are still the most mdely used methods, at least m mdustry. This IS probably due to the fact that, m spite of the many attempts to launch an nnpedance vanatlon hygrometer on to the market [ 1 - 5 3, these mstruments have not still proved then ability to measure rehably humidity from 0 to 100% RH m the temperature range -40 “C to +150 “C necessary to satisfy the undest fields of apphcation. This paper presents the mam results of a research programme still under way that am18 to nnprove this situation. The detruled charactenzation of a first generation of sensors that has reached the mdustnal stage will gwe a good idea of the present state-of-the-art Some prelunmary results on new hygrometers m the process of development w111show that further lmprovements can still be expected

*Based on a Paper presented at Sohd-State May 31 -June 3,1983

0250-6874/83/$3

00

Transducers 83, Delft, The Netherlands,

0 Elsevler Sequola/Prrnted

m The Netherlands

98

2 Technology The technology used 1s that of thm film capacitors [6] The stages of the process relating speclflcally to the hygrometry function consist first m the choice of a hygroscoplc dlelectnc (a polymer, m our case), and secondly m the elaboration of a porous electrode Figure 1 shows the hygrometer The tantalum electrode 1s anodlcally oxldrzed m order to avoid any nsk of short-clrcultmg, even when there are holes m the polymer The Cr-NI-Au electrode ISsimply intended to connect the porous electrode The polymer 1s spread with a photoreslst spmner, a device commonly used for mlcrohthography More detals ~11 be found in an earlier pubhcatlon [ 71

Fig

1 Photomlcrograph

of the sensor

The porous electrode 1s obtamed by chromium evaporation under conditions such that the thm film 1s tensile stressed These stresses generate a very large number of cracks m the polymer These cracks, some 1000 ii wide and spaced a few microns from each other, transform the polymer rnto a lot of small islets, protected by a thick layer of chromium Thanks to this patented technology, permeability 1s achieved mthout unpaumg conductivity or mechanical strength, and the penetration rate of moisture IS mcreased by several orders of magnitude, whatever the thickness of chromium (100 ii to 1 E.rm) Moreover, it allows fast sensors to be made, even with polymers such as polymudes, which exhlblt practically no moisture sensltlvlty mthout cracks

99

3 Charactenzatlon

at room temperature

Charactenzatwn by means of a reference measurement system The sensors were calibrated on a BNM measurement system built by the CETIAT m Lyon [8] The precise measurement of humldlty m the test chamber was carned out by means of a dew-pomt reference hygrometer, calibrated at the NBS The guaranteed precision was + 0 8% of the measured value The charactenstlc cahbratlon curve obtamed 1s gven m Fig 2 for the cellulose acetate based transducer, now commercially avalable The stabxhzmg time between each measurement was about one hour The output IS plotted m frequency, since the sensor was mounted m an RC oscillator 150

temperature !J

140

1tVc to 35°C

I800Cto13o~C

0 A

'4 3 H

130

$ l! 3 I p3

predslon of testing equlpment k 0 8 % 01 value between 15 and 35% ll0

Fig 2 Cabbratlon curve of the cellulose acetate based sensor at room atures on an approved BNM testing equipment (CETIAT)

and high temper-

Accordmg to this charactenstx, the sensor may be considered as almost perfect, gnren the accuracy attamable m relative humidity measurements (refer to the accuracy of the reference system) Nevertheless, some difficulties were encountered when hygrometers were used over long penods of time under real environmental condltrons, and after callbratmg by means of saturated salt solutions Characterazatron by means of saturated salt solutwns, the problem of hysteresrs Usmg salt solutions to calibrate a hygrometer 1s a very cheap and practical method [9] It 1s based on the fact that, above a saturated salt solution,

100

the relative humldlty of ar stabilizes at a well known value, which 1s mdependent of temperature The salts we used were LlCl for 12% RH, NaGl for 76% RH and KpSO.+ for 97% RH A plot of the response obtamed with this method 1s shown by the contmuous hnes on Fig 3. First there IS a two-step response (a very fast phase followed by a very slow one), secondly, a notlceable hysteresis occurs at 76% RH after a long pemod of time (> 1 h) at 97% RH

I r--P

E

low hysteresis

04

sensor

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_--I I

I-_--

-

1 I

I

.----

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0

Fzg

60

150

3 Plot of the response

1eo

240

time (mn)

0

50

of two sensors above saturated

100

HR

time (hour)

salt solutions

In many cases, a non-mltlated user would explam thrs slow response followed by hysteresis as a dnft of the transducer We prefer the word hysteresls, since the phenomenon 1s reversible Indeed, if the sensor 1s kept agam at low humidity for a long enough penod of time (one night at 12% HR, for mstance), it unll nse agam to 76% HR followmg a curve identical to that of the first step To remove any doubt regarding the possible mfluence of the salt solutions, the measurements were repeated m a chmatlc chamber, under condltlons slmllar to those of the reference measurement system (ventilated atmosphere) For these measurements, we used a &@tal portable hygrometer from CORECI company (France) that mcluded our sensor Figure 4 shows the measured RH values plotted agamst actual RH The result obtamed 1s m perfect agreement with the measurements cmed out above the salt solutions The circles on the plot correspond to measurements camed out after less than half an hour, the squares correspond to asymptotic stablllzed values obtamed after more than 20 hours

101

100 -

0 fast answer (less than l/2 h ) 1 asymptotic answer (more than 20 h )

SO-

I

?rn a 0

high callbratfon curve-

L,_

Loz f

50 -

I! low

40 -

t

v

caltbmtton curve

30-

20 -

before point 1, the hygrometer stayed at RH < 35 % for 6 months

10 -

I I

10

I

I

I

I

I

70

30

40

50

60

I 70

I

so

I

SO

I

loo

actual relatie humidity (% RH)

Fig 4 Callbratton curve long period of ttme

of a portable

dlgltal hygrometer

m a chmatlc chamber durmg a

It can be seen that the transducer has a lmear charactenstic for the short term (< l/2 h) that shifts dependmg on the measurement history. The shift 1s only really slgmflcant (> 3% RH) when the value of RH IS higher than 75% It appears as if high humidity values would momentarily modify the structure of the polymer, and consequently its surface of adsorption. Many sclentlsts have endeavoured to explam the ongm of this adsorption hysteresis m polymers [lo], and we ~11 not dwell here on this SubJect. Our purpose 1s rather to show how this drawback can be overcome m practice Thus, we ~11 e;lve below some examples where we have succeeded m measurmg humidity ~th an acceptable accuracy. 4. Some examples of use of the hygrometer In many cases, m spite of hysteresis, accurate measurements can be performed sunply by adoptmg a cahbratmg method adapted to the apphcation. Con trolkng a&r-condohoned rooms In this case, the transducer IS nearly always operatmg m the low RH range (RH < 75%) Thus, it only has to be calibrated once it has been kept

102

for a few days at low humldlty Measurements above 80% RH are not excluded, but must be over a short penod (< 1 h) The fast response of the transducer to small RH vaatlons guarantees the high performance of servocontrol systems This last point 1s still true at more than 95% RH, but the transducer has to be cahbrated after a long penode of time (> 24 h) m an ambiance close to the proposed set value Ground meteorogzcal stafon In the open ar, relative humidity very rarely falls below 50% RH and often exceeds 90% RH Sensor operation 1s therefore mamtamed on Its ‘high cahbratlon curve’ We ascertamed that the humidity value mdlcated by SLX transducers cahbrated after havmg spent a long time at 97% RH and put outdoors mthout any protection did not differ by more than 3% RH over a penod of 150 days from that grven by a psychrometer Plottrng the atmosphertc humldlty proflie wrth a meteorograph baIloon Humidity changes very rapldly, so the charactenstlc of the hygrometer cannot shift, excellent results are obtamed m this type of apphcatlon (see [71) Utlltzatlon m condltlons unknown beforehand In a few cases (for mstance, regulation of a climatic chamber) hysteresis 1s unacceptable To overcome this difficulty, we can increase the temperature of the sensor by a few degrees centigrade (e g , tnth a small heatmg resistor) Increasing the temperature by 5 “C m an atmosphere at 20 “C ~11 lower the measured relative humldlty value to 74% of the real humidity value This IS because the hygrometer actually andlcates relative humidity referred to saturation vapour pressure at the temperature of the sensor and not of the zur[ 111 Nevertheless, If the hygrometer 1s accidentally exposed to RH > 80% w&hout heating, Its charactenstlc will be shifted Thus It 1s worthwhile to try to lower the hysteresis of the sensor 5. Low hysteresm sensor After analysmg the possible ongm of the hysteresis, we tested different polymers that are potentmlly less prone to thrs phenomenon The dashed curve on Fig 3 depicts the plot of the response of one of these hygrometers There 1s almost no hysteresis and the ‘slow phase’ has completely dlsappeared However, this result could only be obtamed at the expense of lmeanty and temperature coefficient, and progress ISSt111 to be made. Nevertheless, m some apphcatlons these drawbacks would be preferred to hysteresis, because of the posslbllltles of correctton now offered by electronic systems (intelligent sensors)

103

6. Charactenzatlon at high temperature- polylmlde sensor The sensor was tested up to 130 “C m a humidified ar generator, belonging to CETIAT [S] The tnangles plotted m Fig 2 correspond to these measurements Although humldlty can still be measured, a decrease m sensitivity IS apparent at low RH values RH being rather low at high temperature (at 130 “C and P = 1 atm, RH cannot be higher than 37%), it IS unportant to keep to a high sensltlvlty and low temperature coefficient m this range With the use of polyunlde m combmatlon with the chromium electrode, we have been able to fulfill these two condltlons Measurements at high temperature on the polylmlde sensor are aven m Fig 5 They are compared with values obtamed above saturated salt solutions at 25 “C Smce polyunlde 1s renowned for its good resistance to high temperature and chemical corrosion, it promrses to be a very useful product for hard environments

7. Resistance to agmg Tests on low-hysteresis type and polyunlde type sensors are now JUSt begmnmg So we shall only report on the rehabllrty of the cellulose acetate hygrometer that 1s now commercially avtiable.

0

calibration

with a dew point hygrometer

n calibration

above saturated salt solutions

A calibration

In a high temperature

generator

(CETIAT)

temperature (” C)

I

10

I

20

I

30 relative humidity (% RH)

I

40

Fig 5 High temperature cahhratlon curve of a polyrmlde sensor

I

50

104

No slgmfrcant change (< + 3% RH) of the charactenstlc has been detected durmg 150 days m a chmatlc chamber under the followmg cycling condltlons one night at 10 *C and 30% RH, one day at 20 “C and 45% RH, one night at 60 “C and 90% RH, one day at 20 “C and 45% RH A test of resistance to SOa was also cmed out It proved to be a very fast method of revealmg defects that are only observed after some months m an outdoor atmosphere No measurable change of charactenstlcs was observed after 50 days at 100% RH and 5 ppm of SO2 8. Conclusions The charactenstlcs described here show that for many applications, a polymer-based capacitive hygrometer can be used to obtam perfectly correct measurements of relative humldlty However, the hmltatlons of the sensor should be assessed v&h accuracy It 1s hoped that the few promlsmg prehmmary results we have reported here have at least shown that a simple and rehable hygrometer gwmg accurate measurements over a urlde range of RH and temperature 1s actually wlthm reach and that, wrth contmued work m this field, improved sensors can be expected m the near future

References 1 A Wexler (ed ), Humldlty und Mozsture, Reinhold, 414 2 K W Mlsemch, Capacitive humldlty transducer,

New

York,

1965, Vol

1,

pp

219 -

IEEE Trans Ind Electron Contr Instrum, IECI-16 (1969) 6 - 12 3 E Salasmaa and P Kostamo, New thin film humldlty sensor, Thzrd Symposrum on Meteorologzcal Observations and Instrumentatzon, Am Meteor Sot , Boston, 1975, pp 4 P 85 5 R

33 - 38 P L Regtlen, Sohd state humldlty sensors, Sensors and Actuators, 2 (1) (1981) - 95 S Jachowlcz and S D Senturla, A thin-film capacitance humldlty sensor, Sensors

and Actuators, 2 (2) (1981) 171 - 196 6 L I Malssel and R Glang (eds ), Handbook of Thm Fzlm Technology, McGraw New York, 1970, Part 4, pp 1927 - 1930 7 G Delaplerre, Hygromatre capacltlf ii base de polymhre,

(1981) 47 - 52

8 J Mengoux

Mes

Reg Autom

HIl,

, 46 (11)

and I3 Cretmon, Mesure d ‘humzdztddans ies gaz, CETIAT, Vtileurbanne, 1982, pp 47 - 52 9 A Wexler (ed ), Humzdjty and Motsture, Remhold, New York, 1965, pp 603 - 536 10 R M Felder and G S Huvard, m R A Fava (ed ), Meth of Exp Phys Polymers, Academic Press, New York, 1980, Vol 16, Part C, pp 315 - 337 11 J Ovarlez, Importance de l’bqulhbre thermxque des sondes senslbles 1 l’humldlt6 relative, Mes Reg Autom , 47 (4) (1982) 43 - 44