1186
Anal. Chem. 1983, 55,1186-1187
gether they provide reliable routine sealing of capillary tubes.
(5) Shoolery, J. N. “Microsample Techniques in ’H and I3C NMR Spectroscopy-Study of Microgram and Submilligram Amounts of Sample with the CFT-gO”, Varlan Instrument Division: Palo Alto, CA, 1979; P5. ~
ACKNOWLEDGMENT We thank S. G. Huang of the Cornel1 University Department of Chemistry for the NMR analysis. LITERATURE CITED (1) Melnwald, J.; Wiemer, D. F.; Eisner, T. J . Am. Chetn. SOC. 1979, 707, 3055-3060. (2) Templeton, J. F.; Jackson, C. C.; Seaman, K. L. SteroMs 1982, 39, 509-516. (3) Dewey, E. A.; Maylin, 0. A.; Ebel, J. G.; Henlon, J. D. Drug Metab. D/SPOS.1981, 9 ,30-36. (4) Skrabalak, D. S.; Maylln, (3. A. SteroMs 1982, 39, 233-244.
RECEIVED for review January 17,1983. Accepted February 28,1983. We wish to thank the Zweig Memorial Fund and the New York State Racing and Wagering Board Drug Testing and Research Program for financial support for this work. We wish to acknowledge the National Science Foundation Instrumentation Program (CHE7904825)for support of the Cornell Nuclear Magnetic Resonance Facility.
Instrument for Relative Humidlty Measurement Knut Irgum Department of Analytical Chemistry, Universlty of Ume 4, S-90 1 87 Ume A, Sweden
Relative humidity is a quantity which it is often desirable to measure in the chemical laboratory. A fairly accurate instrument to measure this quantity is the well-known hair hygrometer. Ita use, however, is often limited by voluminous physical dimensions and the fact that it has to be placed in an upright position in order to give a correct reading. Another and more serious drawback is that it has to be read visually. Another widely used method is the psychrometric measurement, which is based on the temperature decrease due to vaporization of water from a wet thermometer placed in a flow of the air to be measured. The air velocity at the probe must be greater than 3 m/s in order to obtain a correct reading. Moreover, the temperature decrease is not only a function of relative humidity but also of temperature and of barometric pressure. If an electric signal proportional to the relative humidity is required, there are instruments available which will do this, but at a price which will make the chemist think twice before investing in such an instrument. In our laboratory a dynamic gas dilution system was built to produce a controlled atmosphere for testing work environmental sampling devices. One of the variables that must be controlled is relative humidity, to within h5% a t 20 “C. Consequently, an instrument was built to monitor this.
EXPERIMENTAL SECTION A sensor for relative humidity is now available at a cost of less than $10 (Philips, Eindhoven, The Netherlands, Catalog no. 2322 691 90001; Mepco/Electra Inc., Morristown, NJ). The sensor is made from a stretched plastic film, which is gold coated on both sides, forming the dielectric and plates of a parallel plate capacitor, respectively. The dielectric constant of the plastic is changed upon the adsorbtion of water, thereby changing the overall capacitance of the sensor. The manufacturers have assigned it to consumer applications, probably due to the inherent nonlinearity which they have dealt with in their literature (1,2)without greater success. By closer examination of its response, a nearly perfect exponential relationship can be found between the capacitance and relative humidity. This means that linearization can be accomplished by charging the capacitor of a series RC circuit for a time determined by the change in capacitance of the sensor. The circuit described performs the measurement of capacitance difference and the transformation of this needed t o produce a linear voltage output. The primary task is to measure the change in capacitance when relative humidity is varied. An easy way to accomplish this is to simultaneously start two monostable multivibrators, one in 0003-2700/83/0355-1166$01.50/0
which the capacitive humidity sensor is the capacitance of the time-determining RC component. The other monostable is a reference generator with its timing components chosen to match those of the sensor monostable at 0% relative humidity. If the outputs of these two generators are exclusive-OR-wired,an output pulse which is proportional in width to the sensor capacitance change is obtained. The two monostables are triggered by the negative going edge of an astable operating at 1 kHz with 95% duty cycle. The next step to be performed is to convert the exponential pulse width response to a signal which is proportional to relative humidity. This is accomplished by charging capacitor C, with a constant voltage, E,, through a charging resistor, R, for the time which the exclusive-OR-gate output is high, according to
E = Eo(l- e-ton/RC)
(1)
The exponential decay of voltage increase rate across the capacitor with time matches well the relative higher increase in sensor capacitance, and thus exclusive-OR-output pulse width, at higher relative humidities. The parameters Eo and RC have then to be optimized. This is accomplished by giving C a valve of 10 nF, letting a computer step through a series of E , values, calculate the R value necessary to give 1V out at 100% relative humidity, and then compute the RMS error based on every 10% relative humidity for each Eo. The value of Eo which gives minimum error is 1.48 V with a corresponding R of 20.9 k. The capacitance vs. relative humidity data used to perform these calculations is extracted from the manufacturer’s documentation by multiplying the capacitance differences between 0 and 100% relative humidity from Table I at 1 kHz operational frequency with the percent deflection of Table I1 in the technical note (1). The circuit elements which perform this charging/decharging of the capacitor are the two pairs of paralleled 4066 CMOS transmission gates switching the capacitor C to Eo via R or to ground alternately. As the signal of interest is the voltage across the capacitor at the end of the charging pulse, some means of monitoring this voltage has to be applied. In the suggested circuit this is accomplished by a bleeding peak detector. The time constant of the hold capacitor is 1 s, assuring that ripple is kept below 1 mV but still allowing the hold voltage to decrease rapidly enough when a lowering in the relative humidity to be measured takes place. The output signal was fed directly into a CA 3162 based two-digit panel meter, thus giving percent relative humidity directly.
RESULTS AND DISCUSSION As mentioned above, it was possible to calculate the sensor capacitances from the figure given in the technical note supplied by the manufacturer. With the method described 0 1983 Amerlcan Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
1187
+12v
out
Flgure 1. Circuit diagram of relative humidity meter. Part of circuit within dashed lines is the remotely placed sensor monostable: 7555 = Intersil 7555 CMOS timer: 3130 = RCA 3130 P-MOS Input CMOS output operational ampllfier; 3140 = RCA CA 3140 P-MOS input operational amplifier.
-
Table I. Calculated Errors % RH
0 10 20
30 40 50 60 70 80
90 100 a
AC,pFa 0
3.0 6.0 9.3 1i3.2 16.7 20.9 215.8 30.8 3’7.1 46.5
Eout, mV
0 106 2 04 305 412
*O
602
0.2 -0.2 -0.3 -1.1
598 698 789
890 1000
Table 11. Performmce of the Instrument at 20 “C
z
%error medium silica gel H,SO,, P = 1.48 Ca(NO,), (sat) H,SO,, P = 1.20 Na,SO,. 1OH,O (sat)
0.6 0.4 0.5 1.2
-1.1
*O
Calculated from Tables I and I1 in ref 1.
above, the nonlinearity, when using optimized parameters, was calculated hly f i s t computing the theoretical response and then subtracting the signal desired for tlhe particular relative humidity. These calculations are presented in Table I. The actual performance of the instrument was also tested a t 20 “C by using sulfuric acid dilutions or saturated salt solutions in a desiccator (3). These figures are shown in Table 11. It should be noted that the error never exceeds the digital f 1 count error for the values tested. Testing of the instrument at temperatures other than 20 OC was never attempted, as this was not necessary for our purpose. The manufacturer is indirectly stating that tlhe temperature coefficient of the sensor is +lo0 ppm/OC. Left unattended, this should give rise to an error of +0.03% relative humidity/OC, or less than f l % over the operational temperature range, 0 OC to 60 OC. This can, however, easily be compensated for by selecting or paralleling resistors of suitable negative temperature coefficients in the sensor monostable timing component. The hysteresis effect stated by the mmufacturer on cycling the sensor between different relative humidities was never seen in these experiments. The readout unit applied should be designed so that the output is current sourcing. If current sinking is required, the output has to be buffered in some way or the output 3130 be operated from dual supplies. If, for some reason, operational
a
RH/2O0C
readout %RH
0 22
22
0a
55
55
81
80
93
93 a
Calibration points.
amplifiers should be replaced by other types, the alternative must be able to sense down to its negative supply and have a unity gain bandwidth of more than 1MHz. For substitution of the CA 3130, CMOS output is required in addition, to enable output swing to negative supply. Ordinary 555 timers must not be substitued for the 7555. It should be noted that the sensor monostable housing ought to be constructed from a shielding material in order to minimize the effect of stray capacitance changes. The sensor monostable should also be mounted together with the sensor on a probe to keep total sensor capacitance low. This will increase considerably if a cable is used between the sensor and its monostable. Generally, high-quality components like metal film resistors, cermet trimmers, and polypropylene capacitors should be used throughout. Finally, the PC boards and components should be given a spray of a good protective lacquer. Power consumption is 150 mW typically, which makes the instrument suitable for battery operation.
LITERATURE CITED (1)
“Capacitive Humidity Sensor for Consumer Applications”, Technical Note 134; Phlllps Nederland 8. V.: Elndhoven, The Netherlands, 1963.
(2) “Capacitive HumMlty Sensor”,Technical Information 063; Phiiips Nederland B. V.: Elndhoven, The Netherlands, 1980. (3) Weast, R. C. “CRC Handbook of Chemistry and Physics”, 57th ed.; CRC Press: Cleveland, OH, 1976; pp E-45-46.
RECEIVED for review December 29,1982. Accepted February 14, 1983.
