A method and apparatus using a known water-absorbent polymer as a capacitor which is operated at a dc voltage for measuring relative humidity is presented. When formed as a layer between porous electrically-conductive electrodes and operated in an RC oscillator circuit, the oscillator frequency varies inversely with the partial pressure of the moisture to be measured. In a preferred embodiment, the capacitor is formed from Nafion and is operated at a low dc voltage with a resistor as an RC circuit in an RC oscillator. At the low voltage, the leakage current is proper for oscillation over a satisfactory range. The frequency of oscillation varies in an essentially linear fashion with relative humidity which is represented by the moisture being absorbed into the Nafion. The oscillation frequency is detected by a frequency detector

Laue, Eric G.

English

NASA Technical Reports Server

United States Patent 
Laue 
[54] WATER-ABSORBING CAPACITOR SYSTEM 
FOR MEASURING RELATIVE HUMIDITY 
[75] Inventor: Eric G. h u e ,  San Marino, Calif. 
[73] Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 746,809 
[22] Filed: Jun. 20, 1985 
[51] Int. Cl.4 .............................................. GOlW 1/00 
[52] U.S. CI. ................................. 73/336.5; 324/61 R 
[58] Field of Search ............... 73/336.5, 336; 340/603; 
338/35; 324/61 R 
[561 References Cited 
U.S. PATENT DOCUMENTS 
2,412,782 12/1946 Palmer 
3,164,004 1/1965 King, J 
3,287,974 11/1966 Clemochowski .................. 73/336.5 
3,392,348 7/1968 Horwitz ............................. 73/336.5 
3,427,864 2/1969 King, Jr. ................................. 73/29 
3,523,244 8/1970 Goodman et al. 
4.083.765 4/1978 Lawson ............. 
4,143,177 31'979 Kovac et al. ......................... 427/79 
4,160,374 7/1979 Crump et ai. 
4,161,660 7/1979 Gallant 
4,203,087 5/1980 Kovac e 
4,242,096 12/1980 Oliveira et al. ....................... 422/61 
4,317,084 2/1982 Senturia et a1 .... 331/65 
4,482,581 11/1984 Lorin et al. ........................... 427/79 
[ i l l  Patent Number: 4,662,220 
[45] Date of Patent: May 5, 1987 
FOREIGN PATENT DOCUMENTS 
148945 9/1983 Japan .................................. 73/336.5 
,196441 11/1983 Japan .................................. 73/336.5 
196442 11/1983 Japan ..... 
160344 12/1964 U.S.S.R. ....... 
W0/03139 9/1983 PCT Int'l App 
21 1 I684 7/1983 United Kingdom ... 
OTHER PUBLICATIONS 
R. Samuel et al., Electronic Humidity Measuring Sys- 
tem, Patent Associated Literature, 4-1976. 
Primary Examiner-Stewart J. Levy 
Assistant Examiner-Hezron E. Williams 
Attorney, Agent, or Firm-Paul F. McCaul; John R. 
Manning; Thomas H. Jones 
1571 ABSTRACT 
A method and apparatus using a known water-absorb- 
ent polymer as a capacitor which is operated at a DC 
voltage for measuring relative humidity. When formed 
as a layer between porous electrically-conductive elec- 
trodes and operated in an R C  oscillator circuit, the 
oscillator frequency varies inversely with the partial 
pressure of the moisture to be measured. In a preferred 
embodiment, the capacitor is formed form Nafion and is 
operated at a low D C  voltage with a resistor as an RC 
circuit in an R C  oscillator. At the low voltage, the 
leakage current is proper for oscillation over a satisfac- 
tory range. The frequency of oscillation varies in an 
essentially linear fashion with relative humidity which 
is represented by the moisture being absorbed into the 
Nafion. The oscillation frequency is detected by a fre- 
quency detector. 
3 Claims, 3 Drawing Figures 
15 u FREQUENCY 
COUNTER 
(DETECTOR) 
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U S  Patent M a y 5 , i ~  Sheet 2 of 2 4,662,220 
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4,662,220 
1 
WATER-ABSORBING CAPACITOR SYSTEM FOR 
MEASURING RELATIVE HUMIDITY 
BACKGROUND OF T H E  INVENTION 
1. Origin of the Invention 
The  invention described herein was made in the per- 
formance of work under a NASA Contract and is sub- 
ject to the provisions of Public Law 96-517 (35 USC 
202) in which the contractor has elected not to retain 
title. 
2. Field of the Invention 
This invention relates to a new method and system 
for measuring relative humidity by using a known wa- 
ter-absorbent polymer operating as a capacitor circuit 
element in an R C  oscillator circuit which exhibits a 
frequency that varies inversely with the partial pressure 
of the moisture vapor in an area to be measured. I have 
discovered that a water-absorbent polymer, when 
formed as a capacitor between porous electrically-con- 
ductive electrodes, will absorb moisture. The electrical 
properties of an RC oscillator, including the capacitor, 
vary linearly in frequency when compared to variations 
in relative humidity. 
One preferred form of capacitor was subjected to a 
low D C  voltage of about 3 to 3.2 volts. In this preferred 
embodiment the R C  circuit element is formed from a 
Nafion capacitor which is operated in an R C  oscillator 
circuit at the low D C  voltage. At that low voltage the 
reactive current through the Nafion is sufficiently high 
to allow R C  oscillation over a satisfactory humidity 
range. The frequency of oscillation varies in an essen- 
tially linear fashion with relative humidity which is 
represented by the moisture being absorbed into the 
Nafion. The range of oscillation is detected by a fre- 
quency detector. Thus, the amount of moisture varies 
the oscillator frequency and a simple, effective measur- 
ing system is attained. 
3. Background Discussion 
Nafion is water-absorbent and its use in a hygrometer 
as a moisture variable resistor is known in the art, as 
exemplified by U.S. Pat. No. 4,083,765. Various other 
moisture variable resistance structures using absorbent 
materials having a porous layer are shown in U.S. Pat. 
Nos. 4,143,177 and 4,203,087. A water-permeable layer 
in a capacitive hygrometer is disclosed in U S .  Pat. No. 
4 , 4 8 2 3  1. 
The  inventor is aware of two prior art NASA new 
technology reports identified by NPO-13948 entitled 
“Long Lasting Solid Polymer Electrolytic Hygrome- 
ter” (Jet Propulsion Laboratory, May 1976) and NPO- 
15722 entitled “Trace Water Sensing in Space” (Jet 
Propulsion Laboratory, December 198 1). These reports 
describe, as is well known, that typical polymer film 
material such as polyphosphoric acid film and sulpho- 
nated fluorocarbon polymers (DuPont Nafion) have 
excellent water absorption characteristics and superior 
the gases produced thereby tend to make the measure- 
ments unreliable. The electrodes become corroded and 
the corrosion also makes the measurements unreliable. 
Although it  has long been known as a desirable goal 
5 to monitor absorbed moisture on a polymer film by a 
simple, efficient nonelectrolytic system, attainment of 
that goal was not available until the advent of this in- 
vention. 
10 SUMMARY OF T H E  INVENTION 
A water-absorbent sulphonated fluorocarbon mate- 
rial is formed into a capacitor which is connected with 
a resistor into an RC oscillator circuit. This invention 
involves the discovery that the RC oscillator behavior 
varies inversely with the partial pressure of the water 
content, which is a function of relative humidity. Using 
the capacitor in an R C  oscillator circuit running at a 
low DC voltage range of about 3 to 3.2 volts produces 
a variable frequency. The oscillation varies in a linear 
*O manner with relative humidity and, thus, detection of 
the oscillation frequency yields a simple and efficient 
measurement of water vapor adsorbed by the film. 
In a preferred embodiment, the water-absorbing ca- 
pacitor was connected in circuit with a series resistor 
across a low D C  voltage of about 3.2 volts to form a 
simple R C  switching oscillator having a Schmidt trig- 
ger as the switching element. A known frequency 
counter is used as a frequency detector to yield relative 
BRIEF DESCRIPTION OF T H E  DRAWINGS 
FIG. 1 is a schematic diagram of a simple R C  oscilla- 
tor and frequency detector in accordance with this 
FIG. 2 depicts what is presently known to be the 
equivalent electrical circuit of a water-absorbent plastic 
film sandwiched between two porous electrically-con- 
ductive electrodes; and 
FIG. 3 depigts a nearly-linear graph of real time fre- 
quency readings in kHz and relative humidity values in 
percentage. 
DETAILED DESCRIPTION OF T H E  
DRAWINGS 
FIG. 1 depicts the use of a water-absorbing element 
15 formed from a layer of polymer film sandwiched 
between porous and electrically-conductive electrodes 
15A and 15B. The element is used in a well-known R C  
Reference to FIG. 2 depicts what the inventor hy- 
pothesizes is the equivalent electrical circuit for element 
15. As noted in FIG. 2, there is a series connected bat- 
tery 30 and resistor 31 formed in a parallel circuit with 
55 both capacitor 34 and resistor 35. Although the relative 
values of the equivalent circuit components are not 
presently known, they are not deemed significant to this 
25 
3o humidity measurements. 
35 invention; 
40 
45 
50 oscillator circuit 20 as shown in FIG. 1. 
chemical resistance. In these disclosures an attempt is application. The  discovery of this invention is based 
made to  read picoampere current variations as a mea- upon the fact that an R C  oscillator employing the cir- 
surement of local relative humidity. Such small currents 60 cuit element 15 has a frequency which varies inversely 
result in an expensive and highly-sensitive system which with the partial pressure of the vapor that is absorbed 
is not only delicate but suffers from reliability, as dis- by element 15. 
cussed in more detail below. The  oscillator circuit 20 makes use of the element 15 
The problem presented by the NASA prior art ap- connected in series with a resistor 16 to  form an R C  
proaches is that, in both cases, the mode of water detec- 65 circuit. For simplicity, the circuit element is referred to 
tion involves electrolysis of water absorbed on the poly- hereinafter as a capacitor. A Schmidt trigger 17 of any 
mer film and the presence of a chemical electrolyte. The  well-known type such as, for example, an RCA-CD 
use of high voltage in the electrolysis operation and/or 4093 is connected between a direct current voltage Vc( 
4,662,220 
3 4 
of 3 to 3.2 volts and ground. The input and output of the ent, then the water is a good conductor and the leakage 
trigger circuit is connected in commonly-known fash- current prevents oscillation. 
ion to the capacitor 15 and resistor 16 to form a known The output signal amplitude for the circuit of FIG.  1. 
RC oscillator circuit 20. Any well-known frequency is about 0.8 of the applied voltage. Such an output signal 
counter 25 is used to detect the frequency of oscillation 5 is readily readable. Use of a frequency counter 25 obvi- 
and provide a suitable output signal, Either visual or ates the necessity, as in the case of the prior art, of 
electrical output signals are acceptable outputs for such to detect very currents. Frequency 
can be measured with greater accuracy and resolution a counter 25. 
why the than is the case with very small current levels: Finally, 
manipulation. In the case of the reduction to practice of invention has not yet been undertaken, certain theory is this invention, the frequency counter 25 supplied a read- 
study of leakage current must be made based upon the vides an analog signal output, 
material being used. Nafion was formed into a dual plate , 5  Use of the low DC voltage makes the invention par- 
and 15B. The upper and lower surfaces were various locations of spacecraft, such as a shuttle bay. 
four square inches. With this capacitor 15 at a D C  volt- Other foreseen applications include remote soil and 
age of greater than 3.5 volts the leakage resistance, crop monito,.jng which may advantageously be accom- 
which is nonlinear with applied Voltage, prevented 20 plished with this invention, 
oscillation. At 3.2 volts, however, oscillation occurred The above description presents the best mode con- 
over a suitable fIXqLIenCy range t O  provide nearly linear templated in carrying o u t  the invention, The invention 
and reliable relative humidity nw~urements .  At the is, however, susceptible to modifications and alternate 
lower voltage, the frequency of the oscillator changed constructions from the embodiments shown in the 
in direct proportion to the water content absorbed by 25 drawings and described above. Consequently, it is not 
the Nafion. It is believed that, at the lower voltage the intention to limit the invention to the particular 
range of this invention, the water does not disassociate embodiments disclosed. On the contrary, the invention 
into its gases nor does it act as a storage battery. Instead, is intended and shall cover all modifications, sizes and 
it serves as a simple and directly-varying capacitor alternate constructions falling within the spirit and 
which has a leakage resistance that is nonlinear over a 30 scope of the invention, as expressed in the appended 
wide range of relative humidity. At the higher voltages claims when read in light of the description and draw- 
and higher humidity, it  is believed that the known resis- ings. 
tive property of Nafion creates too much leakage cur- 
rent since the water itself acts as a good electrical con- 
ductor. 35 prising: 
FIG,  3 represents an intercomparison of data on rela- a circuit ekment fUnCtiOning as a Capacitor formed Of 
tive humidity from a Belfort temperature/humidity sulphonated fluorocarbon material sandwiched 
between electrically-conductive and porous elec- recorder, as shown by the squares. The dark colored 
trodes for permitting the sulphonated fluorocarbon dots represent data taken from a Honeywell relative 
humidity recorder. In the case of both graphs, a simulta- 40 material to absorb moisture; 
neOuS frequency reading from the detector 25 o f ~ I ~ .  an R C  switching oscillator including said circuit ele- 
ment as a capacitor and a resistor in an R C  circuit was taken in real time for the two different humidity 
for control of the frequency of said switching oscil- recorders. It should be noted that the swings above and 
lator as a function of the R C  time constant of said below the linear dashed line were not as severe in the 
case of the Honeywell unit. a source of DC voltage impressed across said R C  
circuit, said voltage being selected to be suffi- The intercomparison of FIG. 3 shows a nearly linear 
ciently low to not cause current to be conducted relation between the observed signal frequency read by 
through said material; and detector 25, FIG. 1, and the relative humidity values 
which are scaled along the horizontal axis in FIG. 3. for detecting the oscillator frequency of said 
oscillator as a measurement of said relative humid- The data shown in FIG. 3 was obtained with the afore- 
mentioned Nafion capacitor 15 and a D C  voltage of ity. 
about 3.2 volts as the oscillator voltage. 2. A device in accordance with claim 1 wherein: 
has said capacitor circuit element is comprised of upper 
proven very beneficial over a wide frequency range and 55 . and lower electrically-conductive plates with 
a wide range of relative humidity, the invention does openings therein to expose to moisture upper and 
have application at higher D C  voltages. For  example, in lower surfaces of said sulphonated fluorocarbon 
a vacuum where very minute traces of water are pres- material. 
ent, an impressed voltage of up to 15 volts has been used 3. A device in accordance with claim 1 and further 
to achieve oscillation in an R C  oscillator circuit 2Q. In 60 comprising: 
such a vacuum, the device is sensitive and is useful over a frequency counter connected to said R C  oscillator 
a narrowly-limited range of relative humidity. The for measuring the frequency of said R C  oscillator 
small amount of moisture present allows the higher as a determination of relative humidity. 
While an exhaustive study of the 
relative humidity is linear at the low D C  voltages of this l o  frequency are easier to process by computer 
and worthy Of mention. a out as a digital signal, The prior art, in contrast, pro- 
capacitor l5 having upper and lower plate surfaces 15A ticularly attractive for measurement of the water in 
What is claimed is: 
1. A system for determining relative humidity corn- 
45 R C  circuit; 
the low DC Of 3.0 to 3.2 
voltage to be employed. When more moisture is pres- * * * * *  
65 


Water-absorbing capacitor system for measuring relative humidity

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