<*>.\

NBS TECHNICAL NOTE 824
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (I) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3 ) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Nuclear

Sciences 2 — Applied Radiation " — Quantum Electronics " — Electromagnetics a — Time

and Frequency ' — Laboratory Astrophysics " — Cryogenics ". THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational institutions, and Government; provides

advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions: Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute consists of a Center for Building Technology and the following divisions and offices: Engineering and Product Standards — Weights and Measures — Invention and Innovation — Product Evaluation Technology — Electronic Technology — Technical Analysis — Measurement Engineering — Structures, Materials, and Life Safety 4 — Building

Environment * — Technical Evaluation and Application 4 — Fire Technology.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective

utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing

equipment, techniques, and computer languages. The Institute consists of the following

divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data — Office of Information Activities — Office of Technical

Publications — Library — Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg. Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

3 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

1 Part of the Center for Building Technology.
A Laboratory Study of

Some Performance Characteristics

of an Aluminum Oxide Humidity Sensor

National Bureau of Gtsndnrcfc APR 2 9 1974 Saburo Hasegawa, Lewis Greenspan,

James W. Little, and Arnold Wexler

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

/

e

-jfiS %

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued March 1974
National Bureau of Standards Technical Note 824

Nat. Bur. Stand. (U.S.), Tech. Note 824, 28 pages (Mar. 1974) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1974

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:824). Price 65 cents.
TABLE OF CONTENTS

Page

1. Introduction 1

2. Description of Sensor 3

3. Test Procedure 3

A. Results 7

5. Conclusions and Discussion 10

6. Tables 11

7. References 22

iii
\J
A Laboratory Study of Some Performance Characteristics of An Aluminum Oxide Humidity Sensor*

Saburo Hasegawa, Lewis Greenspan

James W. Little and Arnold Wexler

A laboratory study was made of the performance of aluminum oxide humidity sensors over

a range of ambient temperatures from +20°C to -60°C encompassing dew points from +18°C to frost points of -100°C. Information was obtained on such characteristics as sensitivity,

hysteresis, temperature effect, pressure-altitude effect and short-term and long-term

repeatability. The sensors were found to be capable of detecting frost points as low as -100°C at ambient temperatures of -40°C and -60°C. It is estimated that the total

uncertainty inherent in these sensors is approximately 4°C.

Key words: Aluminum oxide sensor; humidity; humidity sensor; hygrometer; measurement

of frost points; moisture measurement; water vapor measurement.

1. Introduction

The Department of Transportation's Climatic Impact Assessment Program (CIAP) [1,2,3]**

includes a global study of the nature of the stratosphere. The present distributions of

temperature, constituents, motions, and circulations are being actively investigated.

Among the constituents that are of concern is water vapor. Measurements of water vapor

concentration in the stratosphere are or will be made with instrumentation carried aloft in

balloon-borne radiosondes and in high-altitude aircraft.

The National Bureau of Standards was asked to select and study the performance

characteristics of several humidity sensors for possible use in CIAP. The choice of sensors

for this laboratory investigation was constrained by the following considerations. First,

the sensors were required to be immediately available from commercial sources, or, at least,

to have progressed to a stage of development that could make them available soon. Second,

they could not exceed a cost of $350 per unit. Third, they were required to be compatible

with the telemetry circuitry of the radiosonde. Fourth, they were required to have a dynamic range of three orders of magnitude in measuring humidity. Fifth, they were required

to have an accuracy of about 10 percent. Sixth, they were required to have a capability of

detecting moisture levels equivalent to frost points as low as -90°C. Seventh, they were

required to have stable calibration curves. Finally, they were required to have a known

temperature coefficient.

A survey of the field disclosed two sensors that met the criteria of immediate

availability and cost and showed promise for use in the radiosonde. These were: (1) the

aluminum oxide sensor and (2) the miniature crystal array. In addition this survey disclosed

that there were three sensors which fell into the category of limited availability or which

required further development. These were: (1) the expendable dew-point hygrometer; (2) the

coated piezoelectric quartz crystal; and (3) the diffusion coulometric cell. There appeared

to be no other device or sensor suitable for stratospheric moisture measurements which met

the specified constraints and performance requirements, at least none that did not require

extensive research and development. An evaluation program was designed to yield the

following performance characteristics: sensitivity, range, hysteresis, temperature effect,

pressure-a1 titude effect, short-term repeatability, and long-term repeatability. The tests were chosen to cover the following range of parameters: (1) ambient temperatures from 20

to -60° C; (2) dew-point temperature from 18°C to frost-point temperature of -100°C; and (3)

ambient pressure-altitudes corresponding to sea level and 20 km.

The purpose of this paper is to report on the results of a laboratory investigation of

two forms of the aluminum oxide humidity sensor.

* This work was supported, in part by the Office of Naval Research acting on behalf of the

Climatic Impact Assessment Program Office, DOT, and by the Laboratory for Meteorology and

Earth Sciences, Goddard Space Flight Center, NASA.

** Figures in brackets indicate the literature references at the end of this paper.
TWO-PRESSURE HUMIDITY GENERATOR

HIGH PRES PRES GAGE PRES GAGE

EXH VALVE

BATH BATH BATH

Fig. 1 Simplified flow diagram of NBS two-pressure humidity generator.

LOW FROST-POINT HUMIDITY GENERATOR

PRES 6AGE O

r®i VALVE

Fig. 2 Simplified flow diagram of NBS low frost-point humidity generator.
2. Description of Sensor

The aluminum oxide sensor is a variable impedance device. It consists essentially of an aluminum base or substrate with an anodized surface on top of which is a thin film of

gold. The gold and aluminum serve as electrodes. Water vapor diffuses through the gold

film to the oxide surface. On sorbing water vapor the oxide changes its impedance. This

impedance is measured with appropriate ac circuitry.

The origin of this sensor can be traced back to the U. S. patent issued to L. R. Roller [4] . It was brought into prominence by Ansbacher and Jason [5] , Jason and Wood [6]

,

Underwood and Houslip [7], Cutting et al. [8,9], MacDowall et al. [10], Booker and Wood [11],

and Jason [12]. There was further study and development work on various versions of this

device by Stover [13,14], Chleck and Brousaides [15], Miyata and Watari [16], Chleck [17],

Morrissey and Brousaides [18], Lai and Hidy [19], Brousaides [20], Sze [21], DelPico [22]

and commercial production both in the U. S. and Great Britian.

Two forms of a commercially available sensor were used in this investigation. One was

a regular or standard industrial type that has been in production for some time. The other

was an experimental type which differed from the first in several fabrication details,

primarily in the method of attaching electrical leads to the electrodes.

The impedance of the sensor was measured with an ac circuit which yielded a variable

low-frequency output. This circuit is similar in most respects to the one used on the

radiosonde. However, the latter is limited to an output of to 200 hertz whereas this one

can yield higher output frequencies. An electronic counter with an overall accuracy of

better than 3 parts in 10^ was used to measure the frequency.

3. Test Procedure

The National Bureau of Standards has two humidity generators which provide a capability

for testing and calibrating humidity sensors and hygrometers with high accuracy. Because

these generators are discussed elsewhere in great detail by Wexler and Daniels [23],

Hasegawa et al. [24], Wexler [25], and Greenspan [26], only a brief description will be

given here.

The NBS two-pressure humidity generator is an apparatus which produces atmospheres of

known humidity. Fig. 1 is a simplified flow diagram which illustrates the principle of

operation and shows the basic components. Air from a high pressure source is cleaned and

dehumidified in a heatless dryer and passed through an external saturator that is

temperature-controlled at 10 to 15 deg C above the desired saturation temperature. The air

picks up water vapor in excess of that necessary for saturation at the desired saturation

temperature. It then flows through a series of three saturators with interconnecting heat

exchangers that are immersed in a temperature-controlled liquid bath. Excess water is

condensed out. The gas leaves the final saturator completely saturated with vapor in

equilibrium with either the liquid or solid phase (depending on the temperature) of

distilled water at some fixed pressure and temperature. It now flows through an expansion

valve and then into a test chamber which is located in a separate temperature-controlled

bath. The pressure in the test chamber is nominally at atmospheric pressure. Measurements

of the temperatures and pressures in the final saturator and in the test chamber serve to establish the moisture content of the air with high accuracy.

This apparatus is capable of generating humidities as low as -70°C in frost point. A

range of ambient temperatures from +60 to -55°C can be achieved in the test chamber. The

flow rate through the apparatus is maintained at 0.0014 m

3 /s (84 1pm). This is equivalent

to a mean air speed of about 0.02 m/s through the test chamber. The uncertainty in frost

point, over most of the operating range, does not exceed 0.1 deg C. The NBS low frost-point humidity generator also produces atmospheres of known humidity.

Fig. 2 is a flow diagram which illustrates its principle of operation. Clean dry air enters

a heat exchanger immersed in a liquid bath and is brought to a desirrd fixed temperature.

It then passes through a saturator, a long helical coil of stainless steel tubing, the

interior of which is coated with a thin film of ice. The path is sufficiently long so that

emerging from this coil the air is completely saturated with respect to ice at the temperature of the surrounding bath. The air then flows into a gold-plated test chamber which is

separately temperature-controlled. A small correction is made for the slight difference in

absolute pressure between the saturator and the test chamber. The frost point produced by
200

-100 -80 -60 -40 -20

FROST/DEW POINT, °C

Fig. 3 Family of calibration curves (isotherms) for a typical standard sensor.

250

-60 -40 -20

FROST/DEW POINT, °C

Fig. 4 Family of calibration curves (isotherms) for a typical experimental sensor,
this apparatus is basically equal to the temperature of the saturator and is measured with a platinum resistance thermometer. With this apparatus frost points from -30 to -100°C can be

generated in atmospheric air and other gases. The test chamber can be operated at ambient

temperatures from +25 to -100°C. Ambient pressures from atmospheric pressure to 500 Pa

(5 mb) can be established within the system. The flow rate is maintained at 3.3 x 10-5 m

3 /s

(2 1pm). This is equivalent to a mean air speed of about 1.7 x 10-3 m/s through the test

chamber. The uncertainty in frost point does not exceed 0.2 deg C. The procedure followed in this work was to use the two-pressure humidity generator for

tests at ambient temperatures from +20 through -40°C and to use the low frost-point generator

at ambient temperatures of -40 and -60°C. At -40°C the operating ranges of the two generators were chosen so as to produce an overlap in the test humidities.

Ten sensors were tested simultaneously, five of the conventional industrial type and

five of the modified experimental type. These ten sensors were selected by the manufacturer

for use in these tests. At each ambient temperature, a reasonable number of humidities was chosen to cover the

desired test range. At each ambient temperature, the highest frost or dew point to which

the sensors were subjected was chosen so that the equivalent relative humidity did not

exceed 90 percent. This upper limit was set to preclude the possibility that humidities

close to saturation might adversely affect the performance of the sensors. Except at the

-60°C ambient temperature each test range encompassed two or more contiguous smaller spans.

The ranges and spans are shown in Table 1. Within each span the sensors were cycled one or more times from the lowest to the highest frost point and then back to the initial point,

except for the -55 to -100°C frost point span at the -40 C C ambient temperature and the -65

to -100°C frost point span at the -60°C ambient temperature. For these latter two spans,

the humidity change was made only in one direction, from the lowest to the highest frost

point. In each span the sensors were exposed successively to three or more different

constant humidities. After each test humidity was established readings were taken

periodically until no further significant change was observed. In the two-pressure humidity

generator the exposure times to reach steady state conditions at ambient temperatures of +20,

0, -20 and -40°C were 30, 40, 60 and 120 minutes, respectively. In the low frost-point

humidity generator, the comparable steady state exposure times varied from 1 day to 9 days

at frost points of -55 to -100°C. In changing from one test humidity to another, the sensor exhibited an initial fast response and then a slow drift to its final reading.

Response times or lags should not be inferred from these observations of exposure times.

No attempt was made to differentiate between the generator and the sensor lag. Furthermore,

such parameters as air velocity and sensor orientation were different in the two generators.

The essential criterion governing the exposure time was the allowance of adequate time for

the system (sensors" and test chamber) to equilibrate after each change in humidity.

Each sensor was electrically connected to the measuring circuit through its individual

shielded 2-conductor cable and a selector switch. It was noted that the output reading was

affected by the length of cable, the relative positions of the separate cables, laboratory

location, and stray capacitances. By keeping the circuit, cables, and surroundings in each

humidity generator fixed and stable, the disturbances to the readings were minimized. Under

these conditions satisfactory repeatability was obtained. Individual capacitance calibrations

were made of the measuring circuit and the specific cable used with each element in each

generator. These calibrations were used to adjust the output readings taken with the low

frost-point generator to compensate for differences arising from the two test set-ups. The

oscillator was operated only while measurements were being taken. Between measurements there

was no excitation of any element.

Two sets of calibration runs were made at atmospheric pressure. The first set included

the entire ensemble temperatures and spans listed in Table 1. This set comprises the initial

runs. The second set was limited to ambient temperatures of 20 and -40°C. At 20°C the same

four spans were used whereas at -40°C only the -41 to -53°C and -56 to -68°C frost point spans were used. This second set comprises the final runs and was made five months after the first

set. After the initial runs were completed, and prior to the start of the final runs, the

ten elements were calibrated at an ambient pressure of 5000 Pa (50 mb) , and ambient temperature

of -40°C, and over a frost-point range of -55 to -90°C at successively lower frost-point

temperatures.
Fig

Fig. 6 Short term repeatability. Typical

plot of two calibration curves at an ambient temperature of -t-20°C obtained on two successive days on

the same sensor.

Typical rested state hysteresis

loop at +20°C for a span from a frost point of about -4°C to a dew

point of about +18°C. The arrows indicate the direction of the humidity change.

45

40

NH

ur>

o

z

ajDa

u

-5 5 10 15 FROST/DEW POINT, °C

20
4. Results

A family of calibration curves is shown in Figure 3 for a typical standard or conventional type of sensor and in Figure 4 for a typical experimental type of sensor. The

output in hertz has been plotted as a function of frost point (below 0°C) and dew point

(above C C) for ambient temperatures of 20, 0, -20, -40 and -60°C. For purposes of this

paper, the term "frost point" will be used to refer to frost points below 0°C and to dew

points above 0°C. The output frequency increases with decreasing frost point. There is a displacement of each isotherm to the left with decreasing ambient temperature. Therefore,

for any given output frequency, the corresponding frost point is a function of ambient

temperature. At ambient temperatures of -40 and -60°C, the sensor has sensitivity to detect humidities with equivalent frost points as low as -100°C. No attempt was made to test the sensor at frost points below -53°C at the higher ambient temperatures.

There is a tendency for the calibration curves of the standard type sensor to merge at

the higher output frequencies and to diverge at the lower output frequencies. The reverse

behavior occurs with the experimental type. The temperature coefficients are given in

Table 2, These express the change in frost-point temperature per deg change in ambient

temperature for fixed frequencies. They indicate the horizontal spread among the isotherms

at a given output frequency. They can be used to estimate the error in indicated frost

point due to an uncertainty in, or a failure to account for, the ambient temperature. The

temperature coefficients tend to be larger for the experimental type of sensor. The sensitivity of the sensor is shown in Table 3. An average sensitivity was

calculated for each element by dividing the overall change in measured output frequency by

the corresponding change in frost point for each ambient temperature. These sensitivities

were pooled for each ambient temperature to yield mean values. The five mean values for

each type were then combined to give a composite mean of 1.8 Hz /deg C frost point for the

standard type and 3.2 Hz /deg C frost point for the experimental type. The -20°C isotherm

exhibits the greatest sensitivity whereas the -60°C isotherm shows the least. The composite

mean sensitivity of the experimental type is almost twice that of the standard type. For the

-40°C and -60°C isotherms the ratio of sensitivities is 2.4 and 3.7.

A typical hysteresis loop for a span extending from a frost point of about -4°C to a dew point of +18°C at an ambient temperature of +20°C is shown in Figure 5. This loop was

obtained without prior humidity cycling over the +18 to -4°C dew or frost point span at ambient temperature of 20°C. This first test loop will be referred to as the "first

calibration cycle" obtained for the sensor for the particular dew or frost point testing span. For any output' frequency the difference in frost points between the increasing and decreasing

humidity branches of the loop is defined as the hysteresis. The average of the differences

in frost points for equally spaced frequencies is called the average hysteresis; the maximum

difference for the entire loop is the maximum hysteresis for the span. A summary of the

average and maximum first calibration cycle hysteresis for each element for various

temperatures and spans is given in Tables 4 and 5 respectively.

There appears to be no significant difference in hysteresis between the standard and

experimental types of sensor. At each ambient temperature the hysteresis is substantially

larger for the span enclosing the higher humidities (frost points) than for the spans

enclosing the lower humidities. Furthermore, the hysteresis is larger at the higher ambient

temperatures.

Figure 6 is a typical plot of two calibration curves covering a frost or dew point span

of -4 to +18°C at an ambient temperature of 20°C. Only the increasing humidity branches of

the hysteresis loops are shown. These two curves were obtained on the same sensor on successive days. Therefore, the displacement may be taken as an indication of the short term

repeatability of the sensor. Shifts of this kind occur to a greater or lesser extent with

each element. This short term repeatability varies with ambient temperature and span.

Average and maximum values of short term repeatability are given in Tables 6 and 7. Again,

there appears to be no significant difference in behavior between the two types of sensor.

As in the case for hysteresis, at each ambient temperature the short term repeatability

(more precisely, the lack of repeatability) is larger for the higher humidity spans than for

the lowers ones; also, the shifts are larger at the higher ambient temperatures.
250

-100 -80 -60 -40 -20

FROST/DEW POINT, °C

20

Fig. 7 Long term repeatability. Typical plot of +20 and -40° C isotherms.

The "final" curves were obtained 5 months after the "initial" curves

250

180

170

~~i—r~i—

r

ATMOSPHERIC PRESSURE

-90 -85 -80 -75 -70 -65 -60 -55

FROST POINT, °C

Fig. 8 Ambient pressure effect. Typical plot of -40°C isotherm obtained at standard atmospheric pressure and at 5000 Pa (50 mb) . Divergence of

the curves may not be significant.
Figure 7 is a typical plot of 20 and -40°C isotherms based on calibration runs made

about 5 months apart. Only the increasing humidity branches of the hysteresis loops are

shown. During the interval between the two runs the element had been subjected to the

humidity spans and ambient temperatures outlined in Table 1 and to an ambient pressure of

5000 Pa (50 mb) . The difference between these two curves may therefore be viewed as the

result of the combined effect of aging, testing, cycling, etc. This difference will be

defined as long term repeatability. Average and maximum values of long term repeatability

are given in Table 8 for the +20 and -40°C isotherms. The long term effect is larger

for the experimental type. The +20°C isotherm displays a much larger effect (from two- to three-fold) than the -40°C isotherm. The change that occurs produces both a displacement and

a rotation of the isotherm, with the two curves crossing at a high output frequency.

For each element, at each ambient temperature, the calibration data were fitted by the

method of least squares to a polynomial equation of the form

5>.

i=0

where t is the temperature in C of the frost point (below 0°C) and the dew point (above 0°C)

and f is the sensor output frequency readings in hertz. Satisfactory fits generally were

obtained for n = 3. Higher degree polynomials yielded no significant improvement in the fits.

The estimated residual standard deviation a r was calculated for each equation using the

formula given by Natrella [27]

a =

r

where r. is the difference between the experimental frost (or dew) point and the

corresponding computed value, k is the number of fitted points, k-j is the number of degrees

of freedom and j is the number of coefficients in the polynomial equation. The residual

standard deviation will be used as a measure of estimated random uncertainty arising from

the use of the element at the appropriate ambient temperature.

Table 9 gives the values of a r for the initial runs. There appears to be a tendency for the residual standard deviation to be larger for the standard type of sensor, except for the 20°C isotherm. At -40°C the mean a r for the standard type is 2 times larger

than for the experimental type over the -41 to -68° C frost point range and 3 times larger

over the -41 to -100°C frost point range. No data were obtained at frost points below -53°C

at the ambient temperatures of 20, 0, and -20°C. Therefore, it is not known whether frost

points below -53°C would increase the or

of these isotherms as it did for the -40°C isotherm.

The overall mean a r for the five isotherms is 1.5 and 1.3 deg C, respectively for the

standard and experimental types of sensor. The final runs only cover two ambient temperatures: +20 and -40°C. Furthermore, as indicated earlier, the -55 to -100°C frost span was not included in the +20°C runs. Therefore, this span was deleted from the initial runs at -40°C and a new a r calculated. The

residual standard deviations for the ensuing initial and final runs are compared in Table 10.

The values of a r for the final runs tend to be larger than for the initial runs. The hysteresis observed in the elements during the final runs was larger in magnitude

than in the initial runs. Table 11 summarizes the average and maximum hysteresis encountered

during the final runs. The typical effect of a change in ambient pressure on the -40°C isotherm is shown in

Fig. 8, over the frost-point range of -55 to -90°C. There appears to be no significant

difference between the standard atmospheric pressure curve and the 5000 Pa (50 mb) curve

for frost points of -55 to -75°C. At -90°C the 50 mb curve dips substantially below the

standard atmospheric pressure curve producing an apparent difference in frost point of
approximately 3 to 7 deg C. However this observed difference may be due, in part or entirely, to reasons other than ambient pressure. The atmospheric pressure curve represents

data taken in the increasing humidity direction, whereas the 50 mb curve represents data

taken in the decreasing humidity direction. Thus hysteresis may be a contributing factor.

The exposure time for a change from a frost point of -100 to -90°C (at atmospheric pressure) was 7 days; for a change from a frost point of -75 to -90°C (at 50 mb) the exposure time was 3 days. Although it was assumed after these exposure times that steady state conditions were reached because of the almost negligible residual drift in the elements, it is possible

that further changes in reading would have occurred with longer exposure times. If steady

state conditions were not obtained then the indicated output frequency for the atmospheric

pressure curve is too high (since the frequency change was from a higher to lower value) and

the indicated output frequency for the 50 mb curve is too low (since the frequency change was from a lower to a higher value)

.

5. Conclusions and Discussion

Several basic differences between the standard and experimental types of sensor emerge

from these tests. The experimental type has greater range and sensitivity. It has larger

temperature coefficients. The isotherms for the standard type tend to coalesce at the low

frost points (high frequencies) and diverge at the high frost (dew) points (low frequencies)

The reverse occurs with the isotherms of the experimental type. The two types perform alike

with respect to such characteristics as hysteresis, short term repeatability, long term

repeatability, pressure effect, and estimated overall uncertainty.

The individual sensors differ from each other in all major respects and individual

calibrations are required.

At ambient temperatures of -40 and -60°C the sensor is capable of detecting frost points

as low as -100°C. Tests were not made at higher ambient temperatures so it is not known

whether there is a similar capability at higher ambient temperature.

The sensor displays hysteresis. It also undergoes a shift in reading on being

subjected to the same monotonic short span of humidity after an elapsed time of 1 to 2 days

(short term repeatability). It undergoes a larger shift in calibration after an elapsed

time of 5 months (long term repeatability)

.

The overall mean residual standard deviation a r in terms of dew or frost point for the

five isotherms comprising the initial run for all standard elements is 1.5 deg C and for all

experimental elements is 1.3 deg C. A reasonable estimate for the overall maximum uncertaint;

inherent in this sensor can therefore be taken as 3 a r or 4.5 and 3.9 deg C for the two types of sensors, respectively. There is also a shift in calibration with time. or use or

both. After subjecting the sensor over a period of five months to a range of ambient temperatures of 20 to -60°C and to frost points as low as -100°C and to ambient pressures of 50 mb,

the 20°C isotherm undergoes an average shift of 5.1 and 6.0 deg C in dew or frost point and

a maximum shift of 8.5 and 12.1 deg C for the standard and experimental types, respectively,

whereas the -40°C isotherm experiences average shifts of 1.4 and 2.5 deg C and maximum shifts

of 2.9 and 4.9 deg C for the standard and experimental types, respectively.

It should be emphasized that the conclusions given here are based on data taken on elements that were not exposed to relative humidities above 90 percent. The data presented

here indicate that conditions tending to degrade performance, such as aging, hysteresis,

etc., tend to have more pronounced effects at the higher humidities. What effect, if any,

exposure to conditions of saturation or near saturation would have, is not known. In addition, it is not known how representative these elements are of normal production.

Another important characteristic that was not investigated is the response time (lag)

.

Knowledge of this property is essential for many humidity measurement applications.

A further observation may be worth adding. These tests were made under laboratory

conditions where care was taken to minimize or eliminate all known sources of error. If similar precautions are not taken when these (or any other sensors) are utilized for a specific measurement problem, then additional systematic errors may be introduced. For

example, in stratospheric measurements using balloon-borne instrumentation such factors as moisture contamination, solar radiation, telemetry errors, shelf life, and lag can contribute

large uncertainties.

10
Table 1

Testing Ranges and Spans

Ambient Temperature, °C

20 -20 -40 -60

Overall testing range in dew/frost point , °C

+ 18 to -53 -1 to -53 -21 to -53 -41 to -100 -65 to -100

Testing spans in dew/frost point , °C

+ 18 to -4

to -18 -1 to -19

-20 to -36 -20 to -36 -21 to -36

-40 to -53 -40 to -53 -40 to -53 -41 to -53

-56 to -68

-55 to -100 -65 to -100

Dew point for temperatures above 0°C; frost point for temperature below 0°C.

11
CN 0)O

o>U

iHX HI

(0

u

H 3

4-1 toM

0)o.e

<u H

m

m CO CN o CN CN <f m

.H

O

in r^ CN o H

to

r~

~3" O

<r -* <r

4J ai c u

a) 3 o 00 r^ 00 e 4-1 vO r-\ ro ro

•H

m to • • • • VJ l-l o

CU <u P- ex X e W <u

4-1

4-1 C o oo CN

m

01 CN en <f <

ro

XI6

tfl u o

00

QJ XI oi-H CN

CNm

i-l 4-1 c

•H o ' * H o O • D.

0)C o

4J

0) CO

05 4-1

O c u o oo r14-1 ai 14-1 vD m i-l O s o C_> O • •

<UH

p.

W 00 ^ CU H XI„

4-1 c in in o

CUm ro .-1 00 O

•H

14-1

14-4 CU o o

CJ

CU

mm o u ro !-H iH vO 3

4-1 CO o '

X>u

M CU CO a a e c CU D a

4-1 tz> c m o r^ <r tfl CU S o"

CN

H

mmO CNI O o .H

S^ocV3

<rN o o O o ai JZ m o m o H <-4 rH CN £

12
X> •H

to CO H c

o> CO

o

vO

I

o

\0

o

o>

oa

0£

0) -o

G

<uCO

a)M

CO

0)

O o r^ r-~ vo

.H O o O O

O

en

u

cfl C

tfl

o

<^ co -» <r <r

r-H CM CM CM CM

CM

CM

c

CO

01

<r o

CM CO

>d- r-» a\ o

co ro co m

0)a

•HM

01a

CM

co

vO CT\ C^ CM CT> CT> CM CM CM CO CM CM

CM UO l-» CT> CO CO CO CO CO CO

H H «* CM CTi i-H CO CO CO CO CM CO

c

cfl <u s

COc

0)coa

cfl e

XIu

cfl T3c

cfl 4-J CO

c

(U I

0>O. w

CM

13
[fl o

01 >, 10 M u a

01 3

<r 4J c l-i 0) o

oi p^ •H rH iH x; 4J t« XI « •H

n) 01

M 4J

H 00X •H

cd •H

C

u rHH

01 rt > U < U

cn •H

01

TJC

•iHOO P- U "M

1*

<D Q

G

01s

•HM

01O.Xw

01

1-1 3u

CO

l-i 0)(XeH

O

ft

01 •a

00

a; -a

to x;

oo

tfl

>-< 01 4

CN

O CN 00

O

O

O cn

Oh in* o o o en o o o en o o <r oo o o o cn

cn

O cn r-»

OOO cn

<n o m <r O O O rH

rH i-t CN CN O O O cn

iH o cn cn o o o rH

.h cn cn

m o o o rH

cn

O •* r-~ o o o cn o cn cn \o O O O r-l N H* CO O o O m O O O 00 <r cn r-i o o o o m vo oo <t

in cn in i i I i o

CN

<T iH CTi O O i-t

iH i-h cn O O CN

VO rH r~ O O rH N H N o o o

cn

O r-»

OO .-I

-* i-i

m

OO r-t

CN r-l CN O O CN

r-lO

O

O

O CN

rH cn oo O O CN

cn

O r~^

OO CN

r-t CN <J\ O O CN O O r-l ~* CN I I I o o o

cn vo cn

1/1 <n rH I I I

cn o O r-t < oo o o

-<r oo o o

CN CN o o

CN 00 o o

<r o O rH

-J- rH O r-l

CN

O

O rH

<3" -* o o o o o ,-t

CNO

O rH

—I 00 o o

cn cr> o o o <& o o o oo o o O 00 o o

cn r~ o o

vO rH O rH o sr O rH

CN

O

O rH

cn cn d d

O rH St CN I I

<n vo m cn I I

vO rH m •* I Io

4-1 4-1 oo <n 1 I o

CN o

14
o

en >v

•H c_> CO

CDM 6 wC

m cu •H 3

4-1 4J

u

ai CO CO

1-1 >>

M

H

x> x: X> crt CO •H •H H R rH 4J 3 CO H e C_> C

•H

H

Hs

4-1 CDu

C cn rH vO cn VO CM

O cn cn VO rH

CO • • • • • • • • • • CU O O O <f O O m O rH O rH S o cn o m H N H rH O cn cn o • • « • • • • • • • •

r-l o o rH

m o o cnO rH o o

iH

CO

4.) VO CM IT) r~- r- cm <r cn o- r-. cn c i-. • • • • • • • • • • CU O O O <f o o cn O rH O rH e

•i-4 U

CUCU

OO u-> vO o cn vo VO CM VO vO

X

m • • • • • • • • • • • W

4-1 C o o o -* rH o cn

O rH rH rH

•HO <r o co

m 0-000 cm cn

m r-» co

4-1 COO o o o <•

O

O CMO rHd o u

U-l LO

O vO ~* vo i—i cn cnm o -*

.-i ^^ • • • • « • • • • • 3 O O O CM O O CM O rH «0 rH

CU

rJ • a O o

co s a C

cu 4-1 c oo

c/o

a CO <u in cm

m <f LO CM CM -st -tf rH rH

CU CU T3 • • • • • • • • • • • 4-1 e S O O O cn o o cn O rH O rH o CU rH CO

<UW -H

p. cn >s CU H u vo o -d- CM oo <r on -st CM

O 00 o> CU • • • • • • • • • • •

4-1

OOO cmOO CMO rHO

O CO>> -Ca m cn sj- cm cn o <f o o O rH

T3 00 3 • • • • • • • • • • M S o o o cn O O CM O rH O rH

CO •H

X)

X

a cO « s 4J oo cm vo rN m<f

m <t -* ON

00 vO o o o cn o o cn

O rH

d

d o <r -<r ~* r- o m 00 ON O CM

-cf o o o cn o o cn

O rH

O CM

«* cn r^ vo cm cn

mm vo cn

m CM o o o sj- O O cn O rH o o

4-1 a

•i-i o

cu o o O 00

O

O rHO rH VO rH

4->

C S3- CM 1-4. <f CM -st CM

m <f

cn co I I 1 1 1 1 1 1 1 O Pl. U u <x> o o o o o O O O o o o o

pn 4J 4-1 4J 4-J 4J 4-1 4-1 4-> 4J 4-1 4-1 ^ cn vo oo <d- cn vo o\ m vo oo cn

CU un cn iH

m cn rH

m cn vo in o 1 I 1 1 I I i I i I I

CU

4-1

U

C 3

cu 4-i •H CO

U o o o o

XI

M CM Ol sS- S CU I 1 s

CU

E-4

15
Xi

co

4-1

cfl <uQ.

OJuO

4=

0100

COu

<u * C vo sr cm oo sr u-i

m sr u~l <r cm

COa)

OO

O iHOO

H o o o o S

r>- «on * m -a- oo st <r sr cm o • • • • • • • • • • H O O O CM O O H ' o o o o H

CO

m co cm cm » CMH sr co CM CM

4J r~ • • • • • • « • • • • c O O O CM O O H o o o o

a) 4-1 6 c H H M O

01 a. »>}HO <t com u-| sr vO co a uo • • • • • • • • • • • X 4J O O O CM O O H o o o o w COou

(4-1 CO "s

m sr cm co CO CMH sT CM CM CM

01

OOO iHOO rHOOOO

03U

60 r». co sr sr I/O CM 00 u-i CO sr cm

fH OJ • • • • • • • • • • a o o o o o o o o o o o u . . o

CO o

4J c •H

01 4-1 c .H

(0 c ct) rl r» vo srm v© U-| 00 vo sr CM u-i

01 OJ JQ • • • • • • • • • • • 4-1 a s tO O O O .H O O iH o o o o o 01

i-H 4J

CO

0)

w 01

a a.

>* -<u H ov M CO -J CM

N 00 vO u-i r-» sr CO ST

B o o o o O

O rH o o o o u

01 4J

4-1 oo in sr sr u-i si- sr UI CO

O sr

00 V4 • • • • • « • • • • O O O i-l O O tH o o o o

.G

CO

OJ T3 00 •*

N CO o> 00 u-l CO vo u-i CM CO

M vO CO • • • • • • • • • • • CO u o o o o O O CM o o o o

•a OJ c >

cd <c

4J

C/> Cft r- co vo s in co r-. co

O co «» O O O i-H O O .H o o d d

in uo ro i-H sr sr cm ST vo <r cm

CN

OO

O CM o O CMd o dd

4Jc

•Ho

PO

O

O 00OO rHO rH VO tH

4-1 c •* CM iH ST CM sr cm U-I ST

0) aj

u 1 1 1 1 1 I I 1 1 o a o M 00 o o o o o o o o o O O

Pn i-l 4-1 4-1 4-1 4J 4-1 4-1 4J 4-1 4-1 4-1 "s CO vO 00 -J co * c^ CO vO 00 CO

(U UI CO

H u-i CO i-H UI CO vO u-|

Q 1 1 1 1 1 1 1 1 1 1 1

<D 4J U C 3

0) 4J •H CO o O o O

J3

M

U CM CM sr 1

ajaE

OJ H 1

16
§

4J

ca(X CO

01 c U 3

01 rH

id H o

CO I

cd

01aHO

El,

0)M

01ft wu

cd ac

cd

CO c c cd

)-> ex

Pk c/> &

ai nI c

cd

cu so

i-H

r~. m

en

t-i oz

4-1

a c cd 0) 01 e X 0) w

o\

oo

vO

-»

CN

00

U

oo

m in a> o o o CN

cmm in cm

i-l o o -*

Ninnn o o o en m vo en en o o o en

p- vo <r o o o o en

oo -* o~\ r-~- o o o o

CJ\ vO vO vO O O O en O «tf -* rH rH O O en

a\ vo vo oo o o o <•

cocoo<t O O rH rH O oo m cm

rH

dd in

r~ oo vom o o O en O O O ao -* CM rH o o o M 4-1 4J 4J ro vO 00 -J m en rH 1 I I I o

cm

vo <r cm o o CM

VO VO CO O O CM

<7i en en O O CM

vo

m

m o o <M

-d- -3- o O O CM

r-» en en O O rH

oo r-» en O O CM

ct\ vo o~> O O rH

t-» oo r- O O rH

o> oo r-. O O CM O O CM m oo p~ O O CM O O rH

-3- CM I I I o o

4J 4J 4J en vO <3\ m en rH 1 I I O

O

vO vo O O

-*

m o o m en o o d d o o

vom o oCM CJV rH O

m en o oen r-. o d

O o

vo r~ O O

dd

en

m o o

oo r-. o o

*d- vo o o

vo

m o o o vo o o m m o o

-* oo

i-i

d

O vo o o

vo en O rHVO -<J- d d

O rH

«tf CM I Io

4-1 4J en vo m en 1 I

vO o m -* I I

oo en

vo

mI I oI

17
1 53EHX

cfl s

-*

-*

CO H

•H

XI

cfl 4-> (0

0)P.

0)U

01

4-> ooco

1-1

T3C

CO iH

Cfl Co

CO •H

t-l COP6O

c_>

O1

013

4-> CO

!-i 0)P.eHI O

CN

60

0) T3

4-1 c

•HO

P.

Xj

cfl 4-1 cd

9JP.

cu

(-1 eu

01

60eoJ

0>60

COU

0)B3EHX

ca s

v£3 C» co CO

m

00

0160

Cfl u

Ot 4 m in

m

VO 00 CN oo

oo

m in «*

C

0) • 6 o

Ol z Hw c

0)

cfl

0)p.

I-COC

cfl 4-1 CO

C

01e

•HM

01P.

X! W

18
.a

TO H >

T3

•3

1-1 TO •3C

CT3

-I

TO3

TJ

•H

to

(U pi

COc3M

r-H

TO

O

vOI ov0ooI o

CMI

00 T3

to3H

CO

01 Pi

r^ oo O O

vO ON O iH o O

oi tn cm cm

en cm co en

lO Is ^ Ol o\

N

d o o o o o m m m

vO 00 CPi c

TO01 2

T3P

<u TO o. TJ

>> s H TO o o O O

00 as O O

in -tf vO O O O

d o

cm

mm -j- o o o o O r~ O a\

r-4

O i-HO

O

CO vO

CM .-Is

TO c

cue

•HU

CDn.xw

O1O

4J in

ini 3c • TO u

00

O

vO o 1 00vO o

4-i o

4J vO

lO vO 1 u-l 3c •» TO vJ u CO

u-> CO 1 u->1 O

4->

O

4-1 .-1 -*

i-H 1 <f

..

COC

0) TO c ex

TO CO CX

CO 4J3

4J •H 3 o

•H o. O i D. 4-1 1 CO

4-1 o

CO

u

O VW u

M-l <ucu O u 3 .c

4J 4-1 3 c o o

X) T5

(!) cu

rn (0

TO TO. PQ w

19
COc3M

co iH C CO O C

•H •H

4-1 M—1

CO •H T1 > c

<U CO T3 o iH

rH T3 CO ^ •H

CU <0 4-1 rH T) •H ^3 c C

cd (0 •H H 4-1 co M-lo

iH

(0 c D o

T3 CO •H •H

CO l-l 01 CO « eo oI

en r-. o o o

CM oI oCM >

CU

T3U

CO XIC

CO

cfl 3

13

•H

CO

0) OS

O

<• -3" O O

u-i -a- vo o o d o

CM

iH o

CM CM

O

CM

(3

0) •

CU 13

i-l w c

cd

01 c

cd

0)

cu

T3

>-< COc

CO 4J

C/3 c

CUSu

CUexXw

00I

COc

COl

COc

nja.

eno

(XI 4-1 COOu£O

T3

0)

0)

CO pa

20
cO H

CD01U

0) 4J

CO>>

PC

COu

.o c m cn on oo rH rH

O r-» cn o O On

CO • • • • • • • • • • • • 01 o o o sr CO CN cn O i-t i-~ m cn X

CN CM «* r-. 00 00 00 o o • • • • 1 1 • • • • 1 1 r-H O O i-l vO O O r-H O

rH

i-H

cfl in -j r^ >t o -*

O Olrl-J 00 rH

(J r» • • • • • • • • • • • • C 4J o o o -vt -* CN 4J cn o rH in m cn

0)

a c 6 •H •H

iH o O

ri a. a.

01 oo cn oo en ON 00 00 in rH vO <r o (X in 4J • • • • • • 4-1 • • • • • • X CO o o o -* m c-4 CO <r o h vo on ~a- w Ou

14-1

Ou

>4H -^ v*^. cn 3 in ci o> cn cn r~- 3 -»onH CM CN

01 • • • • • • 01 • • • • • • ID O O O <T i-H o •V NHrlN CM CM

C_>

u

on 60

01 CI CNI 00 CN cnm 01

O -if CN 00m <r

r-H

a

OOO -vf rH o

13

•vf

O rHm CM CN

ri * "

CO CO

OO •H •H

CO SB CO CO a 01 01 01 4-) C rl u

CO e CO 01 o-jnn CN o 01 oo vo vo

mm -*

0) 01u • • • • • • 4-) • * • • • • CM 6 X CO rH O rH Vf iH rH CO cn r-i rH vO CM <-t o 01

I-l

0)

w

o. 01 e

>v 60 PJ H CO rH m O r^ r-» cn e •st VO vt t^ <r on

on

M • • • • • • •H • • • • • 0) HOHn i-H i-l X m rH rH vO cn i-h > CO < 2

vO U1 ON i-H 00 iH

m vo i-h

m on cn

T3 00 • • • • • > • • • • • • M i-H o o m i-l iH m <-t i-h «* cn i-h a) T3C

CO

4-) i-i in <r oo 00 <!• rH CN ON rH

m t-t

!*) vo

OO i-H -*O rH rH CN rH r~. rH CN

MTHO 100 00

m CN CN o oo -<r -* HOrlfl d d VO CN CN VO rH rH

r~ cn

m o rH CO r~ cnm •* oom CN O O rH vO rH O O O rH 00 rH O

4_) C

•HO

PL.

OOO 00 VO rHOOO 00 vO rH

u <r cn <-i

m -<r Si- CM rH

m -dCO C i I 1 1 1 1 l I o ca u u o. o o o o o o o o o o o o

tii en 4J 4-1 4J 4-> 4-1 4J 4-1 4J 4J 4-> 4-1 4J Is CO vO 00 «* oo cn en vo oo -tf oo cn

0)

m cn i-h vo

m

m cn i-h vom c 1 1 1 1 I i I I I I I I

a)M

4J 3 C 4J

01 CO •H M

u o o o o X 4) o CN -* CN >d- 1 & I I

01 H

21
References

[1]. Cannon, Jr., Robert H. , 1971: Planning a program for assessing the possibility that SST

aircraft might modify climate. Science , 52 , 836-842.

[2]. Grobecker, A. J., 1972a: The DOT climatic impact assessment program. Proc. of the

Survey Conference February 15-16, of the Climatic Impact Assessment Program , A. E. Barrington, Editor.

[3]. Grobecker, A. J., 1972b: Assessment of climatic changes due to flights in the

stratosphere. AIAA 5th Fluid and Plasma Dynamics Conference , Boston, Mass., June 26-28,

AIAA Paper No. 72-658, 16 pp.

[4]. Roller, L. R. , 1941: Electric hygrometer , U. S. Patent 2,237,006, April 1.

[5]. Ansbacher, F. , and Jason, A. C. , 1953: Effect of water vapor on the electrical properties

of anodized aluminum. Nature , 171 , 177-178.

[6]. Jason, A. C. , and Wood, J. L. , 1955: Some electrical effects of the adsorption of water

vapor by anodized aluminum. Proc. Phys. Soc . (London) B68 , 1105-1116.

[7]. Underwood, C. R. , and Houslip, R. C. , 1955: The behavior of humidity-sensitive

capacitors at room temperatures. J. Sci. Inst . , 32 , 432-436.

[8]. Cutting, C. L., Jason, A. C. , and Wood, J. L. , 1955: A capacitance-resistance

hygrometer. J. Sci. Inst . , 32.. 425-431.

[9]. Cutting, C. L., Jason, A. C. , and Wood, J. L. , 1956: A capacitance-resistance hygrometer.

J. Sci. Inst ., 33, 36.

[10]. MacDowall, J., Cutting, C. L. , Jason, A. C. , and Wood, J. L. , 1956: A capacitanceresistance hygrometer. J. Sci. Inst . , 33 , 284.

[11]. Booker, C. J. L. , and Wood, J. L. , 1960: Further electrical effects of the

adsorption of water vapour by anodized aluminum. Proc. Phys. Soc . , 76. > 721-731.

[12]. Jason, A. C, 1965: Some properties and limitations of the aluminum oxide hygrometer.

Humidity and Moisture (A. Wexler, ed.) Vol. 1, 372-390, Reinhold Pub. Corp., New York.

[13]. Stover, C. M. , 1963a: Hygrometer., U. S. Patent 3,075,385, Jan. 29. [14]. Stover, C. M. , 1963b: Aluminum oxide humidity element for radiosonde weather measuring

use. Rev. Sci. Inst ., 34 , 632-635.

[15]. Chleck, D. , and Brousaides, F. J., 1965: A partial evaluation of the performance of an aluminum oxide humidity element. Humidity and Moisture (A. Wexler, ed.) Vol. 1, 405-

414, Reinhold Pub. Corp., New York.

[16]. Miyata, A., and Watari, H. , 1965: A hygrometer which utilizes an anodic oxide film of

aluminum. Humidity and Moisture (A. Wexler, ed.) Vol. 1, 391-404, Reinhold Pub. Corp.,

New York.

[17]. Chleck, D. , 1966: Aluminum oxide hygrometer: laboratory performance and flight results.

J. Appl. Met . , 5_, 878-886.

[18]. Morrissey, J. F. , and Brousaides, F. J., 1967: Comments on "Aluminum oxide hygrometer:

laboratory performance and flight results." Chleck, D. Reply. J. Appl. Met . , 6_, 965-969

[19]. Lai, J. R. , and Hidy, G. M. , 1968: Microsensor for measuring humidity. Rev. Sci. Inst.,

39, 1197-1203.

[20]. Brousaides, F. J., 1968: An evaluation of the aluminum oxide humidity element. Air

Force Cambridge Laboratories , Report AFCRL-68-0547 , Oct., 45 pp.

[21]. Sze, Min-nan, 1970: On the anodized aluminum humidity sensor. Air Force Cambridge

Research Laboratories , Report AFCRL-70-0395, 30 June, 274 pp.

[22]. DelPico, J., 1967: Performance of thin film humidity sensors. Air Force Cambridge

Research Laboratories , Report AFCRL-67-0543, 15 October, 65 pp.

[23]. Wexler, A., and Daniels, Jr., R. D. , 1952: Pressure-humidity apparatus. NBS J. Res .

,

48, 269-274.

22
[24]. Hasegawa, S., Hyland, R. W. , and Rhodes, S. W. , 1965: A comparison between the

National Bureau of Standards two-pressure humidity generator and the National Bureau

of Standard hygrometer. Humidity and Moisture (A. Wexler, ed.), Vol. 3, 455-459,

Reinhold Pub. Corp. , New York.

[25]. Wexler, A., 1968: Calibration of humidity-measuring instruments at the National

Bureau of Standards. Trans. ISA , 7_, 356-362.

[26]. Greenspan, L. , 1973: Low frost-point humidity generator. J. Res., Vol. 77A , No. 5,

Sept. -Oct.

23
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

!D3S TN-S2H

2. Gov't Accession

No.

4. TITLE AND SUBTITLE

A Laboratory Study of Some Performance Characteristics of

An Aluminum Oxide Humidity Sensor.

3. Recipient's Accession No.

5. Publication Date March 197U

6. Performing Organization (ode

7. AUTHOR(S)

Saburo Hasegawa, Lewis Greenspan, James W. Little, Arnold Wexler

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project /Task/Work Unit No. 2130430

11. Contract 'Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Department of the Navy

Office of Naval Research

800 North Quincy Street

Arlington, Virginia 22217

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary ol most significant information. If document includes a significant

bibliography or literature survey, mention it here.) A laboratory study was made of the performance of aluminum oxide humidity

sensors over a range of ambient temperatures from +20°C to -60°C encompassing

dew points from +18°C to frost points of -100°C. Information was obtained on such characteristics as sensitivity, hysteresis, temperature effect, pressurealtitude effect and short-term and long-term repeatability. The sensors were

found to be capable of detecting frost points as low as -100°C at ambient

temperatures of -40°C and -60°C. It is estimated that the total uncertainty

inherent in these sensors is approximately 4°C.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key uord unless a proper name; separated by semicolons) Aluminum oxide sensor; humidity; humidity sensor; hygrometer;

measurement of frost points; moisture measurement; water vapor measurement.

18. AVAILABILITY fc2 Unlimited

_ For Official Distribution. Do Not Release to NTIS

Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20102, SD Cat. No. CI3«i;h;fl2U

Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SE( URITY (LASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

28

22. Price

65 cents

USCOMM.DC 29042-P74


NBS TECHNICAL PUBLICATIONS

PERIODICALS JOURNAL OF RESEARCH reports National Bureau of Standards research and development in physics, mathematics, and chemistry. Comprehensive

scientific papers give complete details of the work,

including laboratory data, experimental procedures,

and theoretical and mathematical analyses. Illustrated with photographs, drawings, and charts. Includes

listings of other NBS papers as issued.

Published in two'sections, available separately:

• Physics and Chemistry (Section A)

Papers of interest primarily to scientists working in these fields. This section covers a broad range of physical and chemical research, with major emphasis on standards of physical measurement, fundamental

constants, and properties of matter. Issued six times

a year. Annual subscription: Domestic, $17.00; Foreign, $21.25.

• Mathematical Sciences (Section B)

Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design,

numerical analysis, theoretical physics and chemistry,

logical design and programming of computers and

computer systems. Short numerical tables. Issued quarterly. Annual subscription: Domestic, $9.00; Foreign,

$11.25.

DIMENSIONS, NBS

The best single source of information concerning the Bureau's measurement, research, developmental, co- operative, and publication activities, this monthly

publication is designed for the layman and also for the industry-oriented individual whose daily work

involves intimate contact with science and technology —for engineers, chemists, physicists, research managers, product-development managers, and company

executives. Annual subscription: Domestic, $6.50; Foreign, $8.25.

NONPERIODICALS Applied Mathematics Series. Mathematical tables, manuals, and studies. Building Science Series. Research results, test methods, and performance criteria of building materials, components, systems, and structures. Handbooks. Recommended codes of engineering

and industrial practice (including safety codes) de- veloped in cooperation with interested industries, professional organizations, and regulatory bodies. Special Publications. Proceedings of NBS conferences, bibliographies, annual reports, wall charts, pamphlets, etc. Monographs. Major contributions to the technical

literature on various subjects related to the Bureau's

scientific and technical activities. National Standard Reference Data Series.

NSRDS provides quantitative data on the physical

and chemical properties of materials, compiled from

the world's literature and critically evaluated. Product Standards. Provide requirements for sizes, types, quality, and methods for testing various industrial products. , These standards are developed co- operatively with interested Government and industry groups and provide the basis for common understanding of product characteristics for both buyers and

sellers. Their use is voluntary.

Technical Notes. This series consists of communications and reports (covering both other-agency and

NBS-sponsored work) of limited or transitory interest. Federal Information Processing Standards

Publications. This series is the official publication

within the Federal Government for information on

standards adopted and promulgated under the Public Law 89-306, and Bureau of the Budget Circular A-86

entitled, Standardization of Data Elements and Codes

in Data Systems.

Consumer Information Series. Practical information, based on NBS research and experience, covering areas of interest to the consumer. Easily understandable language and illustrations provide useful background knowledge for shopping in today's tech- nological marketplace.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey bibliographies are issued periodically by the Bureau

Cryogenic Data Center Current Awareness Service (Publications and Reports of Interest in Cryogenics).

A literature survey issued weekly. Annual subscription: Domestic, $20.00; foreign, $25.00.

Liquefied Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00. Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription: $20.00. Send subscription orders and remittances for the preceding bibliographic services to the U.S. Department

of Commerce, National Technical Information Service, Springfield, Va. 22151. Electromagnetic Metrology Current Awareness Service (Abstracts of Selected Articles on Measurement

Techniques and Standards of Electromagnetic Quantities from D-C to Millimeter-Wave Frequencies). Issued monthly. Annual subscription: $100.00 (Special rates for multi-subscriptions). Send subscription order and

remittance to the Electromagnetic Metrology Information Center, Electromagnetics Division, National Bureau

of Standards, Boulder, Colo. 80302. Order NBS publications (except Bibliographic Subscription Services) from: Superintendent of Documents, Government Printing Office, Washington, D.C. 20402.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington. DC. 20234

OFFICIAL BUSINESS

Penalty for Private Use. $300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE COM-215

'^6-l9lfo