NBSIR 74-477

Performance Characteristics of a

**Bull( Effect" Humidity Sensor

James W. Little, Saburo Hasegawa, and Lewis Greenspan

Humidity Section

Meclianics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

May, 1974

Final Report

Prepared for Dispartment of the Navy

Office of Naval Research

800 North Quincy Street

Arlington, Virginia 22217

NBSIR 74477

PERFORMANCE CHARACTERISTICS OF A

"BULK EFFECT" HUMIDITY SENSOR

James W. Little, Saburo Hasegawa, and Lewis Greenspan

Humidity Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington D. C. 20234

May, 1974

Final Report

Prepared for Department of the Navy

Office of Naval Research

800 North Quincy Street Arlington, Virginia 22217

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

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

TABLE OF CONTENTS

Abstract 1

1. Introduction 2

2. Description of Sensor 2

3. Test Procedure 3

4. Results 4

5. Conclusions and Discussion 10

6. Appendix 13

7. Figures 16

8. Tables 21

9. References .31

Performance Characteristics of a

"Bulk Effect" Humidity Sensor^ [1]

by

James W. Little

Saburo Hasegawa

and

Lewis Greenspan

Abstract

A laboratory study was made of the performance of "Brady Array"

humidity sensors^ over a range of ambient temperatures from -40°C to

+20*'C encompassing relative humidities from 0 to 90 percent. Information

was obtained on such characteristics as sensitivity, hysteresis,

temperature effect, short-term and long-term repeatability.

Key words: "Brady Array" sensors, electric hygrometer, humidity,

humidity sensor, moisture measurement, relative humidity, water vapor

measurement

.

This work was supported, in part, by the Office of Naval Research

acting on behalf of the Climatic Impact Assessment Program Office, DOT.

The "Bulk Effect" humidity sensor also known as The Brady Array is,

to the best of our knowledge, the only sensor of its kind and

identification does not imply endorsement by NBS.
1. Introduction

The National Bureau of Standards was asked to select and study

the performance characteristics of several humidity sensors for possible

use in the Department of Transportation Climatic Impact Assessment

c Program (CIAP) [2,3,4] . The results of tests on two types of aluminum

oxide sensor were reported by Hasegawa et al [ 5] . The purpose of this

paper is to report on the results of the "Brady Array" humidity sensor [ 1] .

2. Description of Sensor

The manufacturer categorizes this sensor as a solid-state semiconductor

device comprising "a precise array of crystals and interstitial spaces."

It is mounted within a slotted 10-5 transistor enclosure. An excitation

oscillator, driven by a dc input of 15 volts, provides a 5-volt rms 1000-Hz

signal to the sensor and a demodulator provides a dc output signal varying in

magnitude from 0 to 5 volts. This dc signal was measured with a digital

voltmeter calibrated to within ± 0.3 mV or less for all measurements reported

here. Each of two excitation oscillators was connected through appropriate

cables to five sensors and demodulators.

Figures in brackets indicate the literature references at the end of this

report.
3. Test Procedure

The test procedure and test equipment have been described in detail

in the earlier paper [5]. Briefly, ten sensors'^ were subjected to

atmospheric air of known relative humidity. First, a series of runs

was made on five of these sensors (Nos. 6 through 10) at ambient

temperatures of -A0° and -60°C in the NBS low frost-point generator [6]

.

It was soon apparent that the sensors were insensitive to changes in

humidity at these temperatures. Therefore, further testing at these low

temperatures in this apparatus was discontinued. During this series of

tests, these five sensors were inadverently exposed to conditions of

saturation (with respect to ice) at temperatures below -60°C.

Other than on this occasion, none of sensors was subjected to relative

humidities in excess of 90.3 percent with respect to either water or ice.

A second series of runs was made on all ten sensors using the NBS

two-pressure humidity generator [7] for producing a range of humidities at

ambient temperatures from +20 to -40°C. A list of the test spans at the

various ambient temperatures is given in Table 1 in both relative humidity

and dew/frost point.

d

Ten 'Brady Array"sensors , Model BR-101, were obtained with the circuitry to

operate these sensors. The manufacturer was informed that these sensors were to be tested and evaluated at temperatures down to -70°C. These

sensors were purposely obtained without built-in temperature compensation.

- 3 -
4. Results

A family of calibration curves is shown in Figure 1 for sensor

No. 2, which had a sensitivity close to the average of all sensors. The

output in volts has been plotted against relative humidity with respect

to water at ambient temperatures of 20, 0, -20 and -40''C. There is a

displacement of each isotherm downward ( a diminution in output voltage

at a given relative humidity) as well as a loss in sensitivity (a decrease

in output voltage change per unit change in relative humidity) with

decreasing ambient temperature. A corresponding family of calibration

curves for sensor No. 2 with the output in volts plotted against dew/frost

point is shown in Figure 2. At -40° C ambient temperature, the sensor is

quite insensitive both in terms of relative humidity and in terms of dew/

frost point.

The manufacturer sells these sensors as relative humidity measuring

instruments and provides calibration curves in terms of relative humidity

vs voltage output or impedance. Therefore, relative humidity with respect

to water will be considered the measurand in all cases and will be used as

the dependent variable in all of the subsequent mathematical analyses.

Where dew/frost points are given, as in Figure 2, they will be conversions

for the convenience of the reader.

- .4 -
Table 2 is a compilation of the sensitivities of each sensor at the

four ambient test temperatures at various relative humidities in units of

mV/percent r.h. The sensitivities were obtained by differentiating the

calibration equation and solving the differentiated equation at voltages

corresponding to the indicated relative humidities. The method of

obtaining these equations as well as others used in the analysis of the

sensor performance is given in the Appendix.

Due to the fact that these sensitivities are based on limited numbers

of calibration points subject to various influences and errors, no great

significance can be placed on individual values. The sensors in general

are most sensitive in the upper (higher) half of the relative humidity

range and at the higher ambient temperatures. At -40°C, the sensitivity

is so small that there is likely no significance to the sensitivity data.

Temperature coefficients are given in Tables 3 and 4 for each of the

ten sensors. These express the average change in relative humidity

per degree change in ambient temperature for selected spans of output

voltage, that is, they are a measure of the shift in indicated r.h. that

one would expect due to a change in ambient temperature. The values

listed in Table 3 were calculated from the 20 and O^C isotherms; the values

listed in Table 4 similarly were calculated from the CC and -20°C isotherms.

The method of calculating the coefficients is explained in the appendix.

- 5 -
In each table, column 2 tabulates the selected span of output voltage,

columns 3 and 4 show the relative humidity limits corresponding to

the end points of the selected span of output voltage for each of the

two Isotherms and column 5 gives the mean temperature coefficient for the

selected span of output voltage.

Hysteresis loops, at ambient temperatures of 20, 0 and -20°C,

for sensor No. 2 are plotted in Figure 3. Each loop was obtained without

prior humidity cycling at the specified temperature and so will be

referred to as the first cycle hysteresis loop. Starting from an initial

"rest" position, the relative humidity was changed monotonically to some

final value and then returned monotonically to the initial point, with

pauses along the loop for reading the output voltage of the sensor. At -AO°C,

completed loops were not obtained in one day and no curve is shown.

Hysteresis data for all sensors are given in Table 5. For any output

voltage, the difference in relative humidity between the increasing

and decreasing r. h. branches of the loop is defined as the hysteresis.

The order of the limits in the span in Table 5 Indicates the direction

of the first leg of the hysteresis loop. The mean value is the mean of

the differences in relative humidity indication at identical output voltages

between the increasing humidity leg and the decreasing humidity leg over

the humidity span. The |Max| value is the maximum absolute difference in

relative humidity over the range of output voltages used. The method of

computation is described in the Appendix.

- 6 -
At -40° C, there Is no true hysteresis loop data, since only single

leg calibrations were made at any one time. Similar calculations were

therefore performed on an open loop which consisted of two first cycle

legs starting at opposite limits of the span and proceeding in opposite

directions -to the other limit.

With the exception of the high humidity span at 0°C, the mean

hysteresis for the ten sensors is approximately 10 percent of the

upper humidity limit for limits in excess of 15 percent r.h. In the case

of the high humidity span at O^C, it will be noted that the direction

of r.h. change of the hysteresis loop is opposite from that of the other

loops. In addition, the relative humidity surrounding the sensor was

brought from a low value to the indicated high value before measurements

were commenced resulting in mean and maximum values of hysteresis

appreciably less than comparable values at other ambient temperatures.

What is calculated as hysteresis at very low values of relative humidity

is in a direction opposite to that which can be assigned to hysteresis

and suggests that other factors besides immediate past history influence

the output of the sensor .

- 7 -
In order to obtain a better estimate of the effect of the direction

of humidity change on the sensor, separate calibration curves were derived

for Increasing and decreasing changes in humidity, using all available

data obtained in a given direction, for the sensors at 20, 0 and -20*0.

Because these curves include data from all test spans they are probably

the best overall calibration of the sensors from the available data. Other

factors Influencing sensor output are more balanced in these curves than

in the hysteresis curves. Analysis of the directional calibration curves

was performed in a manner similar to the analysis of hysteresis and the

results are tabulated in Table 6. The spans used were selected from

the calibration data available meeting the criterion of having previous

points at lower humidities for the increasing humidity curve and previous

points at higher humidities for the decreasing humidity curve. The results

show the Importance of a knowledge of prior history whenever a measurement

is performed with these sensors.

Short term repeatability is shown in figure 4 for sensor No. 2 and

for all sensors in Table 7. The values were obtained by comparing two

similar calibration curves taken one day apart except at -40°C where, due

to the increased time required to perform a calibration, the calibration

curves were taken three days apart. The shift in output over a period

approximating a day was such as to give higher voltage outputs for the same

relative humidities, which has the effect of indicating lower humidities

at the same voltage output.

- 8 -
Although the reliability of any individual value is probably not

very great due to the scatter of individual calibration points, it is

significant that all shifts were in the same direction, except for two

sensors at -AO^C, and that the composite mean of all sensors is

approximately the same at all ambient temperatures.

Long term repeatability is shown in figure 5 for sensor No. 2 and

for all sensors except No. 1 in Table 8. Due to the failure of a

connection in the measuring circuitry, the final data on sensor No. 1

was not obtained. The procedure is the same as that used for short

term repeatability except that in this case the calibrations were

performed five months apart with the sensors unused and stored in their

original containers at room conditions during this period. Room

conditions during storage were approximately 24°C and 40 percent

relative humidity. These long term repeat calibrations were performed

only at 20 and -UO°C. At -AO°C ambient temperature, the long term

shift was so great that there was a duplicated output voltage range

with only three sensors as is shown in Table 8.

With the exception of Sensor No. 4, the long term shift is in the

opposite direction of the short term shift and is approximately twice as

great in magnitude.

- 9 -
An estimate of calibration scatter is given in Table 9. This was

obtained by fitting calibration points taken with increasing relative

humidities to a cubic equation by the method of least squares and calculating

the standard deviation of the fit. This selection of calibration points

gave the smallest estimates of standard deviation and is an indication

of the scatter in a calibration. The larger value obtained for the

standard deviation at -40°C is no doubt due to the very low sensitivity

at that temperature and confirms the fact that there is very little valid

quantitative information available on what effect various factors have

on sensor behavior at -AO°C. nr: a

At the other ambient temperatures, the scatter is much smaller and

fairly constant. The value of this scatter can be used as a measure of

the significance of the effect of the other factors which have been

estimated.

5. Conclusions and Discussion

There was great variability in the set of ten sensors used in this

study. The sensitivity of individual sensors varied by a factor of at

least three. It is therefore apparent that individual calibrations for

each sensor are required. The calibration curves tend to be S-shaped

and can be approximated by cubic equations. This shape is typical of

relative humidity sensors and results in considerably reduced sensitivity

at low relative humidities. The sensor would therefore appear to be

more appropriate for use at moderate or high relative humidities.

- 10 -
There is a considerable ambient temperature effect oi\ the performance

of these sensors. Not only is there a shift in the calibration curve with

temperature, but also a diminution of sensitivity with decreasing ambient

temperature. This diminution is very great at -40°C. In addition, the

ambient temperature coefficient of the various sensors varies by a factor

of approximately three. It is therefore necessary that each sensor be

calibrated over the ambient temperature range of intended use. The

temperature coefficient varies with ambient temperature sufficiently to

require calibration at fairly close ambient temperatures.

The hysteresis of these sensors is appreciable. Although there are

some values of hysteresis given in Table 5 which are very modest, it

will be noted that these values are a large part of the value of the

humidity range to which they apply. The direction effect values given

in Table 6 are probably a more significant indication of the hysteresis

effect and average about 9 percent of the maximum humidity in the

applicable range. No direction effect is given for -40°C because there

were insufficient calibration points to provide the same type of analysis

as provided for the other ambient temperatures. The open loop hysteresis

values for -40° C given in Table 5 can be taken as an approximation to the

missing values in Table 6 and are consistent with the other data.

- 11 -
There is a short term shift averaging approximately six percent r.h.

at all ambient temperatures. It is possible that this is primarily an

exercising effect. Since the long term shift as shown in Table 9 is in

a direction opposite to the short term shift, this effect might also

be reduced by an exercise routine. It is also possible that this effect

could be modified by controlled storage methods. It should be noted that

there was no appreciable change in sensitivity at 20°C after storage for

five months. The primary change that did occur was that the absolute

output voltage decreased over the entire humidity range.

In general, polynomial equations can be obtained that represent the

calibration data with a standard deviation of approximately 3 percent r.h.

except at -40°C where, probably due to the reduced sensitivity, the

standard deviation is 18 percent r.h.

It should be pointed out that in all the calibrations performed, only

the sensor was subjected to the humidities and temperature indicated.

The associated electronics was always maintained at room temperature and

humidity (approximately 24°C and AO percent r.h.).

This sensor is convenient and easy to use. It does not appear to be

as useful at low temperatures or low relative humidities as at higher

temperatures and higher relative humidities. Although there is no

unequivocal correlation, the sensors with high sensitivity appear to behave

more favorably under ambient temperature changes, aging and direction of r.h.

change. It might therefore be possible to choose the more favorable sensors

by a simple selection routine involving calibration at only two relative

humidities.

- 12 -
Appendix

The calibration data for each sensor were fitted by means of least

squares regression to a polynomial of the following form:

where r.h. is the relative humidity in percent, V is output voltage in

volts, and n has a value between 2 and A. As a general rule, n was 4 at

the higher ambient temperatures and higher relative humidity spans.

The values given in Table 2 were obtained by solving equation (1)

for V at fixed values of r.h. These values of V were then substituted

into the following equation to obtain the sensitivity, S; in mV/percent

r.h.:

n

r.h. (1)

i=0

S = 1000.

(2)

- 13 -
Mean differences, A(r.h.)> as given in Table 5 through 8, were

obtained by solving the following equation:

A(r.h.) = i=0 -

b.

1 i+1 „i+V

i + 1 12

V - V

2 1

v: - V

(3)

where a^ and b^ are the coefficients of the two equations being compared

as determined by equation 1 and and V2 are the limits of output voltage

Involved in the comparison.

In Tables 3 and 4 column 3a is given by:

n

r.h. = / .b. i-0

Column 3b is given by

(4)

(5)

Column 4a is given by

r.h. = (6)

Column 4b is given by

n .

(7)

and Column 5 is given by

C = A(r.h.)/20 (8)

- 14 -
where C is the mean temperature coefficient and Ar.h. is obtained from

solution of equation 3.

Values of |Max| as given in Tables 5 through 8 were obtained by

solution of the following equation:

iMax I = |\^(a. - b.) vi I

' ' ' / . 1 1 Max

(9)

V^^is the value of V between V^^ and which gives the largest absolute

value for |Max |

.

Table 9 is a table of residual standard deviations, a, as given by the

following equation:

CT =

-il/2

(m - n ) (10)

where V are measured output voltages obtained at corresponding (r.h.).

humidities and m is the total number of calibration points. The values

of a^ are the values obtained by the least squares solution resulting in

equation 1.

- 15 -
0 20 40 60 80 100

RELATIVE HUMIDITY WITH RESPECT TO WATER, %

Figure 1. Output Voltage vs Relative Humidity for Sensor #2.

- 16 -
DEW/FROST POINT, °C

Figure 2. Output Voltage vs Dew/Frost Point for Sensor #2.

- 17 -
0 20 40 60 80

RELATIVE HUMIDITY WITH RESPECT TO WATER, %

Figure 3. First Cycle Hysteresis for Sensor #2.

- 18 -
RELATIVE HUMIDITY WITH RESPECT TO WATER, %

Figure 4. Short Term Repeatability for Sensor #2.

- 19 -
0 20 40 60 80

RELATIVE HUMIDITY WITH RESPECT TO WATER, %

100

Figure 5. Long Term Repeatability for Sensor #2.
Table 1

Testing Spans

Ambient Temperature ,°C

20 0 -20 -40

Testing spans, percent relative humidity

0.1 to 0.5

0.8 to 4.3

5.2 to 26.0

19.4 to 89.5

0.4 to 2.1

3.3 to 16.4

19.5 to 90.3

2.0 to 9.8

15.4 to 72.9

1.9 to 9.1

13.3 to 58.1

Testing spans in dew/frost point''', °C

-53 to -40

-36 to -20

-18 to 0

- 4 to 18

-53 to -40

-36 to -20

-19 to -1

-53 to -40

-36 to -21

-68 to -56

-53 to -41

1

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

- 21 -
Table 2

Sensitivity

Relative Humidity, Percent

Ambient 10 30 50 70 90 t Temperature

Sensor C Sensitivity, mV /percent r.h.

i

1

1 20 16 23 52 55 24

0 15 21 39 45 23

-20 ^ 7 11 42 12

-40 1.9 1.9 1.9

2 20 12 16 28 117 23

0 11 16 27 59 24

-20 6 9 26 16

-40 .5 .5 .5

3 20 17 25 53 63 26

0 16 22 41 51 25

-20 6 9 27 15

-40 .7 . 7 .7

4 20 21 30 55 76 37

0 19 24 34 55 50

-20 7 11 27 18

-40 1.7 1.7 1.7

5 20 10 13 20 37 27

0 5 7 11 22 13

-20 2 3 6 8

-40 -4.8 -4.8 -4.8

6 20 11 11 13 18 54

0 4 5 8 15 8

-20 2 2 4 5

-40 0.2 0.2 0.2

- 22 -
Table 2 (cont'd)

Sensitivity

1 Relative Humidity, Percent

Ambient 1 n 30 50 70 on Temperature

Sensor C Sensitivity, mV/percent r.h.

7 20 A J 25 30 41

0

1 1 XI 14 20 37 An

5 8 12 15

-0.6 -0. 6 A C -0.6

8 20 — i / 16 "I c 15 18 I

J

0 D 8 11 18 1 7 — ZD 2 3 6 7 0.2 0.2 A O 0.2

9 20 — 1 9 14 14 17 9A

0 Li. 26 32 38

-20 2 2 4 7 0.1 0.1 0.1

10 20 39 51 56 AS

0

9 9 26 32 38

-20 8 13 25 14

-40 1.0 1.0 1.0

Mean 20 11 21 33 50 36

0 13 17 26 38 28

-20 4 7 18 12

-40 0.2 0.2 0.2

- 23 -
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Table 5

First Cycle Hysteresis

Ambient Temperature, °C

20 0

1 -20 -40-'"

Relativ<2 Humidity Span, Percent

0.1 0.8 5.2 19.4 0.4 3.3

9 90.3 2.0 15.8 13.3

to to to to to to to to to to

0.5 4.3 26.0 89.5 2.1 16.4 1 Q S 9.8 72.9 57.8

Sensor Hyst<tresis, Percent r .h.

1 Mean -0.16 0.01 2.09 5.89 -0.16 u . y J

1 Ar\ i . 4U U . 21 O . ZD O . h3

|Max| 0.20 0.21 2.82 10.58 0.40 1. 39 2.52 0.38 8.17 7. 52

2 Mean -0.10 0.05 2.26 10.56 -0.12 U . yj 1 Q "7 1 . o / A /. Q U . 4o *7 iMaxI 0.22 0.27 3.04 15.86 0.31 1.40 3.49 0.50 10.05 7.65

3 Mean -0.15 0.13 2.94 6.67 -0.04 T /A 1. 40 T O O i . oo A A A

u . yo O . DO /. AA4 . UU

)Max| 0.16 0.26 4.00 11.88 0.16 2.02 3.22 1.30 11.71 5.57

4 Mean -0.11 0.06 2.68 7.77 -0.11 1. 42 1 . oO A AO 0 . y J O . ZD A 0

9 . zl

|Max| 0.19 0.19 3.66 10.92 0.32 2.01 3. 19 0.98 7.88 12.40

5 Mean -0.03 0.02 2.17 11.13 -0.05 1.22 2 . 37 0. 52 8.03 -7 . 94

|Max| 0.07 0.21 3.09 17.74 0.19 1.74 4.41 0.53 15.22 22.16

6 Mean -0.04 -0.02 2.05 12.02 -0.12 1.14 2.82 -0.21 9.41 4.11

(Max| 0.09 0.22 2.91 19.28 0.31 1.62 4.68 0.38 15.93 5.65

7 Mean -0.07 0.14 4.01 2.57 -0.61 2.07 1.37 0.05 8.18 7.81

|Max| 0.17 0.51 5.66 16.43 0,62 3.11 3.17 0.13 19.83 12.02

8 Mean -0.03 -0.02 2.31 13.58 -0.09 1.25 2.69 0.05 11.86 11.15

(Max| 0.07 0.25 3.26 23.49 0.25 1.77 4.69 0.35 21.52 17.23

9 Mean -0.03 -0.03 1.51 12.44 -0.11 0.69 1.81 -0.17 7.28 1.62

[Maxl 0.09 0.22 2.18 22.21 0.29 1.03 3.02 0.53 16.14 5.00

10 Mean -0.16 0.14 3.42 4.69 -0.22 1.82 1.66 0.23 6.88 8.94

)Max| 0.22 0.28 4.68 7.84 0.48 2.62 2.48 0.38 13.36 13.80

Mean Mean -0.09 0.05 2.55 8.74 -0.16 1.29 1.97 0.31 8.03 5.02

(Maxj 0.15 0.26 3.53 15.62 0.33 1.87 3.49 0.55 13.98 10.90

Degree of fit 2 3 3 4 2 3 4 2 4 3

This is an open loop.

This is not a first cycle.

- 26 -
i

Table 6

Direction Effect

Ambient Temperature, °C

1

20 0 -20

Relative Humidity Span, Percent

0-75 7-75 15-59

Sensor Diff erence , Percent r. n.

r 1 Mean "7 TO 7 . 18 4. 00 4.34

|Max| 11.47 8.25 10. by

2 Mean n "7 A

9. 79 6. 27 5.56

|Max| 15.10 10.13 O .

3 Mean 6. 85 4.28 5.36

|Max 1 11.05 8.05 13.89

4 Mean 6. 08 / 1 o 4.12 4.77

|Max 1 8.53 6.85 7 . 74

5 Mean 8.55 7.60 5.54

|Max| 12.07 10.02 8. 61

6 Mean 8.44 8.34 6.24

|Max 1 11.03 11.03 8. 68

7 Mean 6.28 8.26 3.85

[Max 1 8.94 10.94 10.82

8 Mean 8.61 10.13 7.89

|Max 1 14.10 14.08 9.91

9 Mean 7.99 8.15 4.26

|Max 1 11.27 11.42 6.79

10 Mean 4.55 3.55 4.79

|Max 1 6.76 4.66 8.70

Mean Mean 7.43 6.47 5.26

|Max| 11.03 9.54 9.46

- 27 -
Table 7

Short term Repeatability

Ambient Temperature, **C

20 0 -20 -40

Relative Humidity Span, Percent

19 to 89 19 to 90 15 to 59 13 to 58

Sensor Difference on repetition. Percent r .h.

1 Mean -2 . 76 -2 . 43 -2. 96 -11.70

|Max 1 5.34 4.36 8.52 23.87

2 Mean -7.07 -5.03 -4. 34 - 9.22

- |Max| 10.13 8.24 8.02 12.56

3 Mean -4.60 -2.01 -5. 15 -11.35

1 1 9.01 2.61 13.43 26.22

i 4 Mean -5.43 -2.47 -2.71 -14.18

\

1 Max 1 8.22 4.40 8.11 32.60

5 Mean -7.32 -7.82 -6.64 - 3.19

1 Max 1 12.10 10.86 10.87 13.01

6 Mean ' -7.73 -8.25 -7.22 - 2.48

1 Max 1 13.21 12.73 12.17

7 Mean -0.48 -6.65 -9.46 1.00

1 Max| 9.95 16.31 20.17 4.74

8 Mean -9.12 -10.88 -9.85 6.14

1 MaxI 16.95 20.17 17.77 19.59

9 Mean -8.82 - 9.14 -8.21 -11.44

i MaxI 16.39 15.12 12.41 32.15

10 Mean -3.10 - 2.65 -4.56 - 7.36 1

1 Max| 4.89 4.81 10.53 15.96

Mean Mean -5.64 - 5.73 -6.11 - 6.38

1 MaxI 10.62 9.96 12.20 18.54

I

- 28 -
Table 8

Long Term Repeatability

Ambient Temperature, °C

I

20 -40

Relative Humidity Span, Percent

5 to 75 27 to 60

Sensor Difference on Repetition, Percent r.h.

2 Mean 14.13

|Max 1 25.39

3 Mean 11.85

|Max 1 16.80

4 Mean -0.94 -24.23

jMax

1 4.07 24.91

5 Mean 0.49 19.31

|Max 1 6.17 20.30

6 Mean 18.89

jMax 1 23.61

7 Mean 22.69

|Max j 31.56

8 Mean 20.99

jMax 1 25.21

9 Mean 22.36

|Max| 28.50

10 Mean 7.31 10.44

[Max 1 9.54 18.86

Mean Mean 13.09

|Max 1 18.98

- 29 -
Table 9

Calibration Scatter

Based on Least Squares Cubic Fits

Ambient Temperature, °C

20 0 — ..... - -20 -40

No. of Points forming Calibration Curve

24 15 10 11 1 Relative Humidity Span, Percent '

0 to 90 7 to 90 10 to 73 5 to 58 1

Sensor Estimate of the standard deviation. Percent r.h.

1 3.6 2.8 1.6 20.2

2 4.3 3.3 1.5 17.0

3 3.1 2.4 2.4 22.7

4 2.2 2.0 2.3 21.1

5 3.6 3.7 2.8 22.4 ?

6 4.8 4.0 3.0 18.3

1

7 2.9 5.0 3.6 13.5

8 4.5 5.7 4.3 13.1

9 5.6 4.8 3.5 16.6

10 1.7 2.1 2.1 17.3

Mean 3.6 3.6 2.7 18.2

- 30 -
References

[1]. Bennewitz, Paul F. , "The Brady Array, A New Bulk Effect Humidity

Sensor," Measurements & Data, January-February, 1972, 104-107.

[2], Cannon, Jr., Robert H. , 1971: Planning a program for assessing the

possibility that SST aircraft might modify climate. Science , 52 ,

836-842.

[3]. 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.

(Available from the National Technical Information Service,

Springfield, Virginia 22151).

[4]. 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.

[5]. Hasegawa, S., Greenspan, L. , Little, James W. , and Wexler, A.,

"A Laboratory Study of Some Performance Characteristics of An

Aluminum Oxide Humidity Sensor." National Bureau of Standards

Technical Note No. 824.

[6]. Greenspan, L. , "Low Frost-Point Humidity Generator," J. of

Research of the National Bureau of Standards-A. Physics and

Chemistry, Vol. 77A, No. 5, September-October 1973, 671-677.

[7]. Wexler, A., and Daniels, Jr., R. D. , 1952: Pressure-humidity

apparatus. NBS J. Res . , 48 , 269-274.

- 31 -
I

I
NBS-1 14A (REV. 7-731

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR RI-^PORT NO. NBSIR 74-477

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

A Laboratory Study of Some Performance Characteristics

of "Bulk Effect" Humidity Sensors.

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

James W. Little, Saburo Hasegawa, and Lewis Greenspan

8. Performing Organ. Report No. NBSIR 74-477

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.

IZ. Sponsoring Organization Name and Gomplete Address (Street, City, State, Z,IP) 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 (actual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.) A laboratory study was made of the performance of "Brady Array"

humidity sensors^ over a range of ambient temperatures from -40° C to

+20°C encompassing relative humidities from 0 to 90 percent.

Information was obtained on such characteristics as sensitivity,

hysteresis, temperature effect, short-term and long-term repeatability.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) "Brady Array" sensors , electric hygrometer, humidity, humidity sensor, moisture measurement, relative humidity, water vapor measurement.

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