NBSIR 74-578

Development of Microwave

Hygrometer Model III

Daniel P. Stokesberry

Microwave & Mechanical Instrumentation

Measurement Engineering Division

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

and

Saburo Hasegawa

Humidity Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington. D. C. 20234

September, 1974

Progress Report

Prepared for Naval Air Systems Command

Department of the Navy

Washington, D. C. 20360

NBSIR 74-578

DEVELOPMENT OF MICROWAVE

HYGROMETER MODEL III

Daniel P. Stokesberry

Microwave & Mechanical Instrumentation

Measurement Engineering Division

Institute for Applied Technology

National Bureau of Standards

Washington. D. C. 20234

and

Saburo Hasegawa

Humidity Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

September, 1974

Progress Report

Prepared for Naval Air Systems Command

Department of the Navy

Washington. D. C. 20360

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

NATIONAL BUREAU OF STANDARDS, Richard W. Rotierts. Director

TABLE OF CONTENTS

Abstract 1

Introduction 2

New Design Features 3

Further Work 5

Figures 6

Reference 11

Development of Microwave Hygrometer

Model III

by

Daniel P. Stokesberry

and

Saburo Hasegawa

Abstract

This is a progress report on the design and construction

of NBS microwave hygrometer, Model III. The hygrometer is intended

for field or laboratory operation with a dew point range of

-40° to 40°C.

Key words: Dew-point, hygrometer, microwave hygrometer

- 1 -
Introduction

The hygrometer contains two microwave cavities. The resonant

frequency of each cavity is determined by its dimensions and the

temperature, pressure and water vapor content of the gas in the

cavity. Sample air is drawn through the instrument in such a manner

that the air within each cavity is at the same total pressure and

temperature; however, one cavity (reference cavity) contains dry air

and the other cavity (sample cavity) contains moist air. Under these

conditions the resonant frequency of the sample cavity differs from

that of the reference cavity by an amount proportional to the water

vapor content of the sample air. - -t v.

I

: , In Model II, Ref. (1), the water vapor content is measured by

the method illustrated in Figure 1 (A paper describing this hygrometer

is now in the final stages of preparation for submission for publication).

Both cavities are excited by a microwave oscillator. The microwave

outputs of the two cavities are fed to two control circuits. A

frequency lock circuit acts to hold the frequency of the oscillator

midway between the resonant frequencies of the two cavities. At the

same time, a servo control circuit tunes the resonant frequency of the

sample cavity so that it is the same as that of the reference cavity.

The servo controls a motor which is mechanically linked to a tuning

probe in the sample cavity. The tuning probe is coaxially mounted on

a precision micrometer screw which is driven by the motor.

- 2 -
The shaft of the micrometer screw is also directly attached to

an electronic shaft position indicator. The least significant digit

of this readout indicates 1/1000 of a turn, which is equivalent to

a change in probe position of 6.35 x 10"*^ mm (2.5 x 10"^ in.).

After the installation and operation of microwave hygrometer

Model II in Townsend , Georgia, to measure humidity for warm fog research

and for calibrating other field instruments, it became apparent that

several mechanical features caused problems under field conditions.

An improved hygrometer designated Model III, was designed to eliminate

these features and also to further ruggedize the instrument without

degrading the overall accuracy.

New Design Features

The major change in the new instrument. Model III, is the

modification of the basic measurement method to eliminate components

which are subject to mechanical wear or sensitive to mechanical shock

or vibration. We have devised a measurement method which requires no

moving parts. This method consists of electronic circuitry which directly

measures the frequency difference between the two cavities. A block

diagram of the. electronic circuit is shown in Figure 2. Each microwave

cavity is fed by a separate microwave oscillator. The frequency of each

oscillator is controlled by a lock circuit which forces the oscillator

to stay at the resonant frequency of the cavity. The outputs of the two

oscillators are fed to a microwave mixer. The frequency of the mixer

output is equal to the difference in frequency between its two inputs.

This signal is fed to a commercial counter which displays the frequency

difference between the two cavities.

- 3 -
The physical appearance of the hygrometer is shown in Figures 3

through 5. Figure 3 is an exterior view. The instrument is contained

in a box measuring 0.46 m (18 in.) by 0.46 m (18 in.) by 0.71 m (28 in.)

with a mass of 47.6 kg . All the components are inside the box for

protection against the weather. The compartment at one end of the box

shown in Figure 4, contains the power supplies, the vacuum pump, the

dryer and much of the plumbing which forms the air path through the

device. This compartment is vented to the outside air so that heat

from the vacuum pump will not unduly raise the temperature. Opening the

opposite end of the box (Figure 5) provides access to the outer oven of

the hygrometer. Most of the electronic circuitry of the instrument is

mounted in this constant temperature compartment to minimize errors

caused by ambient temperature variations. This compartment is

controlled to 50°C. The microwave cavities are mounted in the inner

oven (Figure 5) which is contained within the outer oven. The

temperature in the inner oven is controlled to 60.0°C. Precise control

of the inner oven is possible because of the stable environment

provided by the outer oven. The ovens have been designed to maintain

control over a large range of ambient temperatures.

- 4 -
Further Work

The microwave hygrometer will be compared with the NBS twopressure humidity generator over a minimum dew point range of 25°

to -40°C (possibly 40° to -SS^C) to determine its performance

characteristics. Preliminary tests indicate that Model III performs

as well as or better than Model II.

At present the output of the hygrcsneter is in terms of the

frequency difference between the reference and sample cavities which

is displayed on the frequency counter readout. With appropriate

interfacing, the output can be fed to an analog recorder, ADP equipment,

a minicomputer, etc.

- 5 -
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Reference

(1) Hasegawa, S. and D. P. Stokesberry, Microwave Hygrometer,

NBS Report 10 334, September 1970.

- 11 -
NBS-! 14A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. I'l'MI.K A I ION OK Hi:i^()|< I NO.

NBSIR 74-578

2. Ciov 't Actcs.sion No.3. K(_ L ipifnf ' s AcccsMon No.

4. m ij- AND slih i n i.i:

Development of Microwave Hygrometer Model III

5. I'ubl ic at ion H .tr September 1974

6. l^crforminp Organization Code

7. A\ \ IfORlS)

Daniel P. Stokesberrv and Saburo Hasegawa

8. I'rrtorming Orp:u). Ri port No. NBSIR 74-578

9. l'i:Kr'ORMINC, ORCANIZA I ION NAM1-: ANP ADPH i;S.S NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. l-'ro|i ct/Task/Vl\)rk Unit No. 2130431

11. ( ontrat t /tirant No.

12. Sponsoring O^anization Name .imi < umplcti Address (Street, City, Siiate, ZIP)

Naval Air Systems Command

Department of the Navy

Washington, D. C. 20360

13. Type of Rt-port & P«Tiod

( overed Progress Report

14. Sponsoring Agency (^odt15. SUPPl.F.MENTARY NO I KS

16. AnSTRA( 'I (A 200-word or less (actual summary of most ^igfiilicant information, ff document includes a si^ificant

bibliography or literature survey, mention it here.)

This is a progress report on the design and construction

of NBS microwave hygrometer. Model III. The hygrometer is

intended for field or laboratory operation with a dew

point range of -40° to 40°C.

17. KKY WORDS (six to twelve entries, alphabetical order, capitalize only the firfit letter of the first key word unless a proper name: separated by.semicolons) Dew-point ; hygrometer; microwave hygrometer

18. AVAILABILITY lx ^ Unlimited

For Official Di.stribution. Do Not Release to NTIS

\_

' Order ! rom Sup. of Doc, U.S. (jovernmeni Printing Office

Wa.shington. D.C. 2040.\ SD Cat. No. (13

(

^ Order 1- rom National Technical Information Service (N FIS) Springfield, Virginia ZZl'il

19. SFCURITY CLASS

(THIS REPORT)

()N( 1. ASSIFIFD

21. NO. OF PAGES

13

20. SI ( UKITY CLASS

( I HIS PAGE)

UNCLASSIFIED

22. Price

USCOMM-DC 29042-P74

'^76-191^