Open-ended microwave cavity directly measures the density of flowing liquid, gaseous, or two-phase hydrogen. Its operation is based on derived relations between the cavity resonant frequency and the dielectric constant and density of hydrogen

Smetana, J.

Wenger, N. C.

English

NASA Technical Reports Server

April 1967
	 Brief 67-10115 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Liquid Hydrogen Densitometer Utilizes Open-Ended Microwave Cavity 
Aperture
Rectangular 
J	
Waveguide 
MODEL OF OPEN-ENDED CAVITY 
The problem: 
To develop a method for the direct measurement of 
the density of flowing liquid, gaseous, or two-phase 
hydrogen. 
The solution: 
Microwave cavities have been used for many years 
to measure the densities of fluids and gases. The shift 
in the resonant frequency when a fluid is introduced
into an evacuated microwave cavity gives a measure of 
the dielectric constant of the fluid. The density of the 
fluid can then be determined from the dielectric con-
stant data by using either a suitable theory on electric 
polarization for that particular fluid or an empirical 
or experimentally obtained formula. An open-ended 
microwave cavity has recently been developed to 
measure the density of flowing liquid, gaseous, or two-
phase hydrogen. The operation of the cavity is based 
-.	
(continued overleal) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
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on derived relations between the cavity resonant fre-
quency, the dielectric constant of hydrogen, and the 
density of hydrogen. 
How it's done: 
The cavity consists of a circular waveguide of radius 
"b" that is terminated at each end with thin coaxial 
cylindrical partitions of radius "a" that are separated 
by a distance "L". Because the partitions are ex-
tremely thin, they offer little resistance to the flow of 
hydrogen through the cavity. Microwave energy is 
supplied to the cavity through the small aperture in 
the side wall. If the cylindrical partitions are suf-
ficiently long, they will act as perfect reflectors to the 
microwave energy over a frequency range whose limits 
are dependent on the dimensions "a" and "b". The 
cavity is resonant when the spacing between the parti-
tions is adjusted so that the reflections are construc-
tive. This condition occurs when the spacing "L" is 
approximately an integral multiple of one-half wave-
length. A measurement of the cavity resonant fre-
quency then allows the relative dielectric constant of 
the hydrogen and hence its density to be computed.
Notes: 
1. Hydrogen density measurements that were made 
by using this open-ended microwave cavity were in 
excellent agreement. with density calculations that 
were based on pressure—temperature data. Ac-
curate density measurements can also be taken for 
two-phase hydrogen if the hydrogen gas bubbles 
can be reduced sufficiently in size and distributed 
uniformly in the liquid. 
2. Additional technical details are contained in: 
"Liquid Hydrogen Density Measurements Using 
an Open Ended Microwave Cavity," by Norman 
C. Wenger and Jerry Smetana, NASA TN D-3680, 
October 1966, available from the Clearinghouse for 
Scientific and Technical Information, Springfield, 
Virginia 22151, price $1.00. 
3. Inquiries concerning this innovation may be di-
rected to:
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B67-101.15 
Patent status: 
No patent action is contemplated by NASA. 
Source: Norman C. Wenger and Jerry Smetana 
(Lewis-390) 
Brief 67-10115	 Category 01


Liquid hydrogen densitometer utilizes open-ended microwave cavity

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