Sensing device measures directly the density of compressible fluid existing simultaneously in both liquid and gaseous phases. The device is comprised of a capacitor connected as one leg of a bridge circuit, a power source, and an indicator calibrated to indicate density as a direct measurement

Atkisson, E. A.

English

NASA Technical Reports Server

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Ass
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October 1965
	 Brief 65-10296 
NASA TECH BRIEF H 
NASA Tech Briefs are issued by the Technology Utilization Division to summarize specific 
technical innovations derived from the space program. Copies are available to the public from 
the Clearinghouse for Federal Scientific and Technical information, Springfield, Virginia, 22151. 
Coaxial Capacitor Used to Determine Fluid Density 
Fluid Feedline 
The problem: To measure directly the density of 
compressible fluids existing simultaneously in both 
liquid and gaseous phases. Previous methods involve 
both pressure and temperature measurements, and be-
cause the density of each phase is different, separate 
pressure and temperature measurements had to be 
made. This complicated procedure does not lend itself 
to rapid and accurate measurements. 
The solution: A system comprised of a capacitor 
connected as one leg of a bridge circuit, a power 
source, and an indicator calibrated to indicate density 
as a direct measurement. The system and its operation
are based upon the relationship, discovered by Claus-
ius and Mosotti, between the dielectric constant of a 
fluid and its density. 
How it's done: The sensing device consists of a 
sleeve assembly that is postioned in a section of line 
in the fluid system and contains the capacitor assem-
bly. The center capacitor plate is held in place by outer 
radial support arms that are in electrical contact with 
the center body and sleeve assembly and maintained at 
ground potential. Two additional capacitor plates are 
held in place by inner radial support arms that are in-
sulated from the center body. These two plates are 
(continued overteal) 
-S
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electrically connected, as one leg of the bridge circuit, 
through a glass-to-metal seal in the sleeve assembly. 
The Clausius-Mosotti relation considers the dielectric 
constant, the molecular polarizability, and the ab-
solute density of the fluid. Because the dielectric con-
stant is a: function of the density, the indicator in the 
bridge can be calibrated to read the fluid density di-
rectly. Because the polarizability constant varies from 
fluid to fluid, the indicator is calibrated for each fluid 
to be measured. 
Notes: 
1. This device performs equally well in a static or a 
flow system because fluid flow has no effect on its 
measuring ability. 
2. This invention has measured the density of liquid 
hydrogen over a temperature range of 20°K to 
33 1 K, and of liquid nitrogen over a temperature 
range of 77 1 K to 112° K, each over a pressure range 
of 1 atmosphere to 70 atmospheres with an ac-
curacy of ± 1 percent.
3. This invention should be of interest to organiza -
tions working with compressible fluids, particu-
larly in the cryogenic range. 
4. Inquiries concerning this invention may be directed 
to:
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio, 44135 
Reference: B65-10296 
Patent status: NASA encourages the commercial 
use of this invention. It has been patented by NASA 
(U.S. Patent No. 3,176,222), and royalty-free license 
rights will be granted for its commercial development. 
Inquiries about obtaining a license should be ad-
dressed to NASA, Code AGP, Washington, D.C., 
20546.
Source: Eugene A. Atkisson 
(Lewis-232) 
Brief 65-10296	 Category 02


Coaxial capacitor used to determine fluid density

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