Coaxial arrangement of primary coil and two sensing secondary coils contained inside slender quartz tube inserted into ionized stream permits simultaneous determination of conductivity and linear velocity. System results agree favorably with theory

Cook, G. R.

Posch, R. E.

Rossow, V. J.

Vendell, E. W.

English

NASA Technical Reports Server

July 1971	 Brief 71-10235 
S NASA TECH BRIEF 
Ames Research Center
	
_icv 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Device Measures Conductivity and Velocity of Ionized Gas Streams 
The problem: 
To accurately measure the local electrical conduc-
tivity and linear velocity of ionized streams in plasma 
wind tunnels. Previous methods required two separate 
Sens 
Ionized 
Stream 0 F----
The solution: 
A coaxial arrangement of a primary coil and two 
sensing secondary coils contained inside a slender 
quartz tube which is inserted into the ionized stream, 
f- —	 Ouartz 
-	 M	 ' 
Probe  
I	 L Signa I Separator 
instruments to determine conductivity and velocity. 
The conductivity values yielded by such systems were 
averages over the bulk of the flow field rather than
permitting simultaneous determination of conductivity 
and velocity. The results obtained with this system 
agree favorably with theory. 
a iucaiicu region. in auuiiwii, Ilic pciioiiiiariee ol	 How it's done: 
these methods does not readily relate to theoretical	 The three-coil probe system and associated circuitry 
analysis.	 (see fig.) permit measuring the extent to which the 
cI)I1tiIiLI CLI )erIcLLI 
This documett was prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights.
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magnetic field generated by the primary coil is per-
turbed by the electrically conducting, high velocity 
stream. The field is produced by exciting the primary 
coil with 0.125 W from a 100 kHz oscillator, and 
the distortion of the field is detected by the voltage 
induced in the secondary coils which are equidistant 
from the primary coil. Energy transfer across such 
a loosely coupled transformer depends on the manner 
of excitation of the primary, the positions and direc-
tion of the secondary coil windings relative to the 
primary coil, and the nature of the surrounding 
medium and its velocity relative to the primary coil. 
The sum and difference of the secondary output volt-
ages are linearly related to the conductivity and to the 
product of the conductivity and the velocity of the 
surrounding medium. 
The three-coil system is supported axially by a 
quartz rod mounted inside a quartz tube heat shield. 
The probe is 10.2 cm long and 0.4 cm in diameter. 
Capacitive shielding is accomplished with a single-
layer winding for each coil. The probe assembly 
is mounted on a hydraulically driven rack and pin-
ion to permit a traverse of the plasma stream. A 
variable resistor mounted on the pinion shaft con-
trols the horizontal sweep of the oscilloscope used 
to monitor coil outputs. In tests performed, probes 
traversed a 16.5 cm plasma jet in 0.12 sec. 
The average sum and difference signals of the sens-
ing coil outputs are separated and nulled by means 
of a transformer-coupled bridge circuit, which incor-
porates differentially wound coils in the center and in 
one side leg, with outputs controlled by the posi-
tions of ferrite and steel slugs. The coils effectively 
add "buck-out" voltages of proper amplitude and 
phase to null the system before measurements are 
made. Signals from the bridge circuit are fed through 
amplifier-filter units and displayed on a dual-trace 
oscilloscope having a sensitivity of 0.01 mV/cm. 
Calibration of the nulled system is performed in 
room air by simply reversing the leads to one of 
the sensing coils. Comparison of the probe-deter-
mined conductivity with that of a standard saturated 
salt solution, and probe-determined velocities of the 
plasma stream with those of a nonelectrical measure-
ment, showed good agreement.
References: 
1. Coil Systems for Measuring Conductivity and Ve-
locity of Plasma Streams, Rev. Sci. lnstr., 37, 1232 
(1966). 
2. A Coaxial, Three-Coil Probe for Measuring Local 
Values of Electrical Conductivity and Velocity in 
Plasma Streams, AIAA Journal, Vol. 8, No. 8, 
1399-1404 (Aug. 1970). 
Notes: 
1. Other coil arrangements can be used to measure 
conductivity and velocity in plasma streams. 
2. Computation of the performance of the coil sys-
tem must take boundary corrections into considera -
tion. 
3. The following documentation may be obtained 
from: 
National Technical Information Service 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.95) 
Reference:	 - 
NASA-TN.-13-4538 (N68-21691), Boundary 
Corrections for a Three-Coil Conductivity/ 
Velocity Plasma Probe, 1968 
4. Technical questions may be directed to: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP71-10235 
Patent status: 
This invention has been patented by NASA (U.S. 
Patent No. 3,517,302), and royalty-free license rights 
will be granted for its commercial development. In-
quiries about obtaining a license should be addressed 
to:
Patent Counsel 
Mail Code 200-1 1A 
Ames Research Center 
Moffett Field, California 94035 
Source: E. W. Vendell
Utah State University, and
R. E. Posch, G. R. Cook, and

V. J. Rossow
Ames Research Center 
(XAC-05695) 
Brief 71-10235	 Category 01,03


Device measures conductivity and velocity of ionized gas streams

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