Electrical and Computer Engineering ETDs

Publication Date

3-15-1965

Abstract

Cryogens (liquefied gases with low boiling points) are, in general, excellent insulators. Prior to this investigation, no measurements of conductivity in cryogens had been successful. An upper limit of conductivity in the range 10-15 to 10-17 mho/cm had been established for some cryogens. Data from the organic chemicals and petrol m industries indicated that fluids with conductivities in the 10-11 to 10-15 mho/cm range could present electrostatic potential hazards, particularly in situations of large volume storage and high flow rates. The advent of large capacity dewars and associated high flow rates of cryogens in the aerospace industry led to concern about this hazard. Liquid hydrogen, because of the associated explosion hazard, was of particular interest. Measurements of the electrical conductivity of several cryogens were proposed to determine under what conditions, if any, cryogens could present an electrostatic potential hazard. Apparatus was designed and fabricated which could condense and hold a sample of the cryogen under investigation for a period of several hours. This sample volume incorporated paths for electrical conduction through the cryogen. An equation was derived for the resistivity (reciprocal of conductivity) of the material contained in the sample volume in terms of the geometry and the resistance measured through the sample material. Correction for fringing fields was made by comparison of the calculated and the measured capacitance of the system. The resistance was measured by two methods. The primary method was the measurement of the current reduced by a known potential difference. An electrometer amplifier with a maximum sensitivity of 10-15 ampere fu11 scale was employed for this measurement. The second method used was to observe the decay of an applied charge with an electrostatic voltmeter. The RC time constant of the system was on the order of 6 x 105 seconds so only small voltage increments were obtainable in the time available. Measurements were made on six cryogens. These were liquefied hydrogen, deuterium, argon, nitrogen, carbon monoxide, and chlorotrifluoromethane. The resistivity or conductivity of the cryogens appears to be a function of the applied voltage gradient. In other words, the cryogens tend to act as constant current sources. Electrometer amplifier data indicated a resistivity range of 5.75 x 1015 ohm-cm at 245 volts/inch for deuterium to about 1020 ohm-cm at the same gradient for carbon monoxide. Electrostatic voltmeter data agreed to within a factor of four for the four most conductive cryogens with poorer agreement in the cases of the two least conductive fluids. It is postulated, on the basis of measurements made by others on gases and data taken in this investigation, that cryogens are intrinsically perfect insulators. Observed conduction is believed to be due to the presence of charge carriers produced by ionizing radiation present as natural background. In the case of deuterium, the presence of tritium as an impurity in a concentration of about one part per billion produced enough radiation to make quantitative calculations possible and good support for this hypothesis was obtained. A calculation for the remaining cryogens, based on a reasonable assumption of local cosmic ray activity, indicates sufficient energy availability to account for the observed currents.

Document Type

Thesis

Language

English

Degree Name

Electrical Engineering

Level of Degree

Masters

Department Name

Electrical and Computer Engineering

First Committee Member (Chair)

Jovan Djuric

Second Committee Member

Ahmed Erteza

Third Committee Member

Richard L. Crawford

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