Piezoelectric quartz crystals detect condensable gas contaminants backstreaming into a vacuum chamber when a pump is evacuating the chamber. One crystal acts as a thermometer, the other detects mass change. They are energized by electronic equipment which records frequency changes

Stephens, J. B.

English

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'resent) 
June 1967	 Brief 67-10205 
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. 
Quartz Crystals Detect Gas Contaminants During Vacuum Chamber Evacuation 
Digital 
Oscillator	 Counter	 to Mass 1H IAnalog	 Recorder Converter Crystal 
Thermometer	 Digital	 Strip 
Crystal	 Oscillator
:_•1 Analog Converter
Crystal (Temperature)	 Crystal (Mass) 
C
The problem: 
To develop a device for detecting the presence of 
a condensable gas contaminant backstreaming into a 
vacuum chamber when a vacuum pump is being used 
to evacuate the chamber. Prior art used glass slides 
placed in the line from the vacuum chamber to the 
pump, which allowed the* condensable contaminants 
to condense on the glass slides. The glass slides were 
then taken from the line, washed in a solvent, and 
the resulting fluid analyzed to determine whether a
contaminant was present (such as oil). This was a 
laborious and time consuming process. 
The solution: 
An apparatus which consists of two piezoelectric 
quartz crystals mounted on a cooled support, one of 
which acts as a thermometer, the other detects a 
change in mass. •Associated electronic equipment 
energizes the piezoelectric crystals and records 
changes in frequency from the crystals.
(continued overical) 
This document 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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How it's done: 
To condense the contaminant, liquid nitrogen or 
any liquid cool enough to allow the contaminant to 
condense, is used to fill, the container upon which 
the crystals are mounted. One of the crystals is en-
cased in a shield so that it is responsive only to 
temperature. The other crystal is open to receive 
the condensable contaminants so that a change in mass 
results from the deposit of contaminants upon the 
crystal. Increasing the mass of the crystal decreases 
the frequency with which it vibrates. 
As the air is pumped out, any contaminants back-
streaming into the vacuum chamber will be con-
densed on the mass-determining crystal, reducing its 
vibrating frequency. The other crystal monitors the 
temperature. Since the frequency with which a crystal 
vibrates is dependent upon temperature, it is necessary 
that the temperature be monitored so that a period 
during which there is a constant temperature may 
be used to make an accurate determination of the 
amount and presence of contaminants being collected 
or condensed on the mass-determining crystal. The 
change in frequency is registered by the counter and 
the strip recorder.
Notes: 
I. The device has been fabricated and used at the 
Jet Propulsion Laboratory to indicate the presence 
of condensable contaminants backstreaming into 
a vacuum chamber being evacuated. 
2. The device offers the following advantages: 
(a) it provides a remote and continuous time rate 
of change of contaminants condensing on a 
surface; 
(b) it is sensitive to 1X10 1° grams per sq. cm . 
3. Inquiries concerning this invention may be di-
rected to: 
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: B67-10205 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546. 
Source: J. B. Stephens

Jet Propulsion Laboratory

(NPO-10144) 
Ii 
Brief 67-10205	 Category 01


Quartz crystals detect gas contaminants during vacuum chamber evacuation

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