Gage sensitivity for a particular gas is estimated using the known performance of the gage with a reference gas /nitrogen/ and employing several empirical rules which have usable accuracy. Relative gage sensitivities have been estimated for over 100 gases and vapors

Summers, R. L.

English

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April 1970	 Brief 70-10099 
NASA TECH BRIEF
	
4. 
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 Clearinghouse for Federal Scientific and Technical 
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be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 	 * 
Estimating Sensitivity of Vacuum Gages 
.
Empirical methods of estimating the response (or 
sensitivity) of hot cathode ionization type vacuum 
gages for particular gases have been determined. 
Most vacuum pressure measurements of less than 
1 x 10-3 torr utilize hot cathode ionization gages. 
In operation, the ion gage creates ions by electron 
bombardment of the gas whose pressure is to be meas-
ured. The ratio of the ion current to the electron cur-
rent is then divided by a specific quantity, the gage 
sensitivity, to give the gas pressure. Proper usage of 
an ionization gage requires knowledge of the gage 
response to the gas environment. This gage response 
(or sensitivity) varies with the nature of the gas and the 
gage configuration. Normally, individual calibration 
for each gage-gas combination is required. Unfor-
tunately, the process of calibration is difficult and time 
consuming, so that only a limited number of gage 
sensitivities are known. 
As an alternate approach, gage sensitivity for a 
particular gas can be estimated empirically and with 
reasonable accuracy using. the known performance 
of the gage with a different gas, and either the per-
formance of other gages in the gas of interest or cal-
culations using some property of that gas. 
Using gage sensitivity data from the literature, a 
study of empirical methods of estimating gage sen-
sitivity was conducted. Various empirical rules have 
been previously published, and are in use, although 
their accuracies have never been defined. All of these 
rules define only the relative gage sensitivities, which 
are the ratios of gage sensitivities for various gases to
the gage sensitivity of a given reference gas. In a NASA 
study, two of the previously published rules were 
found to have usable accuracy. In addition, as part 
of the study, a new method of estimating relative gage 
sensitivity was postulated ad tested. 
The most basic and most widely used of the em-
pirical rules states that the relative gage sensitivities.are 
invariant with gage configuration. This rule was found 
to have a probable error of 8 percent. 
A second rule equates the ratio of gage sensitivities 
for any two gases to the ratio of ionization cross sec-
tions for the same two gases. In previous applications, 
the values of ionization cross section utilized were 
chosen at an ionizing electron energy corresponding 
to the gage grid potential. In this study, the maximum 
value of ionization cross section for each gas was used 
and found to be equally accurate. The probable error 
associated with this rule was found to be 10 percent. 
A new empirical rule is proposed which equates the 
ratio of gage sensitivity for any two gases to the ratio 
of the gases' molecular polarizabilities. Molecular 
polarizability is related to, and may be replaced by, 
the molar polarizability or the refractive index. The 
probable error in estimating relative gage sensitivity 
from these molecular properties is 13 percent.. 
Using the above rules, the relative gage sensitivities 
for over 100 gases and vapors have been estimated. An 
estimated gage sensitivity for a particular gage ex-
posed to any of these gases can therefore be estimated 
easily if the gage sensitivity is known for nitrogen as 
the reference gas.
(continued overleaf) 
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.
https://ntrs.nasa.gov/search.jsp?R=19700000095 2020-03-17T00:22:36+00:00Z
Notes: 
1. The following documentation may be obtained 
from:
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.65) 
Reference: 
NASA-TN-D-5285 (N69-29331), 
Empirical Observation on the Sensitivity 
of Hot Cathode Ionization Type Vacuum 
Gages
2. Technical questions may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
• Cleveland, Ohio 44135 
Reference: B70-10099 
Source: Robert L. Summers

Lewis Research Center

(LEW-11007)
. 
Brief 70-10099	 •	 •	 Category 03


Estimating sensitivity of vacuum gages

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