"Ionization detectors have a poor life expectancy" Page 51
FROM PAGE 144:
2. Ionization Chamber
Principle
An ionization chamber is a method of gas analysis to detect an
aerosol which can be formed from the contaminant of interest in a number
of different ways. With air in the chamber, there is a relatively constant
flow of ion current. However, very small concentrations of the particulate
matter cause a pronounced drop in this flow of ion current. Thus, the effect
of the aerosol, or particles, on the conductivity of the ion chamber is a
measure of the concentration of the gas or vapor of interest. Continuous
ionization of the sample is provided by a mild alpha source whose radioactivity
is approximately that of the luminous dial of a watch.
Sensitivity
This method has been used to detect missile fuel and oxidizer
concentrations in the ppm range. (Notice not 0.5ppm range as required by OSHA during the Titan II years)
Comments
The formation of particles can be done in the following ways:
chemical reaction, pyrolysis, combustion of chemical reaction and pyrolysis,
without sensitization if the gas sample is electronegative in a nonelectronegative
process stream such as oxygen in argon.
A sample flow of four to ten liters per minute is required.
The detector is a leg of a Wheatstone bridge.
The M-S-A Billion-Aire instrument which operates on the ionization
principle costs approximately $1300.
Manufacturers of ionization chamber detectors are Mine Safety
Appliance Co.
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