Operational Theory; Thermal Conductivity - Teledyne 275R Instruction Manual

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The Model 275R Turbine Generator Purge Gas Analyzer can measure
either the concentration of one component in a binary stream of gas, or the
purity of a sample stream containing a composite mixture of impurities.
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2.1 Thermal Conductivity

2.1 Thermal Conductivity Thermal Conductivity Thermal Conductivity Thermal Conductivity
The ability of a substance to conduct heat is called thermal conductiv-
ity . A good heat conductor has a large thermal conductivity (as in the case
of hydrogen and helium, which have nearly identical thermal conductivi-
ties); a small thermal conductivity (as in chlorine and carbon dioxide gas)
indicates that the substance may be a better insulator than conductor.
Operation of the 275R analyzer is based on being able to detect the differ-
ence in thermal conductivity between the gas being monitored and the
known reference (which, in this case, is helium).
Thermal conductivity measurements are non-specific by nature, which
imposes certain limitations and requirements. If the analyzer is to detect a
specific component in a sample stream, the sample must be composed of
the component of interest and one other gas in order to be accurate.
If, on the other hand, the user is primarily interested in the purity of a
process stream, and does not require specific identification of the impurity,
the analyzer can be used on more complex mixtures.
Because analysis by thermal conductivity is not an absolute measure-
ment, standardizing gases of known compositions will be required to zero
and span the analyzer.
The detector cell is divided into two halves, with the reference gas
(helium) sealed into one chamber, and the other through which the sample
flows. The difference in thermal conductivity between the reference gas
and the sample is sensed by hot wire elements, which protrude into the
chambers. Each pair of elements is wired into an electrical circuit known
as a Wheatstone bridge.
TELEDYNE ANALYTICAL INSTRUMENTS
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