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Chapter 2
Principle of Measurement
2.1
Determining Concentrations via Thermal Conductivity
Thermal Conductivity Detectors (TCD) are used in the chemical industry since the 1920s as the first
process gas analyzers for the quantitative composition of gas mixtures. Every gas has a typical heat
conductivity governed by its molar mass and viscosity. The measurement is based on the principle that
the thermal conductivity of a gas mixture is dependent on the thermal conductivities of its gas compo-
nents and their fractional amounts in the mixture. Thus, the concentrations of different components can
be calculated from the thermal conductivity.
The main advantage of the TCD’s measurement principle compared with the wide spread infrared
analysis technique is, that it is not limited to gases with a permanent dipole moment. It can identify
noble gases (He, Ar, Ne, etc.) as well as homonuclear gases such as H
2
and N
2
. Furthermore, it is
robust and cost effective.
The principle of thermal conductivity measurement works best if the analyzed gas components’ thermal
conductivities vary greatly. For TC measurement based analysis, one of the following conditions must
be met:
• The mixture contains only two different gases (binary mixture), e.g. CO
2
in N
2
or H
2
in N
2
• The thermal conductivity of two or more components is similar, e.g. measuring H
2
or He in a
mixture of O
2
and N
2
(quasi binary mixture)
• The mixture contains more than two gases, but all but two components’ (or component groups’)
volumetric fractions are constant over time
• The mixture contains more than two gases, of which all but two components’ concentrations
can be determined through other measurement principles (as employed in the FTC 400 through
cross-sensitivity compensation of IR- and TC-sensor information)
The thermal conductivity of gases rises with temperature and the slope of the increase
with temperature is different for different gases. On customer request it can be checked
whether the temperature of heat sink and/or source can be changed in order to improve
the sensitivity of the measurement or to avoid cross-sensitivity effects.
Cross-sensitivity is the sensitivity of the measurement on other gases than the measured
component. Perturbation-sensitivity means the sensitivity of the measurement on other
influences than the gas-composition, e.g. the gas pressure.
File name:
FTC320Ex Operating Manual_1.09KD201009MPO3V02.pdf
Summary of Contents for FTC320
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