Calibration; Figure 2-1: Thermal Conductivity Cell Operating Principle - Teledyne 2000RS Operating Instructions Manual

Operational Theory

Figure 2-1: Thermal Conductivity Cell Operating Principle

If the thermal conductivities of the gases in the two chambers are
different, the Wheatstone bridge circuit unbalances, causing a current to
flow in its detector circuit. The amount of this current can be an
indication of the amount of impurity in the sample gas, or even an
indication of the type of gas, depending on the known properties of the
reference and sample gases.
The temperature of the measuring cell is regulated to 60 °C within 1
°C by a sophisticated control circuit. Temperature control is precise
enough to compensate for diurnal effects in the output over the operating
ranges of the analyzer. Temperature is displayed on the front panel
display.

2.2.2 Calibration

Because analysis by thermal conductivity is not an absolute
measurement, calibration gases of known composition are required to
fix the upper and lower parameters ("zero" and "span") of the range, or
ranges, of analysis. These gases must be used periodically, to check the
accuracy of the analyzer.
During calibration, the bridge circuit is balanced with zero gas
against the reference gas at one end of the measurement range, and it is
sensitized with span gas against the reference gas at the other end of the
measurement range. The resulting electrical signals are processed by the
Analyzer's electronics to produce a standard 0-1V, or an isolated 4–20
mA dc, output signal, as described in the next section.
Teledyne Analytical Instruments
2000RS
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