Principles Of Operation; Table 1-2 Relative Thermal Conductivity Of Common Gases - Honeywell 7866 Operation And Maintenance Manual

1.4

Principles of Operation

Thermal Conductivity Principles
Although basic thermal conductivity principles need not be included here, it may be appropriate to consider
the following expressions.
Thermal Conductivity of Gas
The thermal-conductivity of a gas mixture is approximately the sum of the products of the mole fractions
of each gas mixture. Therefore, using the letter K to represent thermal conductivity, a gas mixture of 15 %
CO in air can be defined by the following expression:
2
Kmix = 0.85 Kair + 0.15K CO
Thermal Conductivity of Air
Expressing thermal conductivities in air can be defined by the following expression:
Kair is 1.00 and KCO
Kmix = 0.85 x 1 + 0.15 x 0.704 = 0.91.
Temperature Difference
Using a thermistor as the detector in a sample gas and another thermistor in the reference gas, the
difference in temperature of these two detectors can be estimated when the thermal conductivities of the
gas mixture of the sample gas and reference gas are known. This temperature difference can be expressed
as follows:
t = Kref – Kmix x (t1–t2)
Where t1 is the reference thermistor temperature and t2 is the cell-block temperature. The above expression
applies only when the measuring and reference thermistors are heated to at least 120 °C, which is the
minimum temperature for which these thermistors will linearly sense thermal conductivity changes. It
applies when current input is constant and the heat losses are by thermal conductivity only.
Common Gases
Table 1-2 lists the common gases which can be measured by this method or can be present as components
in the background gas of the mixture being measured. All thermal conductivities are referred to air at 120
°C.

Table 1-2 Relative Thermal Conductivity of Common Gases

Component
Air
O
2
NH
3
CH
4
He
H
2
9/06 7866 Triple Range Digital H
2
is 0.704. Therefore
2
Kmix
Thermal Conductivity (K)
1.000
1.028
1.040
1.450
5.530
6.803
and CO
2 2
Component
Cl
2
SO
2
H S
2
Ar
CO
2
H O
2
CO
N
2
Analyzer/Indicator - Operation and Maintenance
Introduction
Thermal Conductivity (K)
0.342
0.350
0.540
0.665
0.704
0.771
0.958
0.989
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