Teledyne 9700 Instruction Manual page 23

Flue Gas Analysis System
The temperature of the active bead will be influenced by other factors
such as initial gas temperature, gas thermal conductivity, flow rates and the
temperature of its housing. The reference bead, having similar electrical
and thermal properties and being heated by the same current, but lacking
the catalytic material will be similarly affected by these extraneous factors
but not significantly affected by oxidation of the combustible gas.
These two beads are placed in close proximity. to one another so that
they are affected by the same environmental factors. Thus, the differences
between the changes in resistance of the two coils is directly related to the
concentration of combustible gases.
TELEDYNE BROWN ENGINEERING
Analytical Instruments
Theory of Operation 2
2-5