Theory of operation

Two ultrastable, precision glass-coated thermistors are used—

one in contact with the sample gas and the other in contact

with the reference gas (such as air in a sealed chamber). The

thermistors are mounted so that they are in close proximity

to the stainless steel (or Hastelloy®) walls of the sample

chamber. The entire transmitter is temperature-controlled,

and the thermistors are heated to an elevated temperature

in a constant-current Wheatstone bridge. The thermistors

lose heat to the walls of the sample chamber at a rate that is

proportional to the thermal conductivity of the gas surrounding

them. Thus, each thermistor will reach a different equilibrium

temperature. The temperature difference between the two

thermistors is detected in the Wheatstone bridge, and the

resulting bridge voltage is amplified and converted to a linear 4

to 20 mA output proportional to the concentration of one of the

constituents of the binary or pseudo binary gas mixture.