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Figure 2.1: Scematic drawing of thermal conductivity measurement. The sensor is comprised in
the thermally stabilized stainless steel block.
The FTC400 sensor determines the quantitative composition of gas mixtures by measuring
the thermal conductivity between a heat source and a heat sink through the gas mixture. The
measuring gas is conducted to the sensor through a stainless steel block that is kept at a constant
temperature of 63
◦
C. This temperature a stabilized using a control loop such that the block can be
used as heat sink. A thin membrane of micro-mechanical silicon chip serves as heat source. Two
thin film resistors are integrated into the membrane, one is used for both the membrane the other
for measuring its temperature. A second control loop stabilizes the membrane temperature at
135
◦
C. Above and below the membrane two small cavities are etched into the silicon, which are
filled by diffusion with the measuring gas. The surfaces opposite to the membrane are thermally
connected with the heat sink. Dependent on the thermal conductivity of the gas more or less
energy is led off the membrane. The voltage needed to keep the temperature of the membrane
constant is a measure for the thermal conductivity of the gas. Both resistors on the membrane
are protected by an inert coating in order to prevent chemical reactions with the measuring gas.
“Basic range“ is the largest possible measuring range and is set by default. The linearization
is performed over the basic range. The smallest measuring ranges at the beginning and the
end of the basic range can be deduced from calibration. The smallest possible range between
the basic range and the smallest ranges at the end beginning and the end of the range can be
estimated by linear Interpolation. The Multi Gas Mode (MGM) is a configuration that allows for the
consecutive measurement of different gas pairs. The gas pair can be switched at via the control
panel or via the RS232-interface. Gas pairs labeled “Yes“ in the table above are commonly used.
Gas mixtures labelled “On request“ can also be implemented with little effort.
2.2
Determining Concentrations via Infrared Measurement
Selective measurement of infrared-active gases Molecules like CO
2
, CH
4
, C
2
H
6
, CO, NO, SO
2
and H
2
O absorb infrared radiation. The range of absorption in the wavelength spectrum is a
fingerprint for the identification of the sort of molecules. The level of absorption is a measure of
File name: Manual_FTC400_1.09KD181129CWI1V01.pdf
