Possible heated bodies include heater wires, structured heater resistors on films, and heater resistors on microstructured dielectric diaphragms. The dielectric diaphragms which are produced by silicon micromechanics processes are becoming increasingly important because of their rapid response time, small size, and batch processability. Large numbers of thermal conductivity sensors will be needed in the future for use in hydrogen-powered vehicles, for example. Hydrogen has a very high thermal conductivity in comparison with air and therefore is readily detectable using a thermal conductivity sensor.
For example, the change in the heater resistance under the influence of the thermal conductivity of the surrounding gas is analyzed as the measuring signal for determining the thermal conductivity of a gas, with the heating power being kept constant. Another method of measurement involves regulating the heater resistance at a constant level, i.e., at a constant temperature, and analyzing the power required to do so as the signal. Based on the measurement of the change in resistance or of the controlled variable of power, it is possible to calculate the thermal conductivity of the gas.
One problem in analyzing such sensor modules, however, is that the sensor signal of the known thermal conductivity sensors depends not only on the thermal conductivity of the gas surrounding the heated body, but also on the heat dissipation through the mount of the heated body on the thermal radiation. The heat losses, which are undesirable for the application and are attributed to dissipation of heat through the mount for the heated body and due to radiation are minimized by using materials that are well insulated thermally and low temperatures at which the thermal conductivity sensors are operated.
It is furthermore known that it is possible to provide a second heated body which is identical to the first heated body and is acted upon by a reference gas. By comparing the signals of the two heated bodies, the sensitivity of the thermal conductivity sensor may be improved using a bridge circuit, for example. Such a sensor design is often used for laboratory measurements, but it seems too complicated for a small, compact and sturdy sensor module such as that needed for use in the automotive industry, for example.
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