TABLE OF CONTENTS The overwhelming majority of micro flow sensors described so far work in the thermal domain. It is also thermal flow sensors that are produced commercially million fold. They are placed in car air intake systems used for motor efficiency control and in air conditioning systems. The commercial production of flow sensors began only about 8 years ago with the replacement of conventional flow sensors in cars [40]. In this section, mostly recent publications have been cited, but there are numerous other publications from the last 20 years that deal with thermal flow sensors. Thermal flow sensors have been classified into three basic categories (see Figure 9.5 [41]):
Anemometers;
Calorimetric flow sensors;
Time of flight sensors.
For most materials, the electrical resistivity changes with temperature. Therefore, this parameter has been chosen for the thermal flow measurements. Various materials have been used to form resistors. The higher the TCR, the better the sensitivity to temperature changes and thus to flow rate. Platinum [17, 29, 42], gold [43], polysilicon [44, 45], Ni-ZrO 2 cermet films [46], amorphous germanium [47, 48], and silicon-carbide [49] have been used. Also, thermistors made of germanium ( thermistor: an electrical resistor making use of a semiconductor whose resistance varies sharply in a known manner with the temperature) were employed [47, 50, 51].
