Thermal signal behaviour for air flow measurements as fundamentals to Time-of-Flight

Thermal flow measurement is currently based on the principle of heat energy displacement caused by a flowing fluid (mass flow measurement). The heat is induced by a continuous heating element immersed into the fluid. This kind of sensor is only applicable for fluids with known homogeneity properties. The presented investigation is based on a discontinuous heating element using the pulsed Thermal Time-of-Flight principle (TTOF). The aim is to analyse a measurement technique which determines the flow velocity of any kind of fluid with unknown properties (volume flow measurement). The authors characterise the fundamentals of a measurement principle for determining the air velocity flow in a range from 0.1 m/s to 2 m/s. Therefore, analysing the thermal transfer behaviour of a differential volume of flowing air in a pipe is crucial. For this purpose a sensor model consisting of a filament as heating element and a movable thermocouple as detection unit in a pipe construction under flow conditions is employed. The air flow model represents a thermal transmission system. A filament serves as a heating element and describes the input signal into the system. The output signal of the system is obtained by the thermocouple. Considering a linear time-invariant (LTI) system the impulse responses are measured for different flow velocities by applying an approximated Dirac delta function to the filament, and additionally the 3dB cut-off frequencies are obtained.