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.

1 Introduction

The use of mass and volume flow measurement techniques is essential in industries for process control purposes, e.g. of gases and liquids. The kind of measurement system used, at most depends on pressure, temperature, density, viscosity and homogeneity of the fluid. Hence, flow sensors must be individually adapted and rapidly calibrated.