Coating diagnostics for thermal mass flowmeters

Thermal mass flowmeters are based on the measurement of the heat transfer from the sensor into the moving fluid, which is given by King's law. A coating of the sensor element changes the heat transfer from the sensor to the fluid and leads to an error in the flow measurement. We have developed a possible diagnostics algorithm to detect a sensor coating in the μm range. For this the effect of a small time dependent variation of the heating power is analyzed. The measurement approach is confirmed by a comparison of experimental results and FEM simulations of the quasistationary heat conduction. In addition an analytical model was developed, which can be used to determine the coating thickness and correct the flow value if the thermal properties of the coating material are known. As the relevant frequencies are below 1Hz an adaptive online algorithm was developed, which is able to detect the coating without disturbing the flow measurement.