Schrag, Daniel; Hencken, Kai; Pape, Detlef; Ott, Simon; Barlocher, Adrian; Kramer, Axel (ABB Schweiz AG, Corporate Research, 5405 Baden-Dattwil, Switzerland)
The thermal mass flowmeter principle is based on the forced cooling of a heated sensor element in a fluid flow. The ratio of temperature to power (thermal impedance) into the fluid is a measure of the mass flow rate. A coating on the sensor causes a change of the heat transfer coefficient and therefore an error in the flow measurement. Based on measurements and FEM simulations, the thermal properties of the sensor are investigated. The effect of a harmonic heating power signal is investigated with respect to its ability to acquire information of a possible coating. It can be shown that one of the dominant effects of coating is a change of the sensors heat capacity. This increase is visible through a change of the thermal impedance in the AC-domain. Based on the results of the measurements and simulations, a simplified physical model is developed, which is able to separate from the thermal impedance at one frequency the effect of coating and flow. Using these models we have developed an algorithm that can detect the coating of the sensing element with a known material down to sub µm of deposition thicknesses, corresponding to flow measurement error in the order of a few percent. In addition a control algorithm was developed, which allows an undisturbed concurrent measurement of the fluid velocity while diagnosing the coating on the sensor surface.