Thermal, flow measuring device with diagnostic function

A thermal flow measuring device for determining and/or monitoring the mass flow and/or the flow velocity of a flowable medium through a pipeline, comprising at least three sensor elements and an electronics unit. Each sensor element is in thermal contact with the medium, and includes a heatable temperature sensor. The electronics unit is embodied to heat each of the three sensor elements with a heating power, to register their temperatures, to heat at least two of the at least three sensor elements simultaneously, to ascertain the mass flow and/or the flow velocity of the medium, from a pairwise comparison of the temperatures and/or heating powers and/or at least one variable derived from at least one of the temperatures and/or heating powers, to provide information concerning a change of the thermal resistance of at least one of the at least three sensor elements, from a response to an abrupt change of the heating power supplied to at least one of the at least three sensor elements, to provide information concerning a change of the inner thermal resistance of the at least one sensor element, and in the case that a change of the inner and/or outer thermal resistance occurs in the case of at least one of the at least three sensor elements, to perform a correction of the measured value for the mass flow and/or the flow velocity and/or to generate and to output a report concerning the state of the at least one sensor element mass flow and/or the flow velocity.