The disclosure relates to a thermal mass flow meter for determining a material flow through a vessel. The mass flow meter has at least two measuring elements immersed into a vessel with a flowing medium, with one of the measuring elements being heated. It is proposed that the heated measuring element is arranged in front of the unheated measuring element in the flow direction of the medium for at least part of the time, and for part of the time the unheated measuring element is arranged in front of the heated measuring element.