Thermal flowmeter and method for operating a thermal flowmeter

A thermal flowmeter for measuring a mass flow of a flowable medium in a measuring tube, the flowmeter comprising:

• the measuring tube defined by a measuring tube wall and having a measuring tube axis;
• a sensor including four probes, which project from a main sensor body into the measuring tube, wherein the probes are configured to heat the medium, determine a temperature of the medium, or influence a flow of the medium in the measuring tube;
• and
• an electronic measuring/operating circuit configured to operate at least three of the four probes and to generate and provide flow measurement values by operation of the at least three probes,
• wherein each probe includes a main probe body and an active probe body, wherein the main probe body comprises a proximal side adjacent the main sensor body, and wherein the active probe body comprises a distal side facing away from the main sensor body,
• wherein each active probe body is operable to heat the medium, determine the temperature of the medium, and/or influence the flow of the medium in the measuring tube,
• wherein each main probe body is columnar,
• wherein the four main probe bodies define a rhombus on a surface of the main sensor body, wherein the rhombus is defined by surface centroids of cross-sections of the four main probe bodies,
• wherein a first diagonal of the rhombus is parallel to the measuring tube axis, and wherein a second diagonal lies in a measuring tube cross-section,
• wherein a first probe and a second probe of the four probes are configured to heat the medium,
• wherein a third probe of the four probes is configured to determine the temperature of the medium, and
• wherein the first probe and the second probe configured to heat the medium define a group of probes and are arranged on the first diagonal parallel to the measuring tube axis, and wherein the third probe is arranged on the second diagonal such that, for both a first flow direction along the measuring tube axis and a second flow direction opposite to the first flow direction, a first member of the group is directly exposed to the flowing medium, and a second member of the group is arranged in a shadow area of the flowing medium, and
• wherein the electronic measuring/operating circuit is configured to:
• determine a first power coefficient using the first probe and the third probe;
• determine a second power coefficient using the second probe and the third probe;
• identify the first member of the group, based on the first power coefficient and the second power coefficient, and thereby determine whether the flowing medium is flowing in the first flow direction or the second flow direction;
• use the identified first member and the third probe to define a pair of probes adapted to measure the mass flow;
• and
• when a change from the first flow direction to the second flow direction, or vice versa, is detected, exchange the identifications of first member and the second member with each other.