Thermal flowmeter designed to measure the mass flow of a medium in a measuring tube, said flowmeter comprising: a measuring tube with a measuring tube wall (11.1) and a measuring tube axis (11.2); a sensor with four probes (12.2), which extend into the measuring tube from a sensor base body (12.1), wherein the probes are designed to heat the medium, determine its temperature or influence a flow of the medium through the measuring tube; an electronic measuring/operating circuit, wherein said circuit is designed to operate at least three probes and to create and provide flow measured values via their operation, wherein each probe has a probe base body and a probe active body, wherein the probe base body is arranged, in each case, on a side of the corresponding probe facing towards the sensor base body, and wherein the probe active body is arranged in each case on a side of the corresponding probe facing away from the sensor base body, wherein the probe active body is designed to heat the medium, determine the temperature of the medium and/or influence a flow of the medium in the measuring tube, wherein the probe base bodies are designed in the form of a column, wherein the probe base bodies form a rhombus on a surface of the sensor base body, wherein the rhombus is defined by area centroids of cross-sections of the probe base body, wherein a first probe (12.21) is designed to measure the temperature of the medium, wherein a second probe (12.22) and a third probe (12.23) are each designed to heat the medium and determine the temperature, wherein the first probe is arranged on a first side of the first diagonal, and the third probe is arranged on a second side of the first diagonal, wherein the second and a fourth probe (12.24) are arranged on opposite sides of the second diagonal, wherein the fourth probe is designed to prevent a partial flow of the medium between the first probe and the third probe, characterized in that the probe active body of the fourth probe (12.24) has a first width parallel to the first diagonal and a second width parallel to the second diagonal, wherein the second width is greater than the first width by a factor of at least 1.1 and particularly of at least 1.2 and preferably of at least 1.3, and wherein a geometric center of the probe active body of the fourth probe is offset in the direction of the second probe. a measuring tube with a measuring tube wall (11.1) and a measuring tube axis (11.2); a sensor with four probes (12.2), which extend into the measuring tube from a sensor base body (12.1), wherein the probes are designed to heat the medium, determine its temperature or influence a flow of the medium through the measuring tube; an electronic measuring/operating circuit, wherein said circuit is designed to operate at least three probes and to create and provide flow measured values via their operation, wherein each probe has a probe base body and a probe active body, wherein the probe base body is arranged, in each case, on a side of the corresponding probe facing towards the sensor base body, and wherein the probe active body is arranged in each case on a side of the corresponding probe facing away from the sensor base body, wherein the probe active body is designed to heat the medium, determine the temperature of the medium and/or influence a flow of the medium in the measuring tube, wherein the probe base bodies are designed in the form of a column, wherein the probe base bodies form a rhombus on a surface of the sensor base body, wherein the rhombus is defined by area centroids of cross-sections of the probe base body, wherein a first probe (12.21) is designed to measure the temperature of the medium, wherein a second probe (12.22) and a third probe (12.23) are each designed to heat the medium and determine the temperature, wherein the first probe is arranged on a first side of the first diagonal, and the third probe is arranged on a second side of the first diagonal, wherein the second and a fourth probe (12.24) are arranged on opposite sides of the second diagonal, wherein the fourth probe is designed to prevent a partial flow of the medium between the first probe and the third probe, characterized in that the probe active body of the fourth probe (12.24) has a first width parallel to the first diagonal and a second width parallel to the second diagonal, wherein the second width is greater than the first width by a factor of at least 1.1 and particularly of at least 1.2 and preferably of at least 1.3, and wherein a geometric center of the probe active body of the fourth probe is offset in the direction of the second probe.