Patent ID: 7775104

Claim:
A thermal flowmeter comprising a heat resistor and a temperature sensor element, wherein the heat resistor is arranged so that power supplied from an engine control unit makes the temperature of the heat resistor higher than a temperature of a fluid flow being measured by a temperature increase ΔTh (° C.), and the temperature sensor is arranged to output a value from which a fluid flow rate is measured, and provided that a maximum permissible power to be supplied to the heat resistor from the engine control unit is denoted by Phmax(W), a length Wh(m) of the heat resistor in a direction of flow of the fluid and a length Lh(m) of the heat resistor in a direction perpendicular to the direction of flow of the fluid are set so as to satisfy a formula: Ph max≧35·ΔTh·Lh·√{square root over ((Wh))}.