An air flow measuring device comprising a flow detection valve located in a conduit and a servo-mechanism operable to maintain the difference in pressure across the detection valve at a control value, the servo-mechanism including a pressure-sensitive amplifier for sensing any deviation of the pressure difference from the control value by means of a pressure-responsive diaphragm and producing an amplified actuating pressure in response to such deviation and a valve actuator connected to the amplifier for receiving the actuating pressure and operatively connected to the flow detection valve for positioning same with the actuating pressure, wherein a resiliently deformable bellows filled with a given mass of a gas and having an effective pressure-sensitive-area equal to (the effective pressure-sensitive-area of the diaphragm).times.(the control value under a reference condition of air).div.(the gas pressure in the bellows being equal to the reference pressure of air) is provided so that it is surrounded by air and that the force produced by the diaphragm due to the pressure difference is balanced by the resultant force of a force due to the resilient deformation of the bellows and a force produced by the bellows in response to alteration in temperature and pressure of air, whereby the control value becomes proportional to the specific gravity of air being measured, ensuring the determination of the mass rate of flow of air past the flow detection valve from the area of opening of the flow detection valve.