thermal mass flowmeter is to measurement of fluid flow in closed conduits. The flow meter with two sensors mounted on a probe, inserted through the process conduit wall, into the gas stream. An amount of heating power P is applied to one of the sensors, causing its temeprature to raise to a measured value T2, the other sensor measures the gas temperature T1. The gas mass flowrate can be determined from the temperature difference between the heated sensor and the gas(△T=T2-T1), and the amount of heating power P applied.