The thermal mass flow sensor which is used to measure air flow under the absolute pressure of 35

4

1kPa

should be calibrated.The Venturi nozzles is used as standard instrument

the system of gas pressure adjusting is built up

the gas pressure can be adjusted to absolute pressure of 40±0.1kPa

so the system of low gas flow measuring standard device is founded

the flow sensor is calibrated by comparing the output value of sensor to the standard flow of Venturi nozzles.Whether the critical Venturi nozzles can be used as calibrating facility or not have been analyzed and tested.From the results of experiments

it is concluded that the stability of the facility is 0.6%

the repeatability is 0.6%

the critical pressure ratio is 0.5

the means of low gas pressure flow calibration is feasible.