a returnable by pass circuit of a pump or vacuum pump set 8 of which input is connected by means of a technological cut off valve 9 to a vacuum furnace 10 what is more the by pass circuit branch is switched on between the input and output of vacuum pump set 8 one containing in series device connection a first cut off valve 1 a gas filter 2 a second cut off valve 3 a mass flow signal transducer 5 a departure gas sample calibration valve 6 and a third cut off valve 7 while a reference valve output is switched on between the cut off valve 1 and gas filter 2 by a reference valve 4 supplying from outside reference gases set for system calibration the estimation of volume reference flow in the system is performed through the gas method with reference to the value of the fixed mass flow of the calibration gases e g nitrogen helium or the air through the reference valve 4 mass flow signal converter 5 calibration valve 6 and cut off valve 7 in the fig2 variant the by pass circuit contains in series connection the first cut off valve 1 gas filter 2 the second cut off valve 3 a supporting vacuum pump 11 a pressure stabilization.