Development of a constant pressure flowmeter for He standard leak calibration
Arai, Kenta; Yoshida, Hajime; Akimichi, Hitoshi; Hirata, Masahiro
Japan

For a quantitative evaluation of a leak rate, sensitivity of a He leak detector should be calibrated against a calibrated He standard leak. A reference micro He gas flow (10-7~10-5 Pa m3/s) is necessary for the calibration of the He standard leak by a comparison method. A constant pressure flowmeter for the micro He gas flow was designed and fabricated in National Metrology Institute of Japan (NMIJ). In the flowmeter, the pressure drop due to the outflow of the gas is compensated by reducing the volume to maintain the constant pressure. The flow rate is calculated from the volume change rate and the pressure. The flowmeter consists of a main chamber and a reference chamber connected with a differential type capacitance diaphragm gauge [CDG, full scale (FS): 133 Pa] and a valve each other. An absolute type CDG (FS: 133 Pa), gas outlet and inlet valves are also attached to the main chamber. The volume change in the main chamber is carried out by changing the length of the DC-servo motor driven bellows.
Prior to the operation of the flowmeter, the relation between the volume of the main chamber and the length of the bellows was obtained "in-situ" by Boyle's law. A typical volume change with the shortened length of 25 mm was 32.19 ml and the uncertainty of the measurement was 0.30%. During the operation, the pressure was kept constant from 100 to 1 Pa by a real-time control of the volume change rate according to the output of the differential type CDG with a PID algorithm. A fluctuation of the pressure in the flowmeter was 0.03~0.20%. Keeping the temperature constant during the volume change is essential feature. The flowmeter was surrounded by gel packs to increase its heat capacity and a foam-polystyrene to insulate from ambient temperature change. A fluctuation of the temperature was less than 10 mK for 1 hours. Finally, micro He gas flows ranging from 10-7 to 10-5 Pa m3/s at 23 °C were generated by changing the pressure in the flowmeter from 1 to 100 Pa. Uncertainty of the flow rates is 1.7~1.0% (k=2), which depends on a operation pressure.
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