Comparison of an ultrasonic interferometer manometer and a static expansion system using a capacitance diaphragm gauge

The paper describes the results of a comparison of two primary low-pressure standards, (i) the Ultrasonic Interferometer Manometer (ULM) and (ii) the Static Expansion Apparatus (SEA), maintained at the National Physical Laboratory (NPLI), India. The comparison was carried out by employing a capacitance diaphragm gauge (CDG) of 1333 Pa full-scale range as the transfer standard over the pressure range 1 Pa to 1000 Pa. For pressures of 20 Pa and above, the values of the calibration factor of the CDG obtained on the SEA deviate 0,10 % from the values by the thermal transpiration equation and this deviation increases to over 0,4 % for pressures lower than 20 Pa. For the UIM, the deviation is found below 0,05 % at pressures of 100 Pa and above, but increases to 0,8 % for pressures as low as 1 Pa. The comparison (i) establishes the mutual compatibility of the two primary standards in the region of pressure overlap and (ii) sets limits to the uncertainty of measurements made on the two systems, which are 0,2 % at 1000 Pa and 1,0% at 10(-4) Pa for the SEA and 0,0014 % at 1000 Pa and 1,4 % at 1 Pa for the UIM.