Determination of pumping speed and its impact on the overall uncertainty budget of the CMI orifice flow standard

Abstract

The pumping speed of a pump in an orifice flow standard was measured to enable calculation of the effective pumping speed in the calibration chamber. The main problems are as follows. (1) The pumping speed of a pump connected to a chamber of finite size is not unambiguously defined; different measuring arrangements lead to different values. It is necessary to measure the pumping speed of the pump in the same arrangement as for the orifice flow standard. (2) If this is to be a primary standard, gauges related to calibration of the gauge to another standard cannot be used for the measurement for the same range of pressures.The necessary precision of the measurement was determined. The orifice method and the flowmeter method were tested directly in the arrangement for the orifice flow standard. These two methods were used to avoid the necessity of using gauges calibrated to another standard.The pumping speed of the pump for the CMI orifice flow standard was measured with sufficiently low uncertainty, so that its effect on the uncertainty budget of the entire standard was negligible.