Paper VT-TuP6
Method of Measuring the Volume Flow Rate of Vacuum Pumps Using CFVN

Tuesday, November 10, 2009, 6:00 pm, Room Hall 3

Critical flow Venturi nozzles (CFVN) have been widely used in most of national metrology institutes for the precision measurement and calibration of pressurised gas flow. They enable the relaisation of the critical flow speed equal to the speed of sound at the throat of the Venturi nozzle. The critical flow is exploited in this study not only to achieve the noble stability and repeatability of gas flow but also to minimize effects of the fluctuation of upstream and down stream pressures for the measurement of the volume flow rate of vacuum pumps. These singular properties of CFVN has not fully utilized to measure the pumping speed widely used in the vacuum-related academic and industrial sectors. On the onset of this work, it became apparent that the use of CFVN unfolds new findings for precision measurement of the volume flow rate.

One of the most technical challenges in measuring the volume flow rate was to design a set of Venturi nozzles that can cover the five decades of inlet pressures from 1 bar to 0.001 mbar. Preliminary tests were carried out to examine the practical range of upstream pressure Venturi nozzles can be used within the desired measurement uncertainty of 0.2 %. They revealed that Venturi nozzles were well calibrated in the three and half decades without loss of measurement uncertainty. This observation is very significant since two different sized nozzles are sufficient to cover the desired inlet pressure test range of vacuum pumps, specifically from 100 mbar to 0.001 mbar. This point has encouraged authors to develop a new CFVN-based measurement system targeted for the measurement of the volume flow rate of vacuum pumps. This paper will introduce the details of the developed measurement system, including the configuration of mechanical parts and measurement instruments. Test results obtained from the CFVN-based measurement system are compared to those from the conventional throughput method. The pros and cons of both measurement methods are also discussed. Finally, potential applications of developed CFVN-based volume flow rate measurement technologies for vacuum pumps are briefly pointed out for instance the MFC market for gas flow control and the on-site performance analyzer of dry vacuum pumps in the semiconductor and flat display production lines.