A comparison of pumping speed measurement methods

A comparison of pumping speed measurement method was made using three types of apparatus. These were: (i) the Fischer-Mommsen system, developed at CERN in which the gas flow rate is measured by the pressure drop across an orifice of known conductance at the test vessel inlet and the pressure gauge is arrayed to serve as a molecular flux transducer, (ii) the three gauge system used by some pump manufacturers in which the gas flow rate is determined by measuring the pressure drop across a long tube whose conductance is calculated from its dimensions and (iii) the constant pressure/pipette system in which the time is measured to evacuate a known quantity of gas. A sputter-ion pump with Ti and Ta cathodes and 32 Penning cells was used as the test pump. The Fischer and Mommsen and pipette procedures gave good agreement for the pumping speeds of N/sub 2/ (within 1%) but allowance had to be made for mass discrimination in the gas flow through an orifice when air was pumped. The three gauge method consistently gave a pumping speed of 8% higher. (15 refs).