Design of a Pumping Speed Measurement Apparatus Based on a Symmetric Structure Test Dome Coupled with a Standard Conductance

The pumping speed of high vacuum/ultra-high vacuum (HV/UHV) pumps is commonly tested by a combination of the flow-rate method and the conductance method under the pressure range of 10⁻¹ to 10⁻⁶ Pa. Nevertheless, this technique is constrained by drawbacks including the narrow inlet-pressure range of the test dome and large measurement uncertainty of test results. This paper proposes a novel pumping-speed measurement approach for high-/ultra-high-vacuum pumps, which employs a symmetric-structure test dome coupled with a standard conductance element. With a symmetric configuration, two identical test domes are separately fitted with the pump-under-test and the standard conductance element. Vacuum gauges are utilized as comparators to measure gas pressures induced by the identical gas flow rate within the two test domes. The pumping speed of the pump-under-test for a given gas is calculated by taking the molecular conductance of the standard conductance element for that specific gas as the reference standard. The proposed method overcomes the limitations of conventional techniques, such as considerable deviation in absolute-pressure readings from vacuum gauges and the capability only for equivalent-gas pumping-speed measurement. Furthermore, it eliminates the requirement for a known standard gas flow rate and enables pumping-speed measurement within the pressure interval of 10⁻¹ to 10⁻⁷ Pa in the test dome. The combined standard uncertainty of measured results is reduced to 2.5%.