Compression ratio and leakage through stages in turbomolecular pumps

The presence of leaks through the stages of a turbomolecular pump affects its actual compression ratio. The measured compression ratios are in many instances much lower than calculated ones if this effect is not considered in the calculation. A theoretical evaluation is done for the effect of the leakage from high pressure stages to the low pressure stages. A simple model is developed to calculate the decrease in the compression ratio caused by leakage. Two cases are studied in detail, referring to a particular design of the pump: (1) the conductance in the region between the envelope of the pump and the spacer rings is much larger than the conductance between two adjacent spacer rings; (2) the conductance between two adjacent spacer rings is much larger than the conductance in the region between the envelope of the pump and the spacer rings. In the latter case, compression ratios might be very close to the maximum (without leakage). Above considerations show a way to improve compression ratios in turbomolecular pumps. Finally an experimental verification has been performed, the calculated configurations have been tested, and the experimental and theoretical results are compared.