When considering what kind of vacuum pump to use in a vacuum system, first of all, it should be based on the requirements of the system, and secondly, it should have a detailed understanding of the performance characteristics of the pump, and then decide after comparing the advantages and disadvantages of various pumps, so as to choose the appropriate one. In order to facilitate the selection of the vacuum pump to be used, it is necessary to summarize the general situation of various types of vacuum pumps produced in my country, and briefly compare their characteristics, advantages and disadvantages.

1. Oil-sealed mechanical pump
These types of pumps are used as backing pumps and pre-pumps for obtaining low vacuum or for high vacuum pumps. With gas ballast device to extract humid air. But the ultimate vacuum is an order of magnitude lower than pumping dry air (without gas ballast). The pumping speed of the pump refers to the pumping speed when the inlet pressure is 760 Torr. When the system pressure drops, the pumping speed also decreases until it is close to the ultimate vacuum of the pump, and the pumping speed is also close to zero. There are certain provisions for its pumping coefficient.

2. Mechanical booster pump (Roots pump)
The working pressure range is 10~5x10-4 Torr, the common range is 2~5x10-3 Torr, and the pumping speed range is 150~2000 liters/second. There are already a series of products in my country. The maximum pumping speed of this kind of pump is between 2 Torr and 10-2 Torr, which requires a fore pump and cannot be directly exhausted to the atmosphere. Ultimate vacuum varies with the backing pump. Compared with the oil booster pump, the working pressure range is wider, the maximum exhaust pressure is higher (10~30 Torr for the mechanical booster pump, 1~2 Torr for the oil booster pump), and the backflow of oil vapor is very small. It is more suitable to exclude condensable gases such as water vapor.

3. Oil booster pump
The working pressure range is 1~1x10-4 Torr, the common range is 0.1~1x10-2 Torr, and the pumping speed range is 200~40000 liters/second, which makes up for the small pumping speed of mechanical pumps and diffusion pumps in this pressure range. The same pumping speed for inert gases as for other gases.

4. Oil diffusion pump
The working pressure range of ordinary oil diffusion pump is 10-2~5x10-7 Torr, the common range is 10-8~5x10-6 Torr, and the pumping speed range is 20~100000 liters/second. Diffusion pumps with metal and glass structures are produced in series. The ultimate pressure of ultra-high vacuum oil diffusion pump is about 10-10 Torr. Except for a slight difference in structure and ordinary oil diffusion pump, it is mainly due to different working fluids and different structural materials, that is, 275 silicone oil is used as pump fluid, and structural materials are used Stainless steel. The newly designed ultra-high vacuum pump is quite different.