According to the scope of use and pumping efficiency, vacuum pumps can be divided into three categories: 　　
(1) For general water pumps, the pressure can reach 1.333~100kPa (10~760mmHg) for "rough" vacuum.　　(2) Oil pump, the pressure can reach 0.133~133.3Pa (0.001~1mmHg), which is the "sub-high" vacuum.　　(3) Diffusion pump, the pressure can reach below 0.133Pa, (10-3mmHg) is "high" vacuum.
There are two types of pressure reducing pumps commonly used in organic chemistry laboratories: water pump and oil pump. If low pressure is not required, a water pump can be used. If the pump is well structured and the water pressure is high, the pumping efficiency can reach 1067~3333Pa ~25mmHg). The lowest pressure that the pump can pump is theoretically equivalent to the water vapor pressure at the current water temperature. For example, when the water temperature is 25°C, 20°C, and 10°C, the pressure of water vapor is 3192, 2394, and 1197Pa (8-25mmHg), respectively. When pumping air, a safety bottle should be installed in front of the water pump to prevent the water pressure from falling and the water flow is sucked back; before stopping the air pumping, the air should be vented first, and then the water pump should be turned off.　　 If you want a lower pressure, you need to use an oil pump. A good oil pump can pump below 133.3Pa (1mmHg). The quality of the oil pump is determined by its mechanical structure and the quality of the oil. It must be protected when using the oil pump. If the volatile organic solvent is distilled, the organic solvent will be absorbed by the oil and the vapor pressure will increase, thereby reducing the evacuation efficiency. If it is acid gas, it will corrode the oil pump, if it is water vapor, it will make the oil emulsion. The vacuum pump is damaged due to the turbid liquid.

Therefore, the following points must be paid attention to when using the oil pump:
An absorption device must be installed between the distillation system and the oil pump.
Before distillation, the vapor of the organic solvent in the system must be completely removed by a water pump.
If you can use a water pump to pump air, use a water pump as much as possible. If the distilled substance contains volatile substances, you can first use a water pump to reduce pressure, and then use an oil pump.
The decompression system must be kept airtight, all rubber plugs must be of appropriate size and orifice, and rubber hoses must be vacuum hoses. The ground glass is coated with vacuum grease.