The vacuum pump can be divided into three categories according to the scope of use and the efficiency of the pumping:
1. The general water pump, pressure can reach 1.333~100kPa (10~760mmHg) for the "rough" vacuum.
2. Oil pump, pressure up to 0.133~133.3Pa (0.001~1mmHg) for the "second high" vacuum.
3. Diffusion pump, pressure up to 0.133Pa, (10-3mmHg) for the "high" vacuum.
To lower the pressure, it is to use the oil pump, a good pump can be pumped to 133.3Pa (1mmHg) the following.
Commonly used in organic chemistry laboratory vacuum pump water pumps and vacuum pumps, if does not require a very low pressure, can pump, if the structure of the pump is good and the water pressure and high evacuated efficiency up to 1067 ~ 3333pa (8 ~ 25mmhg).
The lowest pressure that the pump can pump is equal to the water vapor pressure at the time of the water temperature.
For example, the temperature of 25 degrees Celsius, 20 degrees Celsius, 10 degrees Celsius, water vapor pressure were 3192, 2394, 1197Pa (8-25mmHg).
When pumping gas with water pump, the pump should be installed on the safety bottle, the water pressure drop, water flow back suction; stop pumping gas before, should first bleed, and then shut the pump.
