The properties of diffusion pump oil have a great influence on the pumping performance of diffusion pump, so the basic requirements for diffusion pump oil are as follows:
1) The molecular weight of pump oil should be large;
2) In order to reduce the limit pressure of the pump, the saturated steam pressure of the pump oil at room temperature is required to be low;
3) In order to make the pump work at a higher outlet pressure, the saturated vapor pressure of the pump oil at boiling temperature should be as large as possible;
4) The thermal stability (not easy to decompose under high temperature) and oxidation resistance (not changing the performance of pump oil due to oxidation when contacting with atmosphere) of pump oil are better; the freezing point and low temperature viscosity are lower; and it is non-toxic, corrosion-resistant and low cost.
The influence of pump oil on limit pressure
The type and composition of pump oil have a great influence on the ultimate pressure of the pump. In the diffusion pump with the same structure, if No.3 diffusion pump oil is used, the ultimate pressure can only reach 10-5 Pa; if no.275 silicon oil is used, the ultimate pressure can reach 10-7 PA, and the ultimate pressure can be reduced by two orders of magnitude.
The partial pressure of the pumped gas in the vacuum system also varies with the pump oil, which is the effect of the pump oil composition. For example, it is difficult to get a cleaner vacuum in the pumping system of oil diffusion pump without special measures.
Saturated vapor pressure of pump oil
In order to obtain high vacuum, it is hoped that the pump oil will have a lower saturated vapor pressure at room temperature. At the same time, the pump is required to work at a higher outlet pressure, and the pump oil is expected to have a higher saturated vapor pressure at the boiler temperature. Therefore, the slope of the curve of the saturated vapor pressure of the pump oil which is most suitable for the operation of the diffusion pump with the change of temperature is larger.
The metal oil diffusion pump is used in high vacuum and ultra-high vacuum pumping to remove the gas which has no serious harm to the working liquid in the high vacuum system. It can not exhaust the air directly, so it needs to be equipped with a front pump.
After pumping the diffusion pump to a pre vacuum of about 10 Pa, the working liquid with low saturated vapor pressure at room temperature is heated by the heater, and the oil vapor generated moves upward along the steam conduit, and is directed and ejected at high speed through several stages of nozzles, bringing the gas diffused in the oil vapor to the front stage end. After being discharged into the atmosphere by the front stage pump, the oil vapor is injected into the pump wall, and then flows back to the bottom of the pump after being cooled, so as to repeat the work and achieve the purpose of high vacuum.