In principle, diffusion pumps are suitable for all diffusion pump oils. To protect against overtemperature (too low oil level, too little cooling, too high fore-vacuum pressure), diffusion pumps should have an overtemperature switch that is controlled by the oil temperature. If this switch (often a bimetal switch) is not immersed directly in the oil, but is located outside the pump housing near the heating elements, temperatures that are significantly higher than the actual oil temperature can be measured, which leads to the pump being switched off, especially when using high-quality silicone oils can although the oil level, cooling and fore-vacuum pressure are OK. In this case you should contact the pump manufacturer.

Basically you can mix silicone oils with one another. Silicone oils deliberately contain portions of the higher or lower quality oil. Silicone oils must never be mixed with mineral oils. If a pump is to be converted from mineral to silicone oil or vice versa, it must be completely cleaned (see Q 4)

When changing from mineral to silicone oil, the pump must be completely cleaned. The nozzle system must be cleaned with acetone and completely freed from cracking products. Then the entire vacuum-side pump must be washed out with isopropyl alcohol and blown out with oil-free compressed air. All seals should be checked for damage and wear.

When changing the oil, no temperature settings have to be changed, as the necessary temperatures are set automatically. If the excess temperature switch trips after changing the type of oil, please note Q2.

The achievable ultimate pressure of a diffusion pump is extremely dependent on the pump oil used (10E-6 to 10E-11 mbar).

However, this does not apply to the pumping speed (or pumping speed) of the pump. The different diffusion pump oils have different molecular masses from approx. 450 (light mineral oil) to 550 (INDOMET silicone oil), therefore also different boiling points and different evaporation energies. For the same heating power, slightly more oil is evaporated when using light mineral oil than when using heavier silicone oil. However, the heavier silicone oil has better momentum transfer in the elastic collisions with the gases to be pumped out. Therefore, the pumping speeds (pumping speeds) of diffusion pumps are almost independent of the oils (blowing agents) used. With all diffusion pumps, the pumping speed is considerably dependent on the gases to be pumped off. The pumping speed of diffusion pumps is usually standardized for nitrogen. For hydrogen or helium, the suction capacity is significantly higher.