Frequently Asked Questions:

1. What is a diffusion pump?

A diffusion pump is a vacuum pump without moving parts, in which a special oil is evaporated by heating elements. This oil vapor or better oil gas is expanding through several ring nozzles, thus forming a (supersonic) gas jet in the direction to the prevacuum (from top to bottom of the pump). From the high vacuum side ( chamber) gas atoms or gas molecules can diffuse into this vapor jet (hence the name of the pump) in which they obtain a mechanical impuls towards the pre vacuum flange and there they will be sucked into the backing pump. This impuls transfer is the more efficient the lighter the gas is. The suction speed of the pump therefore is higher for Helium than e. g. for Nitrogen. The oil vapor condenses on the (water)-cooled pump walls and flows back to the heater to be evaporated at the bottom again. The generated suction speeds (in l / s) are significant, however the mass flow rates (in g / s) are low because of the low pressure, thus also the compression energy is low, that means that the electrical heating power disappears - with the exception of pump heat radiation- nearly almost into the cooling water.

2. What is the temperature of the liquid oil?

The temperature of the liquid oil can be explained most clearly by a cooking pot on the kitchen oven.
Independent whether you switch the oven to half or full power,  after a while the water starts boiling (on sea level) at 100 degree Celsius. At full power of the oven the evaporation temperature is reached earlier, and it boils violently, but always at 100 degrees.
When we perform this experiment on a high mountain, the water boils at lower temperatures, or if you put a tight cap on the pot the water boils at a higher temperature (pressure cooker).
If you repeat the same experiment not with water but with alcohol, the alcohol already starts boiling at about 70 degrees.
The oil in a diffusion pump shows exactly the same behaviour:
If you run the pump with mineral oil, the oil may start boiling well below 200 degrees and if operated with high-quality silicone oils it starts boiling considerably above 200 degrees.
The exact boiling temperatures of the different oils depend essentially on the vapour pressure right above the oil boiler. This vapour pressure depends on the pre vacuum pressure ( and a little on the oil level and cooling power) and is approx. 1 mbar. At this pressure level the INDOMET silicone oil evaporates at approx. 245° celsius.

3. Is my diffusion pump suitable for silicone oil?

Generally, diffusion pumps are suitable for all kinds of diffusion pump oils. As a protection against over temperature (oil level too low, insufficient cooling, excessive prevacuum pressure) diffusion pumps should be equipped with an over-temperature switch controlling the oil temperature. In case this switch (most often a bimetallic switch) does not dip into the oil, but is mounted outside of the pump housing nearby the heating elements, much higher temperatures can be measured compared to the actual oil temperature, leading in particular when using high grade silicone oils to a shutdown of the pump , although oil level, cooling and vacuum pressure are in order. In this case, you should contact the pump manufacturer.

4. What oils can be mixed?

Basically you can mix different kinds of silicone oils. Silicone oils purposely contain fractions of the relevant higher-or lower-oils. Silicone oils should never be mixed with mineral oils. Should a pump be changed from mineral oil to silicone oil or vice versa, it must be completely cleaned (see section 5)

5. Change from mineral oil to silicone or vice versa.

Should a pump be changed from mineral oil to silicone oil or vice versa, it must be completely cleaned. The nozzle system needs to be cleaned with acetone and cracking products must be completely removed. Then, the entire inside of the vacuum pump side must be washed with isopropyl alcohol and blown out with oil-free compressed air. All seals should be checked for damage and wear.

6. Do you have to make changes at set points when changing the kind of oil?

After changing the kind of oil there is no need to change any temperature settings at the pump, as the oil temperature automatically reaches the boiling point and stays there. If the over-temperature switch is released after changing the kind of oil, please refer to the point 3 above.

7. How depends the suction speed of a diffusion pump on the type of the oil?

The ultimate pressure of a diffusion pump is extremely dependent on the type of the used pump oil (10-6 to 10-11 mbar). This is not the case for the suction speed of the pump. The different diffusion pump oils have different molecular masses of about 450 (light mineral oil) to 550 (INDOMET silicone oil) and thus also have different boiling points and different evaporation energies. If using the same heating power a light mineral oil slightly more evaporating than a heavier silicone oil. However, the heavier silicone oil has a better impuls transfer during the elastic collisions with the gases to be pumped. Therefore, the suction speed of diffusion pumps is nearly independent of the type of used oils. For all diffusion pumps, the suctionspeed is considerably dependent on the pumped gases. The suction speed of diffusion pumps is usually calibated to nitrogen. For hydrogen or helium, the suction speeds are much higher.