Types of Diffusion Pumps
There are a few main types of working fluids used in diffusion pumps. The choice of working fluid determines the operating temperature range, ultimate vacuum level, and suitability for different applications.
Most modern diffusion pumps use a specialised high molecular weight oil as the working fluid that generates the high-velocity vapour jet.
Oil Diffusion Pumps
Silicone Oils
- Chemically inert, exceptional resistance to oxidation and decomposition
- Exhibit outstanding oxidation resistance in high temperatures
- Allow better ultimate vacuums
- More expensive
Silicone oils are most common due to better stability, although they require more careful hot operations.
Hydrocarbon Oils
- Cheaper
- The break down at high temperatures. It may clog the vapour jet.
- Risk of explosion upon air exposure
- Limit ultimate vacuum level
- Need frequent changing of oil
Hydrocarbon oils trade off safety for lower costs. Presently, hydrocarbons are rarely utilised.
Mercury Diffusion Pumps
- Mercury was one of the earliest working fluids used. It has characteristics like:
- Inertness to oxidation and do not decompose
- Mercury diffusion pumps require cooling to a temperature of 15°C
- Mercury may get deposited in the vacuum chamber.
- Toxic risks requiring careful handling
Due to the toxicity of mercury and associated disposal/contamination issues, it sees very specialised usage now.
Some other fluids worth mentioning are Polyphenyl Ether & Perfluoro Polyether. Perfluoro polyethers are fully fluorinated fluids known for their chemical inertness and outstanding lubricating properties. In vacuum applications, they offer an additional advantage as solid films are seldom deposited when their vapours undergo electron and ion bombardment.
🔗Applications advantages and limits of diffusion pump