Diffusion Pump Oil is a critical and costly consumable. So it is important that we get required service life from it. Premature failure of Diffusion Pump oil can cause defects in metalizing, contamination of vacuum chamber and loss of money for its replacement. For Silicone Diffusion Pump oil the service life should be about 1 year for film metalizing and about 6 months for batch metalizing.
Following are 5 major reasons for premature failure of Diffusion Pump Oil.
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Not Topping up at the required time –
A very small amount of Diffusion Pump Oil is continuously lost in vapor form. This results in reduction of volume of Diffusion Pump Oil. But since heaters continue to supply same amount of heat to this reduced volume of oil, so Diffusion Pump oil gets over heated and this results in accelerated loss of Diffusion Pump oil. Ultimately most of diffusion Pump oil is either lost or damaged by overheating.
In my experience this is the number one cause of Diffusion Pump oil failing prematurely; although, this can very easily be prevented by timely topping up.
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Power Failure –
If backup power is not available, power failure can cause rotary pumps to stop abruptly. Whereas oil in diffusion pump will remain hot for next about 10 minutes; this causes back streaming of Diffusion Pump oil into vacuum chamber. This in turn reduces the volume of diffusion pump oil and its subsequent overheating and failure as explained in point 1. In extremely rare occurrences, with sudden power failure, the diffusion pump sucks the rotary pump oil into it and this in turn contaminates the diffusion pump oil.
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Mistakes in Operating of Vacuum System –
Although, modern day vacuum systems are automatic but sometimes because of some defect in system or manual over riding of the system can result in wrong sequencing of opening of valves. If fore vacuum is not sufficient, this can damage the Diffusion Pump oil by back streaming.
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Cooling Failure –
Diffusion Pump oil works on a thermal cycle of vaporization and condensation. Therefore, on one hand it needs continuous heating and on the other hand it needs to dissipate heat. Cooling failure can thus result in oil getting over heated and failing prematurity.
This is easily preventable by regularly descaling the cooling coils and measuring the temperature of cooling water at exit. Normally, the temperature of cooling water should not be more than 22°C at exit.
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Pumping out reactive gasses
Sometimes very corrosive or highly reactive gasses are being evacuated by the vacuum system. The reactive substances can react with diffusion pump oil and contaminate it.
In such circumstances a very stable oil like SV- Sigma Plus should be selected for diffusion pump to minimize contamination.
Conclusion –
Just by taking these few simple precautions we can save ourselves from much bother and money wastage associated with emergency pump maintenance.