Diffusion Pump Oil is one of the costliest consumables required by vacuum users. In the present day scenario of cut throat competition, engineers are under a lot of pressure to reduce costs without compromising on quality.
Based on my experience of almost 20 years in the vacuum line, I have compiled a list of do’s and don’ts as to how one can get maximum life from the diffusion pump oil. For the benefit of our valued customers, the facts are given below –
1.Topping up at regular intervals is most important factor in getting maximum life from Diffusion Pump Oil. Care should be taken to top up the oil in the pump in a timely manner. In the absence of topping up, volume of oil in the Diffusion Pump reduces, whereas heaters keep producing the same amount of heat. This causes overheating and accelerated loss of DP oil. This further reduces the volume of oil, which in turn causes more overheating and loss of DP oil is further accelerated and so on.
2.Keep track of the water flow through the cooling lines. Check the inlet and outlet water temperature as well as the flow rate. Lines can become clogged or scaled over time. As Diffusion Pump oil vapor can only condense on coming in contact with cold walls of DP. Any reduction in cooling causes overheating and increase of pressure inside DP. This may result in loss of oil by back streaming.
3.Strange as it may sound a very low fore- pressure can also cause loss of DP oil. This can be prevented by switching on heaters before fore pressure becomes too low.
4.Faulty gauges for pressure/heater too, can cause accidental overheating of the oil or the oil not getting the required heating. Check that all the gauges are functioning properly.
5.Silicone diffusion Pump Oils have a life of about 4 times that of Hydrocarbon Diffusion Pump Oils. Also Silicone Diffusion Pump oil does not blacken and clog the jets of DP so Silicone DP oil works out more economical in the long run.
6.Poorly working Rotary Pump will put extra burden on diffusion pump. It must be ensured that Rotary pump is working properly to get full life from Diffusion Pump Oil.
7.Do not expose the oil to air when it is heated. Doing so will affect the viscosity of the oil – it thickens and vacuum reduces. To avoid air exposure valves must be checked from time to time.
8.To avoid having the oil migrate into the vacuum chamber, a cold cap should be fitted at the top of the nozzle assembly inside the diffusion pump. This cold cap condenses the vapor in that area.
9.A concentric circular chevron baffle should be present at the mouth of the pump. It allows air molecules to wander in, but traps the heavier oil vapors as they try to escape.
10.There should be no gross vacuum leaks as this will result in deterioration of oil quickly. Vacuum leaks also cause the pressure to drop and in extreme cases may cause stalling of Diffusion Pump. Stalling will cause loss and deterioration of diffusion pump oil. Such leaks can be plugged by applying silicone vacuum grease to O rings and joints.
11.Sometimes back streaming of contaminants from the vacuum chamber takes place causing the oil to form sludge. This is preventable by installing filters above the chevron baffle of the diffusion pump.
12.Silicone diffusion pump oil starts to decompose if it comes in contact with even trace elements of alkali metals (e.g. hydroxides of sodium, potassium, cesium etc.). If process requires use of these then care should be exercised to prevent them reaching silicone DP oil.
13. In processes like PVD, coatings requiring plasma or transparent metalizing, gas load can cause vacuum of diffusion pump to fall below 10-1Torr. This can result in back streaming of DP Oil or even stalling. This is preventable by reducing the process speed or increasing the capacity of backing pumps.