Diffusion Pump oil is of two types – Silicone and Hydrocarbon.
Most people these days prefer silicone DP oil because of the following reasons.
* Good quality silicone diffusion pump oil will have a high vapor point and therefore deliver a higher vacuum level.
* Will
be thermally stable.
* Compared
to Hydrocarbon oil, silicone DP oil will perform better over long periods of
time with little to no degradation.
* This
oil is also relatively safe from the standpoint of toxicity and flammability.
Diffusion
pump oil is one of the costly consumables in a Metallizer. With a few
precautions, its life can be prolonged considerably and much money saved.
Dos and Don’ts:
1) Never
open the pump when the oil inside is still hot. Doing so will affect the
viscosity of the oil – it thickens and eventually turns into gel form.
2) Care
should be taken to top up the oil in the pump in a timely manner. In the
absence of topping up, the reduced quantity of oil will accidently over heat
and in extreme circumstances, can even crystallize into blue-grey powder or
crystals.
3) 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.
4) 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.
5) 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.
6) There
should be no gross vacuum leaks as this will cause the pressure to drop. In a
lay man’s language, this means that from some point air is getting inside the
pump, not letting proper vacuum to form. Such leaks can be plugged by applying silicone
vacuum grease to O rings and joints.
7) 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
corroded over time.
8) Often
back streaming of contaminants from the vacuum chamber takes place causing the
oil to form sludge. This can also
happen if the Rotary vacuum pump is letting its vapors to escape into the
diffusion pump. This is preventable by installing filters above the chevron
baffle of the diffusion pump as well as on the line in between the rotary and
the diffusion pump.
9) Silicone
diffusion pump oil starts to decompose if it comes in contact with even trace
elements of alkali metals (e.g., sodium, potassium, cesium etc.). Use of the
latter should be avoided in the presence of silicone DP oil.
Written by: Anshuman Punj
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