Q.21 What is the alternative of CVC4 diffusion pump oil.
Ans. Supervac Industries, New Delhi manufactures silicone diffusion
pump oil SV-SIGMA. This oil is a perfect alternative of CVC4. Infact, SV-SIGMA
has a much larger satisfied customer base and has been in use much before CVC4
started getting promoted.
Q.22 Can somebody supply me DC-702/DC-704/DC-705 diffusion pump oil?
Ans. Dow Corning the company that manufactured DC-702/DC-704/DC-705 has
stopped production of this oil since last 2 years as such this oil is not
available. Supervac Industries a leading supplier of vacuum oils worldwide
supplies the following-
1) SV-SIGMA FAST which is an alternative
of DC-702.
2) SV-SIGMA which is an alternative of
DC-704.
3) SV-SIGMA PLUS which is an alternative
of DC-705.
Even when DC-702/DC-704/DC-705 was
available a large number of customers were using SV-SIGMA, SV-SIGMA+ and
SV-SIGMA FAST because of considerable technological and cost advantage.
Q.23 Do I need to clean jets of my diffusion pump, every time I
change diffusion pump oil in it?
Ans. If you are using good quality silicone diffusion pump oil then you need not waste precious machine time in
cleaning of jets every time you change diffusion pump oil. Cleaning them once
in about 5 to 7 years is enough.
Q.24 I am using silicone diffusion pump oil. Can I top up with
hydrocarbon diffusion pump oil?
Ans. No, Silicone diffusion pump oil should never be mixed with
hydrocarbon diffusion pump oil. So if you are using silicone diffusion pump oil
then top up with silicone diffusion pump oil and if you are using hydrocarbon
diffusion pump oil then top up with hydrocarbon diffusion pump oil.
Q.25 What should be the basic properties of silicone diffusion pump
oil?
Ans. Silicone diffusion pump oil should have very low vapor pressure,
should have high thermal stability and should be resistant to oxidation. These
are very basic properties of silicone diffusion pump oil.
Q.26 What should the be basic properties of Hydrocarbon diffusion
pump oil?
Supervac Hydrocarbon
Diffusion Pump Oil
Ans. Hydrocarbon D.P. Oil should have very low vapor pressure, should
have high thermal stability and should be resistant to oxidation. These are
very basic properties of hydrocarbon D.P. oil. Apart from these qualities,
hydrocarbon diffusion pump oil should not have aromatic compounds and
artificial additives as these may turn to gel form or black tar like substance
and harm the pump interiors.
Q.27 What is Holding Pump?
Ans. Holding pump is a simple rotary vacuum pump
of small capacity. Its function is to create holding vacuum for diffusion pump,
hence the name Holding Pump. It is invariably connected to diffusion pump and
not to the vacuum chamber.
Q.28 Can I use diffusion pump without using a rotary pump in my
vacuum system?
Ans. No, Diffusion pump cannot be used alone. Reasons are explained
below.
A diffusion pump merely creates a vacuum gradient along its length
where vacuum in top portion connected to vacuum chamber is very high and vacuum
at the bottom of diffusion pump is low. Since even this low vacuum is better
than atmospheric pressure so diffusion pump cannot discharge in atmosphere.
Rotary pump therefore is added to evacuate the air from bottom portion of
diffusion pump.
Q.29 What is stalling of Diffusion Pump?
Ans. A Diffusion pump works on the principle of gas diffusion. This
pump requires Diffusion Pump Oil, which evaporates, gets compressed and then
escapes through jets, trapping air on its path down to the bottom of Diffusion
Pump and from there the air is evacuated by a Rotary Pump or a Booster
Pump-Rotary Pump combination.
For this arrangement to work, vacuum of minimum 10-1Torr
in required to be maintained in the main body of Diffusion Pump. Otherwise the
Diffusion Pump Oil escaping from jets in gaseous form collides with too much
air available and further raises pressure in Diffusion Pump body. This stops
creation of vacuum and is called Stalling of Diffusion Pump.
Q.30 What is the work of baffles in a
diffusion pump?
Baffle in diffusion pump
Ans. Baffles are provided at the top
of diffusion pump. They stop diffusion pump oil from escaping into vacuum
chamber. Baffles should be properly water cooled to perform their function
effectively.
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Written by: Anshuman Punj.
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