Boiling Temperature of Diffusion Pump Oil Explained

A lot of confusion exists in the vacuum industry about certain things, for example-

·         What should the optimum temperature be set in diffusion pump heaters so as to get maximum level of vacuum in shortest possible time?

·         How can the temperature in diffusion pumps be set in such a way that the engineers are able to get maximum life from costly diffusion pump oil?

·         Is boiling temperature of silicone diffusion pump oil different from hydrocarbon diffusion pump oil?

Before attempting to understand optimum boiling temperature of diffusion pump oil, let us take a brief look about working of diffusion pump.

Since efficiency of diffusion pump depends solely on the working cycle of diffusion pump oil (diffusion pump has no moving parts) so this is evident that a higher diffusion pump temperature will mean a higher ultimate vacuum. Obviously a question comes to mind at this stage that why don’t we increase the boiling temperature to highest possible level. Answer to this is that increasing the temperature of diffusion pump oil involves financial implications (cost increase) because of the following reasons -

1.   Diffusion pump oil works in very difficult environment where there is vacuum and high temperature. So to increase the boiling temperature, superior (obviously costlier) oil will be required which is capable of withstanding these adverse conditions.

2.   At higher temperature O-rings and oil seals may give way so better quality sealing material is required (Costly again).

3.   For handling higher temperature, construction of diffusion pump also has to be stronger.

4.   For attaining higher temperature higher energy charges will be incurred.

5.   For obtaining higher temperature longer time is required so vacuum time increases.

Decision about boiling temperature is basically a balance between level of vacuum required and cost incurred. Our guideline in line with the established practice in the industry is as below -

S. No.	Vacuum Required	Temperature in Diffusion Pump	Supervac Products	Equivalent Products
1.	10-9 to 10-10 Torr	245oC	SV-SIGMA+	DC-705
				Hivac F5
2.	10-7 to 10-8 Torr	220oC	SV-SIGMA	DC-704
				CVC4
				Hivac F4
				Ulvoil D31
3.	10-5 to 10-6 Torr	180oC	SV-SIGMA FAST	DC-702

Some myths are prevalent in the industry about diffusion pump oils. These are clarified as under-

Myth no.1: Boiling temperature of hydrocarbon diffusion pump oil is different from boiling temperature of silicone diffusion pump oil- This myth is baseless as boiling temperature of both hydrocarbon and silicone diffusion pump oil is same. For example- Well known hydrocarbon diffusion pump oils like Supervac SV-DELTA, Lion S, Ulvoil D11 and CVP300, all have boiling temperature of 220^oC.

Myth no.2: My diffusion pump has been designed for hydrocarbon/silicone diffusion pump oil only- This is also false. You can use any diffusion pump oil (hydrocarbon or silicone) in any diffusion pump based on your requirement. For details about merits of hydrocarbon and silicone diffusion pump oil please read my blog post-http://supervacindustries.blogspot.in/2013/04/silicone-diffusion-pump-oil-vs.html

Myth no.3: My equipment manufacturer asks me to use only a particular brand of diffusion pump oil which is exorbitantly priced- Your equipment manufacturer is trying to fleece you. For details about equivalent vacuum oils manufactured by reputed and established manufacturers anywhere in the world please read our blog post at- http://supervacindustries.blogspot.in/2014/03/vacuum-oil-equivalents.html. In case you don’t find it there then please contact us. We will help you find an equivalent product.

Myth no.4: There is no need of topping up- Absolutely false and dangerous myth prevailing in the industry. Topping up at regular intervals is of utmost importance to get proper vacuum and avoiding damage to the equipment.

Myth no.5: Charging less diffusion pump oil in diffusion pump reduces vacuum time- Absolutely false. Charging less oil than required will cause back streaming, burning of diffusion pump oil and damage to costly equipment.

Myth no.6: My diffusion pump oil works at 170^oC- Diffusion pumps do not work at temperature below 180^oC. Your oil supplier is supplying you cheaper vapor jet booster pump oil.

Conclusion:-

Based on the information given above I am sure that you will be able to choose the right diffusion pump oil as per your requirement. For example- if level of vacuum required is only up to 10^-5 Torr and equipment is old then hydrocarbon diffusion pump oil will work more economical. Similarly if vacuum level required is 10^-7 Torr then there is no point in using SV-SIGMA+. Furthermore by using simple precautions like frequent topping up you can get a much longer life and value for money from your diffusion pump oil.

For more helpful tips about how to get your money’s worth from your diffusion pump oil, you can read my blog post- http://supervacindustries.blogspot.in/2013/09/how-to-get-maximum-life-from-your.html

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Written by: Anshuman Punj. 

  

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Graphite Foil/Tape - SV-GF

Supervac manufactures Graphite foil (SV-GF). These foils are manufactured by a patented process of EXFOLIATION and hot pressing: a technology which very few companies in the world possess. Because of its excellent electrical conductivity, anti-corrosion and thermal resistant properties coupled with compressibility and flexibility, SV-GF works outstandingly in various applications.

fig.1

As shown in fig.1, graphite foil is used as an electrically conducting gasket between the hot evaporation boat (up to 1500⁰ C) and the copper clamps. It serves various important functions here. 

1.    It has to let as much as 700 amps. of current pass without any resistance.

2.    On one side, it has to protect the expensive evaporation boat from getting over-heated and cracking.

3.    On the other side, it has to protect the copper clamp from overheating and melting.

Good quality graphite foil performs all these functions effortlessly. SV-GF’s pure, compressive strength packs in denser amounts of graphite per sq.cm.

If, for the sake of cost cutting, cheap plumbing grade graphite foil is used, it causes immense damage to the machinery, the process and the product involved. These low grade graphite foils contain various binders like rubber compounds, adhesives and resins etc. in them. Therefore they are not 100% pure. On being used inside the vacuum chamber, these binders release various gases which affect the vacuum adversely. Also, these gases adhere to the thin film being metalized and bring down the quality of metalizing. Because of binders conduction is not 100%. This causes current loss, sparking and overheating of Evaporation boats as well as of Copper Clamps.

Following are the properties of a good quality graphite foil -

• Should be very flexible

• Very compressible

• Very Pure – should contain no bonding agents like adhesives, rubber compounds or resins.

• Corrosion resistant

• Temperature resistant- in the absence of any binders, pure graphite SV-GF can withstand very low temperatures of cryogenic liquids like liquid oxygen to very high temperatures like those of molten metals like molten steel and aluminum.   

• High tensile strength for a paper-like product

• Should not melt even at very high temperatures

• Excellent sealing characteristics

• Excellent insulating barriers

• No cold flow or creep phenomenon

• Self-lubricating – should have a very low co-efficient of friction to prevent cleaving and grazing between machinery parts.

SV-GF comes with all the above qualities. It is available in all the sizes for the convenience of our valued customers.

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                                                                                      Written by: Anshuman Punj.      

 

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   Click Here