We have obtained an old mercury-vapor diffusion pump for high vacuum. However, we do not want to operate it with mercury in our lab due to health concerns (which is presumably the reason it was scrapped by the original owner).

Instead, we plan to fill it with a silicone oil of the DC-704 type or similar. The pump has thermal regulation and a large baffle to stop oil vapor counter-propagation.

Has anybody tried replacing mercury with oil? What are possible problems with this approach?

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    $\begingroup$ One concern is that the pump is still contaminated with mercury. $\endgroup$
    – wwarriner
    Mar 30, 2016 at 17:01
  • 2
    $\begingroup$ @starrise This is a good point, we have been cleaning it by different ways to get rid of Hg as much as possible. $\endgroup$
    – dominecf
    Mar 30, 2016 at 17:07
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    $\begingroup$ What's the max temperature that's going to be applied (i.e. will the silicone break down)? $\endgroup$ Mar 30, 2016 at 19:41
  • $\begingroup$ @CarlWitthoft ... another good point, thank you; we can however adjust the heater temperature in order not to burn the oil. This basically depends on the oil datasheet values. $\endgroup$
    – dominecf
    Mar 30, 2016 at 21:38
  • $\begingroup$ How does vapour pressure of the oil compare with that of Mercury? $\endgroup$ Apr 1, 2016 at 11:31

1 Answer 1


I did some internet searching and did not find anyone that had done a direct swap out. Granted, probably the majority of these replacements occurred before the existence of the internet ;-). As I am sure you are already aware (but to be thorough), modern diffusion pump designs do use synthetic working fluids.

It will probably take some fine tuning of the temperatures of the heater and cooler, but I don't see any reason why it wouldn't work. If you have means of measuring the vacuum level, you could run through some different temperatures to empirically find its new ideal operating conditions.

Worst case, the pumping efficiency will be less because the nozzle diameter was designed for higher density particles. I am not a vacuum physicist however, so who knows, it may work even better than before ;-)

Another possible issue is that it may have more backstreaming of the oil into the vacuum chamber resulting in unacceptable levels of oil contamination. Maybe the baffle you mentioned will take care of this; hard to say without some experimentation.

Just as long as no oxygen reaches the oil during operation, it wont fail catastrophically, so give it a try and let us know how it went!

Good overview of diffusion pump working fluids
Some good history or mercury as a diffusion pump working fluid

  • $\begingroup$ Thank you for a nice answer, I also tried searching about the Hg-silicone switch with no relevant results and I like your explanation that most people did it before the internet era. $\endgroup$
    – dominecf
    Mar 31, 2016 at 7:42

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