#2 FAQ - Mechanical Pump - Diffusion pump?

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John Futter
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by John Futter » Mon May 11, 2009 9:32 am


yes I know the data sheet says less water in litres / min. But the cooler the outer jacket the better it works. We use a closed circuit water system at work and the figure used is what we have found to make these pumps suck really well. Ie inlet temp 8 degrees Celsius for the water and the flow rate at what I said. I know that when the circulation pump needs servicing ie flow rate goes down pumping speed also goes down. Maybe we are pumping at a far greater rate than the pump name plate suggests. I also Know how big a puddle on the floor is formed when the diff pumps are serviced ( disconnected from the system) ie buckets of water for a few tens of seconds.

To put this in perspective we have a few air cooled 50L/sec Diff pumps on mobile pumping stations. These are slow compared to 50L/sec water cooled Diff pumps.

IE the cooler the condensation surface for the pump the happier it is---my thoughts


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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by daghike » Tue Mar 01, 2011 10:55 pm


I have a problem with a diffusion pump and mech pump that I need some help with.
I read previous posts but couldnt find a similar problem.

the mech pump brings the pressure down to 1*10-2 torr. but when the diffusion pump is on it rises to 5*10-2torr.

the diff pump is cooled with a chiller and is sufficient
mech pump oil was just replaced. and the diff pump temps are not that high ~200F for the diff oil to degrade.

would be grateful for some advice on the potential culprits.
- the christmas tree is faulty and the diff pump vapor process is not working as it should?

i have elimnated a lot of causes by now and it really has to be something with the diff pump oil vapors not going up the tree as it should but going up from the sides ..

Thanks for your time

Richard Hester
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Richard Hester » Wed Mar 02, 2011 12:29 am

This is true up to a point. If the outside walls of the diff pump are too cool, the oil gets too viscous to return quicky to the heater portion and do its job after condensing out on the pump walls. This is especially a problem with the the more viscous silicone oils like DC-705. Using a chiller sounds too much over the top to me. O'Hanlon mentions an optimum wall/inlet water temp. If I remember correctly, it's somewhere in the vicinity of 40C. I'll look it up tonight if I remember and have the time, and if someone else doesn't beat me to it.

I just had another look, and if your diff pump heater temp is only 200F, the pump won't do anything of note. Proper operating temps are more like 200-220C.

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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by daghike » Wed Mar 02, 2011 3:21 pm

Thanks Richard.

yes the temps are ~ 230F.

the pressure rises when it heats up and didnt go back down till its cooled down.

this leaves the tree and internal pathway of the vapor and condensate not behaving properly.

is it possible it could also be a leak? (ive checked by spraying methanol in most places not too close to the heater)

im pretty stumped on this but will keep at it till its overcome.

Thanks for your help. will post what i find.. if theres any more suggestions they are welcome !

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Richard Hull
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Richard Hull » Wed Mar 02, 2011 5:56 pm

The other Richard is correct 200 + degrees C not F is needed.

Check your voltage on the stack. Many are 220 volt. If you are feeding 120 into a heater made for 220 then you will never hit operating temps. I operate a 220 volt stack

One characteristic of all pumps is that the pressure tends to rise against the mechanical pump as low temp volatiles vaporize as the stack heats. It is always minor, but a rise nonetheless. Finally, once the stack's boiler nears operational temps near 120-150 deg C the pressure plunges rapidly and deeply. My system with Dow silicone 704 runs at about 195 deg C. My normal back pressure yield by the mechanical pump is about 20 microns ( 2X10e-2 torr) and the diff pump drops the chamber down to the 10e-5 torr range or ~ 3 orders of magnitude.

Richard Hull
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Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by daghike » Wed Mar 02, 2011 6:43 pm

Thanks Richard *2

a few things to try. I have fresh mech pump oil and mech pump by itself was pumping down fine to its usual limits.
the voltages are ok. this system has worked before so its definitely something with the oil or oil levels or leaks ( although leak tests didnt show anything ).

it heats up to ~230F (110C) and for this pump it has worked well before down to 10-8torr using santovac 5. (other diff pumps definitely contributed)

i do hear a popping sort of noise inside the diffusion pump chamber .. and i have not heard it before so this likely related to the increase to ~ 5*10-2 torr seen when pump is hot.

any experience with a similar situ?

i will share what happens with this.

Thanks again

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Chris Bradley
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Chris Bradley » Wed Mar 02, 2011 7:43 pm


Just to, perhaps, update this FAQ given recent availability of some parts...

It was once the case that turbo-molecular pumps were as rare as hen's teeth on ebay, but they are becoming more readily available. Several folks here have now acquire one, or even a few, off ebay.

Today we might see the more modern turbo-molecular pumps in combination with molecular-drag pumps coming up as 2nd hand deals with the frequency that turbo-molecular pumps once used to (i.e. very infrequently!).

The difference is that these newer pumps have an 'additional' pump, Holweck or Gaede types, which are types of scroll pump, that sit on the same axis as the turbine blades of a turbo pump and act to pump gases at pressures just at the point between 'molecular' and 'viscous' flow which reduces the necessary size of the backing pump. These pumps will start 'pumping' at higher pressures than 'straight' turbo-molecular pumps (that require lower than 100 microns before they 'bite'). There is also a consequent increase in the permissible backing pump pressure - sufficiently so that the best diaphragm pumps can now back a good turbo-drag pump.

What this means for the amateur is that in future it may not necessarily be the case that the only backing pumps are clunking great two-stager rotary types. Some suppliers offer desk-top 'mini-vacuum stations' that consist of exactly such parts - a small diaphragm pump that backs a turbo-drag pump - and can pull high vacuum straight out to atm, all there right on your desk!

So this might be expected to be a good 2nd hand buy in the future for the amateur, but what about now? Well, you *can* get these things if you keep your eye out. I have acquired a KNF Neuberger 4 stage diaphragm pump that can pull 500 microns that does a reasonable job at backing my Varian V70. It is not brilliant, you loose out on outright compression ratio performance, but is just fine for pulling into the sub-micron range.

I will posit a speculation on what could be done with such a set-up: Low power and quieter operation is not the biggest advantage that I think might be realised by the amateur, with such pumps. These diaphragm pumps have a specific purpose in life – all the internal parts that the process fluids flow by are usually all-viton, or some other choice of vacuum and chemical compatibility specific to the application. The pumps are designed to pump chemicals without contaminating them, and recompress them. I think it is not inconceivable that a fusor gas circuit could be constructed consisting of a diaphragm pump backing a turbo-drag pump that recompresses the evacuated gasses back into a reservoir. That resovoir could then be used to feed the fusor. So, once the reservoir has been charged with deuterium, the system can be sealed and operated without any further admit of deuterium. Of course, over time the reservoir of deuterium will become contaminated with the outgassings from the chamber and system, and some will no doubt leak. But in principle I think it could work as a fully sealed system for as long as one's skill at achieving joint sealing and chamber cleanliness will permit, and it would surely be better than just pumping the D straight to outside!

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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by ggombert » Thu Mar 03, 2011 12:30 am

Attached is a picture of a small air cooled diffusion pump system that I put together over the last year and a half with most of the ‘big ticket’ items from EBay, the rest of the vacuum fittings are from Lesker’s. This is an illustration of what Richard discusses in his FAQ and what can be done with a modest investment of time and a little net ‘bargain hunting’.

The two stage 6CFM pump will pump down the entire system to ~18mTorr and the diffusion pump once turned on will bottom out the high vacuum gage at 10-5 in about three minutes. I have a Varian Cold Cathode gauge as well but do not have all the fittings (yet) to hook it into the system

Diff Pump System.jpg
Diff Pump System.jpg (148.19 KiB) Viewed 2124 times

Richard Hester
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Richard Hester » Thu Mar 03, 2011 2:05 am

Note - Air-cooled diffusion pumps generally need a fan blowing right on them to work properly. The nice little Edwards pump shown in the previous post (I have one too, love the KF input flange) requires a minimum air flow of 2.7 CFM.

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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Alexi » Thu Mar 03, 2011 3:35 am


I had encountered similar problem with a little Edwards diff pump. There was a popping sound and no pumping. It happened after mechanical pump oil got sucked into the diff pump. An oil change fixed the problem.


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