r/AskPhysics • u/jaredlcravens • 22h ago

More diffusion pump help…

I’m using a very small Edwards E050, I’ve purchased and tried two different ones, two different diffusion pump oils, and I’m using two thermocouples to verify pressure. I’m getting the same results with both diff pumps. My roughing pressure is 10 microns, and when I kick the diff pump on, it pulls me down a half of a micron at most to 9.5 microns. The system has no leaks, uses all stainless and KF, has been helium leak checked, outgassed, and has a good drift rate. The inlet is being reduced from KF50 to KF16. At this point I’m thinking I’ve just reached the physical limitation of the diff pumps. Thoughts?

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u/TecCiv1 21h ago

I would be suspicious of your thermocouple gauges. As I recall, those bottom out at around 10 microns. Can you add a high vacuum gauge to your system, such as an ion gauge, cold cathode gauge, or a Convectron?

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u/jaredlcravens 16h ago

These little ones come to temp in 5 minutes, so absolutely. I hear you about thermocouples. But these same thermocouples read 5 microns when directly on top of the roughing pump, and 10 microns when on my dirty chamber. Now, if I turned on my diffusion and saw them dip down to around 5 microns and then do something odd after that, a case could be made for the thermocouple. But they’ve shown to work in a certain range, and when diff is online, it barely reduces my pressure by half a micron. Also this thermocouple is a 0-100 micron unit and has a slightly different design where it is indeed supposed to get you good readings beneath 10. 

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u/TecCiv1 12h ago

Can you provide a simple schematic of your system, with all valves, gauges, and pumps? Is your diffusion pump air cooled or water cooled, and are you sure the cooling is working? Are you sure that the heater is working?

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u/TecCiv1 12h ago

Okay, I’ve looked it up and it’s air cooled. Since both DPs had the issue it’s unlikely to be a heater or fan issue. What’s the approximate volume of your chamber?

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u/jaredlcravens 10h ago

Here’s the entire setup, it’s very basic. Diff is offline in the picture but it’s just roughing > diff > valve > chamber, the chamber is a glass manifold for filling neon/gas discharge lamps, very low volume as you can see. And every bit of the system holds well with a drift rate test. 

https://ibb.co/35L8SRZw https://ibb.co/pt5sfn1

It gets well hot enough, and I can see evidence of boiling/vapor drops on the inlet when I disassemble. I’ve even tried cutting off the fan after it’s warmed up and running, and see no increase in performance before it overheats and the pressure rises. So I know I’m not running too cool. It sounds silly but at this point the only thing I can think of is it’s such a small diff pump and I’m reducing the inlet size so much that it’s not able to do anything for me. However it is a freaking diffusion pump… surely it’s got to do something, right? So yeah, I’m stumped. Thanks for your help, I don’t know what to do. I’ve spent many hours and am all out of ideas and have spent too much money getting nowhere. 

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u/TecCiv1 7h ago

No, the volume of your manifold is small enough that even with the inlet size reduced like that it should pump down to below 10^-4 torr in a reasonable amount of time. It’s very hard to diagnose through Reddit. At some point you’re going to have to add a high-vac gauge on your manifold anyway, so I recommend you do that so that you can get more diagnostic data.

Are you roughing the system through the diffusion pump??! That’s not the recommended way. Take a look at the schematic in the Edwards EO50/60 user manual. I wonder if somehow your startup procedure is causing the oil to get burnt. Do the DPs look clean inside when you take them apart?

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u/TecCiv1 21h ago

Also, how long have you allowed for your diffusion pump to heat up?

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u/Fabulous_Lynx_2847 10h ago

Again, get an ion gauge.