Does anyone here have any ultra low hvac experience? I purchased a thermo fisher scientific ult freezer at an auction and need a little help. It’s a-90°c unit. First stage is r404, second is r508b. Both compressors fire up and sound fine. Plugged it in yesterday for the first time and after a couple hours it got down to -64°c, I checked it in the morning and it was at -12°c. It now doesn’t cool at all. Any help would be appreciated. Seems to be a silly problem.
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Well not that silly
Turn it off right away
2 stage refrigiration is dependent of one to the other
The r404 cools the r508b cirquit for at a lower temp it has a lowerboiling point
And a lower vapor pressure
Making it possible for the compressor to compress
I hope you have a problem with the r404 cirquit that is fixable
If your problem is in the r508 then i have little hope for saving the unit
Are the cooling fans working ?
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I found ultra low repair guys on craigslist
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All fans are on, it’s been unplugged since. I haven’t found anyone local to take a look at it yet. Not sure where to start testing. My hvac buddy has never even seen a cascade system.
This kind of stuff is why I avoid buying used freezers these days. After many, many issues with units we bought for $400, I’d just go new.
If you live anywhere near revco or Fisher you might be able to get a tech out. Theres also usually independent biotech repair guys local if you live near pharma facilities.
Unfortunately when I’ve been in this situation it usually ends with a scrapyard…
I’ll call around Monday and see if I can find anyone. I payed $15 for it, so I figured why not.
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One of my old maintenance guys had luck with this company, https://www.frigitemp.com/scientific-refrigeration-service/, probably going to cost much more than you paid though. Best of luck
Well don t scrap it
The unit s are fairly simple and the heat exchangers are nice for cooling your butane direct on a chiller
If the compressors are still running you might be lucky in finding a gas that is
Has the same property s if there turns out to be a leak
R404 is $80 for a 24lb tank. R508b is a little harder to find and more expensive. My hvac knowledge stops at r410a 
Sounds like a leak, the compressors are VFD/ECM controlled and will compensate for non-condensibles so they’ll always sound fine. You can get an ultrasonic leak detector, it would work well for the 508b stage, it’s probably losing pressure from a hairline crack in a fatigued brazed connection. Do any of the lines look oily? A lot of scientific gear uses a shiny resin coating on all their copper that makes oil spots really difficult to see without feeling around.
If it is a braze failing, you just reflow the joint with a MAP gas torch after evacuating the lines (both stages). Trouble is, it’s usually impossible to get in there without burning up whatever is around the line. Keep some scrap sheet metal around to make an impromptu heat shield and wet rag everything in the vicinity.
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I see no oil anywhere, someone was in here at some point and put a piercing tap on the 2nd stage. I’m assuming it sat for a couple years. But ran fine for 4 hours or so. Strange.
Is it going into defrost. Is the condenser coil clean. Are the condenser fans working. What is the t-stat set right.
I’ve got a million of them😂
I’m a refrigeration contractor.
Where are you located. Maybe I can help.
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Condenser is clean filter is clean, both fans run. I had it set on -90°c got to -64 and I checked it the next morning to find -12. I see no oil, it won’t drop 1 degree from ambient now.
Also I’m in AZ.
If you have the model and serial number you can send it to me. I can get a manual on the unit. Maybe I can help you get it working properly.
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Send me your email address
Rent an FLIR camera and have a look at the lines while its running. If its a leak from fatigued copper line youll be able to see where.
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R508b 2k for 20# we just bought some needed a little over 1# lol
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I’m looking for r508b refrigerant. We can buy it for $70 a barrel from cheena but it will take a month to ship via ocean freight. We may only need a pound also, not sure.