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Helium leaks are a nightmare. During my PhD I worked with a self-built dilution cryostat that would often have leaks in the custom-built Indium seals. To find t
by ThePhysicist 3y ago
Helium leaks are a nightmare. During my PhD I worked with a self-built dilution cryostat that would often have leaks in the custom-built Indium seals. To find the leaks you'd have to pump the cryostat to high vacuum, hook up a portable mass spectrometer tuned to Helium to the pump circuit and then spray different parts of the cryostat with Helium from a regular gas canister. The He would then get sucked in through the leak and show up on the mass spectrometer, which was coupled to a loudspeaker so it would cause a sound whose pitch increased with the measured He density. Once you found the leak you had to vent the entire system, remove the faulty seal and replace it with a new one. All seals were handmade, i.e. you took a small filament of Indium, placed it between the flange and the housing (after carefully cleaning everything with Acetone) and carefully screwed it shut, turning each screw only a tiny bit at each turn and going around all the screws until you could see the Indium squeeze out of the edges.
Even worse, some leaks would only show up when the system got cooled down to liquid Helium temperature. When that happened you were out of luck as you can't cool the system down to 4K before spraying it with Helium, so you had to just guess where the leak might be and replace all seals in that area until you found the right one. Going even deeper in temperature would eventually turn the He4 suprafluid, which means that it loses all internal friction. In that state it would squeeze through even the tiniest molecular cracks, so again if that happened you just had to redo all the seals and hope they would hold.
- user3939382 3y agoThis is a more extreme description of why I hate plumbing.
- nazgulsenpai 3y agoAs someone who is currently in a game of cat and mouse with an intermittent drip from underneath the kitchen sink, this was exactly the first thing that came to my mind also!
- lotsofpulp 3y agoI feel like I would just replace all of it than play cat and mouse. Surely it would be cheaper in time, and low cost until you figure out you have to replace the whole sink.
- civilitty 3y agoGraduate and postdoc labor is cheap, pressure vessels are not.
- gus_massa 3y agoEven a new vessel need new plumbing to connect it to the vacuum pump (or pumps, sometimes you need one to to go from ambient pressure to low pressure and another to go from low pressure to very low pressure) and the pressure measuring device(s) and also sealing the holes where the wires go (I guess they have some sensor inside the vessel connected to things that are outside). So a brand new vessel means restarting all the seals from zero.
- finnh 3y agoi believe the comment you are responding was talking about actual home sink in GP comment, not the LHC =)
- pjc50 3y agoNuclear reactors offer all the fun of leak tracing and invisible cracks with the added bonus that the fluids involved are extremely radioactive and contaminate everything they touch. This is why all the "just mass produce small reactors" and "just try thorium / molten salt" stuff hasn't taken off, and may never: all the beautiful theory disintegrates into man-years of laborious leak testing.
- PaulHoule 3y agoLWR coolant is not that bad. They keep the water very clean with ion exchange resins because otherwise you can have problems with corrosion. Workers sometimes float in the water during refueling where they flood the area about the reactor and spent fuel pond and open the lid of the reactor vessel. Leaks in liquid metal fast reactors are much more obnoxious but still manageable. Sodium catching on fire when it hits air frequently isn't as bad as it sounds (a "pool fire" isn't particularly hot or dangerous but a "spray fire" can be) but it is important to catch sodium leaks quickly without false alarms and many development projects had trouble with that.
- yborg 3y agohttps://en.wikipedia.org/wiki/Monju_Nuclear_Power_Plant https://en.wikipedia.org/wiki/Monju_Nuclear_Power_Plant I always enjoy the handwaving by nuclear enthusiasts. These are some of the most difficult engineering systems routinely built and operated by mankind, where a coolant loss accident doesn't just destroy the reactor but can turn the facility into a multi-billion dollar cleanup effort in minutes.
- PaulHoule 3y agoMonju was a poorly designed machine which was particularly badly run. I recently found a book they wrote about it and when I looked at the plans I thought "I can't believe they built that in an earthquake zone". I don't know if anyone is ever going to build another loop-type LMFBR as many of the problems you can have with a loop-type can't happen in pool-type reactors. What was most shocking about the fire at Monju wasn't that it happened but that they tried to cover it up. Neither that nor the incident where they dropped the refueling machine into the reactor vessel were dangerous to people off site but the latter sure convinced everyone they didn't know what they were doing. Contrast that to Superphenix where the fuel transfer drum failed and they struggled with a steam turbine system that was procured under corrupt contracts but overall had a good operational record. Or the three pre-1970s cases where there was significant fuel damage in the US but they found that a core melt in a LFMBR isn't as bad as it sounds because the iodine (most dangerous radioactive element in the fission products) reacts with the sodium and the NaI dissolves in the sodium so it doesn't go anywhere. Or EBR-II and the FFTF which performed flawlessly, or the highly successful fast reactors in Russia.
- allenrb 3y agoSeriously. It is annoying when some ancient pipe in my radiator plumbing decides to leak. This sounds orders of magnitude worse. The things we humans put up with to try and understand the universe!
- arbuge 3y ago...and central air conditioning. 2 evaporator coil leaks in my 5 year old house already, another one currently suspected.
- DANmode 3y agoWhy oh why doesn't everything use radiant by now?
- simonjgreen 3y agoThat's sounds horrific. It makes me wonder what proportion of time you'd spend massaging and debugging your lab equipment vs. actually using it?
- gaze 3y agoFor an experimental physicist you basically debug until you get the data for your paper or until you find your sample is bad. So you’re debugging 99% of the time. In many fields it’s rare to build new instrumentation. You fab with well known techniques with one subtle modification, so it’s not even clear what part of your time isn’t spent debugging.
- causality0 3y agoIt's like a high-end fighter jet. An F-22 needs 43 man-hours of maintenance per flight hour.
- gus_massa 3y agoAnd that does not include the problems with electric wires. We had an unofficial rule: after half an hour debugging the electric wires just change all of them. I still have nightmares about BNC connectors.
- gaze 3y agoThat’s a pretty terrible rule. Unless you’re rolling over them with chair wheels, a bad BNC cable should be easy to spot by visual inspection. In my 20 years of instrumentation work it’s rarely the cable.
- gus_massa 3y agoIt was a students' lab, where each device is connected and disconnected 2 or 3 times per day, 5 days per week. The wires get a lot of use and abuse. Usually the problem is the tip that got slightly loose and the signal is now intermittent or null. IIRC, when we detected the wrong wire we had to put it in a box and the guy in charge of the lab equipment would fix it later.
- londons_explore 3y agoOne would imagine there must be some kind of paint you can paint onto the outside of a vacuum flask, and have it be sucked into any gaps and solidify there. It seems they already make a product for that - vacuum grease. It can get sucked into even a fairly large crack, and plug it up, as long as the depth of the crack is much longer than the width, then atmospheric pressure won't provide enough force to send it deeper into the crack. And the grease itself is designed not to evaporate into the vacuum. Whats wrong with using that?
- remus 3y agoJust speculating, but helium molecules are pretty small, so will be able to leak through tiny gaps where the paint/grease would not penetrate.
- beowulfey 3y agoCryogenic temperatures maybe?
- hwillis 3y ago> paint onto the outside of a vacuum flask, and have it be sucked into any gaps and solidify there. Stuff like paint will tend to be very permeable- if it relies on a solvent drying out, then obviously its permeable enough to let the solvent through. Even if it does dry to a very solid state, there will be a very very long tail of evaporation of volatiles inside the substance. > It seems they already make a product for that - vacuum grease. Vacuum grease isn't perfect- it's not recommended for the high end of high vacuum or for UHV. Or for single digits of kelvin. Another big factor is that if you're doing science, you probably want to go out of your way to eliminate contamination like that. Guessing about the possible chemical interactions of a secret proprietary substance under extremely low temperature and pressure is not a fun way to get your doctorate.
- ripperoni 3y agoYes, a huge factor is the outgassing of materials. While just solidifying and a long time after that, it gases off solvents and other small particles that you constantly have to pump away and that raise your pressure noticeably. In cryogenic environments this is less of an issue because things are more sticky, but in a room temperature apparatus targeting ultra high vacuum this is troublesome or impossible.
- phkahler 3y agoMaybe we should "just" build this in space. ;-)
- wiml 3y agoSpace is far, far too hot at 3 Kelvin, and most of it is too high-pressure, too.
- COGlory 3y agoThis sounds miserable!
- javajosh 3y agoI think I'll bookmark this comment and come back to it the next time I feel annoyed by tracking down a software bug.
- YakBizzarro 3y agoOh yes, way too familiar situation... dry cryostats were really a game changer. Apart of the incredible advantage of not having to refill them frequently and on Sunday, they are much more reliable and without any cold seal
- jerf 3y agoAlphaPheonix on YouTube has an excellent and informative video of a very similar process of testing seals, though the vacuum is pulled for a different reason: https://www.youtube.com/watch?v=VD69crOFx10 https://www.youtube.com/watch?v=VD69crOFx10
- throwaway290 3y agoBased on you description... Couldn't you just go so cold He goes superfluid so you find all the leaks right up?
- dreamcompiler 3y agoNot my area, but I know He is very expensive (as is getting it cold enough to make He4) so using He4 as a leak detector is likely going to blow the entire experiment's budget.
- willis936 3y agoA good friend of mine made a leak checker from scrap parts while I was working at a university lab. It was just for a UHV system (no cryo). I wish I still worked with him. The whizz sound leak checkers make is good fun.
- NotYourLawyer 3y agoHa, I also remember using a helium canister to look for leaks in a vacuum system in the lab. At least ours wasn’t at cryo temperatures though. Sounds like a real pain.
- latchkey 3y agoCould you hear the indium scream as it was being squeezed out? https://youtu.be/mjiP5Q6g_aM?t=153 https://youtu.be/mjiP5Q6g_aM?t=153
- mensetmanusman 3y agoWas it ever an option to cycle the system with temporal atomic layer deposition chemistry to slowly build up aluminum oxide barriers?
- throwawaymobule 3y agoHelium leaks are especially annoying, because they'll brick most modern electronics for a day or so. iPhones especially. If they have a PCB oscillator, the gas will seep its way in there and cause a clock failure.