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Natural Science Forum / Physics / Research / February 2008helium gas flow cryostat tips?
Hi, I tried this in s.p.condensed-matter, but maybe I'm better off here because this group is actually active. Here we go... So if you are doing microscopy though a gas flow-cryostat and it looks like your subject is below the surface of a pond, it likely means you have liquid helium flowing over your sample when you should really only be getting cold gas helium. I think the best way to avoid this is to choke off the helium flow a little and turn up the resistive heat slightly, to make sure you boil off the incoming helium before it gets into your optical path. I tried this today and built up enough pressure to blow out the sample insert out of its position (luckily, nothing was damaged). How do I avoid this? My guess is that I need to make sure to get the helium flow down to a minimum before I go heating up the element. Does anyone have any other cryo tips? There's precious little literature on this subject, so I'm making an appeal to the great usenet. snip-- >So if you are doing microscopy though a gas flow-cryostat and it looks >like your subject is below the surface of a pond, it likely means you >have liquid helium flowing over your sample when you should really only >be getting cold gas helium. what is the cryostat ? ie are you taking cold gas off a dewar and putting it into the cell? or introducing liquid from a dewar into an intermediate chamber, evaporating it there before the gas goes into the optical cell ? i think you will have a factor of 800 or so in volume expansion if you turn up the heat to boiling. what it your helium consummption ? with that and the latent heat of evaporation (20 J/g if i recall) you should be able to calculate a heat input ... >I think the best way to avoid this is to choke off the helium flow a are you introducing liquid ? have you a needle valve ? expansion valve ? is the cell at atmospheric pressure or is it pumped down ? >little and turn up the resistive heat slightly, to make sure you boil >off the incoming helium before it gets into your optical path. I tried sounds like you need less heat, and a pressure gauge in the cell >this today and built up enough pressure to blow out the sample insert >out of its position (luckily, nothing was damaged). heehee >How do I avoid this? My guess is that I need to make sure to get the >helium flow down to a minimum before I go heating up the element. watch your thermometers, when there is no liquid in the cell the thermometer (you have one ?) will respond very quickly to heat input; when there is liquid it will hardly change at all and will sit at 4.2K (at atm. pressure) >Does anyone have any other cryo tips? There's precious little >literature on this subject, so I'm making an appeal to the great usenet. see if you can get the Lounaasama book or the cornell book on experimental methods in low temperature fizix the first reference is primarily for 1K or lower, but has all maner of data for crypgenics. sidd >> So if you are doing microscopy though a gas flow-cryostat and it looks >> like your subject is below the surface of a pond, it likely means you [quoted text clipped - 4 lines] > an intermediate chamber, evaporating it there before the gas > goes into the optical cell ? -cut- > are you introducing liquid ? > have you a needle valve ? > expansion valve ? > is the cell at atmospheric pressure or is it pumped down ? Well, from what I can tell (I inherited this lab from a doctoral student who ran off after giving me only a cursory explanation of how the lab worked), we're working with liquid helium that should evaporate before it hits the cell. Liquid helium enters the cryostat via a coaxial transfer arm and needle valve. We pump the return helium out via the same coaxial arm (this time on the outside), back into a recycling system. Somewhere along the line, I think the helium is supposed to evaporate---and before it hits the sample. There is no intermediate chamber that I know of. Is this possible? Or do I think I have a cryostat design that was never invented? This is an old off-the-shelf Oxford system, so it shouldn't be anything crazy. The cell is at atmospheric pressure flooded with warm helium, but the flowing cold helium should feel the under-pressure from the helium recycling system pump downstream. Sometimes the pumping is strong enough to overpower the evaporation pressure of the boiling helium, and introduces a vacuum in the dewar. So to your bullet points: yes, yes, not that I know of, and it varies. > i think you will have a factor of 800 or so in volume expansion if you > turn up the heat to boiling. what it your helium consummption ? > with that and the latent heat of evaporation (20 J/g if i recall) > you should be able to calculate a heat input ... Without heat, I can put away roughly 1 cubic meter of gas helium per hour. We measure this directly through the recycling system. So that's about 180 grams, or 3600 J, right? >> little and turn up the resistive heat slightly, to make sure you boil >> off the incoming helium before it gets into your optical path. I tried > > sounds like you need less heat, and a pressure gauge in the cell The first part is easy, second part not so much. I do at least have a 0.05 bar over-pressure safety valve, although it seems like that valve is cut-off from wherever the pressure builds when the helium boils, hence... >> this today and built up enough pressure to blow out the sample insert >> out of its position (luckily, nothing was damaged). > > heehee >> How do I avoid this? My guess is that I need to make sure to get the >> helium flow down to a minimum before I go heating up the element. [quoted text clipped - 3 lines] > input; when there is liquid it will hardly change at all and > will sit at 4.2K (at atm. pressure) There was definitely lag between changing the heater voltage and the temperature readout. It wasn't at 4.2K though, more like 6K, which I think just means the sensor isn't positioned as well as I would like it to be. >> Does anyone have any other cryo tips? There's precious little >> literature on this subject, so I'm making an appeal to the great usenet. [quoted text clipped - 3 lines] > the first reference is primarily for 1K or lower, but has > all maner of data for crypgenics. I've only read Ekin "experimental techniques for low-temperature measurements", which seemed a little too general. I will see if I can get a copy of Cornell or Lounaasama. Thanks for the help. From what I can tell, I'm not that far off to try reducing flow and being more careful with the heater...well, back into the lab I go... >who ran off after giving me only a cursory explanation let im get away, did you? find a fone # is my first step >Liquid helium enters the cryostat via a coaxial transfer arm and needle >valve. We pump the return helium out via the same coaxial arm (this >time on the outside), back into a recycling system. snip-- >is no intermediate chamber that I know of. Is this >possible? oh yes >This is an old off-the-shelf Oxford system, so it shouldn't >be anything crazy. well...i might not go quite that far... >Without heat, I can put away roughly 1 cubic meter of gas helium per >hour. We measure this directly through the recycling system. So that's >about 180 grams, or 3600 J, right? mmm i think the 20J/g is for just latent heat to change he4 from liquid to a (very dense) gas.. then you have the expansion to atmospheric pressure and temperature which eats many joules so i think you have to put that in as well snip-- >There was definitely lag between changing the heater voltage and the >temperature readout. It wasn't at 4.2K though, more like 6K, which I >think just means the sensor isn't positioned as well as I would like it >to be. at 6K i tink you are to the right of the (normal) liquid gas critical point at 5.sumpn in the P-T plane sidd |
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