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  1. Standard member sonhouse
    Fast and Curious
    16 Jan '14 21:38 / 1 edit
    In my job we have high vacuum systems using mechanical roughing pumps and high vac pumps like turbo pumps and cryo pumps. Cryo pumps work by taking an array of activated charcoal down to around 10 degrees Kelvin. But I just noticed for the first time, I replaced a cryo pump and we first have to evacuate the inside of the pump to around 50 microns from atmosphere, 760 torr. So we get down there, the system performs a ROR (Rate of Rise) check, to see if the vacuum will stay somewhere around 50 microns, not rising higher than about 100 microns in a two minute period.

    When it passes that test, the helium cryo compressor turns on and starts its little refrigeration bit inside the cryo pump.

    So I see the temperature readout which is in Kelvin, starts at around 299 degrees K and starts lowering as the helium does its magic.

    So when it gets down to around 273 degrees K, 0 degrees C, 32 degrees F, freezing point of water, the vacuum reading which had been hanging out at around 50 microns, starts dropping and eventually reaches about 20 microns.

    So here is the question: I know that is WAY to hot to be actually trapping gasses in the charcoal array but it has gotten to the freezing point of water.

    So when the vacuum reading goes to 20 microns, a bit lower than half its previous pressure, is it ok to assume the only thing being pumped at that point is water vapor, humidity in other words.

    That being the case, if it drops from 50 to 25 microns, say, would it be correct to say the humidity level was around 50% inside the pump when the pump started cooling down? It would seem to me to be so because it seems to have lost about half its original pressure and at that point, like right now the cryo is down to 180 degrees K and no more water is there to be frozen out and it stays at 20 microns, till it gets down to about 80 degrees at which point it will start freezing out the first gas, nitrogen at around 78 degrees K. Till it reaches that point, it will still read 20 microns because water vapor has frozen out leaving, I presume, dry air inside the pump, the vapor having frozen out on the charcoal.

    So from say, 200 degrees K to 80 degrees K, it stays around 20 microns, maybe a drop from good old Boyle, but not too much less, till it starts freezing out N2 first, then O2 and so forth, the gasses check in but they don't check out, stuck to the charcoal array which at the end is around 10 to 13 degrees K and at that point becomes a real pump and you can get down to the minus 8 range which one of my machine reaches regularly or low 7 range on the machine I am working on today.

    So would I be safe in saying the humidity of the original atmosphere inside the cryo pump, starting at 50 microns and working its way down to 25 microns, is it safe to say the relative humidity had to have been around 50 %?

    Or is my head just screwed on backwards as normal
  2. 16 Jan '14 22:01
    Originally posted by sonhouse
    In my job we have high vacuum systems using mechanical roughing pumps and high vac pumps like turbo pumps and cryo pumps. Cryo pumps work by taking an array of activated charcoal down to around 10 degrees Kelvin. But I just noticed for the first time, I replaced a cryo pump and we first have to evacuate the inside of the pump to around 50 microns from atmos ...[text shortened]... ve humidity had to have been around 50 %?

    Or is my head just screwed on backwards as normal
    Assuming it is water vapour freezing out..

    There are different measures of humidity.

    If you are talking about humidity as a % then you are talking about 'relative humidity'
    which is the "ratio of the partial pressure of water vapor in an air-water mixture to
    the saturated vapor pressure of water at a prescribed temperature".

    In your example, if the saturated vapour pressure was 50% water vapour 50% air,
    then prior to freezing the sample would have been at 100% humidity.

    Absolute humidity is given in mass per unit volume of water vapour.

    http://en.wikipedia.org/wiki/Humidity