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It's getting late in the lab and I want to head home as fast as I can. The only problem is, that there's a far-infrared bolometer operating at a temperature of 1.8K. This low temperature is attained by pumping away the helium gas from the LHe reservoir in the bolometer.

Now normally we would vent the bolometer with He gas from the dewar, stop pumping, refill the liquid helium and be done for the day, but today I'm too tired to go to all that trouble. However I wouldn't like the entire reservoir of helium to evaporate overnight leaving me with a warm bolometer in the morning.

Therefore I'm left with two options.

Keep pumping as hard as possible, keeping the helium under near-zero pressure and at 1.8K

or

Reduce the pumping speed, bringing the helium vapor pressure to about 500mbar.

Which of these two options is most likely to leave me some liquid helium in the morning?

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    $\begingroup$ Heavily dependent on too many factors. I will say that rushing as you are, or feeling under any time pressure, is a major factor in accidents across all walks of life. Keep that in mind. If you kill the equipment because you are rushing, that will not be a good thing. $\endgroup$
    – Jon Custer
    Sep 14, 2016 at 20:04
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    $\begingroup$ @JonCuster don't take this question too seriously. We had bolometers run out of helium several times, it is merely a nuisance that the guard vacuum needs to be restored before refilling them if they become warm. The narrower question is as follows: my colleagues widely hold the belief, that bolometers operating at 4.2k hold liquid helium for longer. However, I believe (according to some tabulated properties of He) that He at 1.8K evaporates at a slower rate. So maybe if the He is already at 1.8K, it's better to leave it at that? $\endgroup$
    – LLlAMnYP
    Sep 14, 2016 at 21:43
  • $\begingroup$ I spent yesterday afternoon in a corporate electrical safety committee going over several recent incidents where people got shocked. Earlier this year we had $100,000 of damage done to equipment, holding up key projects. Go read any annual Accidents in North American Moutaineering. Time pressure, and your joking attitude towards it, is a root cause in accidents that injure and kill people. When feeling time pressure, alarm bells should go off in your head. $\endgroup$
    – Jon Custer
    Sep 15, 2016 at 12:46

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