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
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?