How do you know when the liquid nitrogen is low (DT-GCMS)

[breitung2]

I have a GL Sciences Optic 4 inlet and cryotrap. It runs on ~35 psi liquid nitrogen, but there's some level of liquid which is not enough to cool the trap to -140. The tank pressure gauge will read 35-40 psi, and the liquid level gauges rarely work.

Do you have a method or tip for knowing when you're about to have too little to run your trap?

I've consider a continuous read scale but am hoping there are simpler or better options.

[Steve Reimer]

On my 200 liter Dewars I use the rock and shake method to check the fill level. On my outdoor 2000 liter tank the gauge works.

[James_Ball]

On my 200 liter Dewars I use the rock and shake method to check the fill level. On my outdoor 2000 liter tank the gauge works.

The gauge was always terrible on ours too, so I would rock it and tilt it at about 15-20 degrees and if it still had liquid you could feel it sloshing back and forth, after a while you can learn about how much is left in them. If it doesn't slosh then you know it is pretty much only gas left.

If the humidity is high, you can also judge by the frost line at the bottom of the dewar, though there really shouldn't be one if it is insulated properly.