How to measure liquid helium level in a dewar?

Answer 1

Dip a glass rod in distilled water so that it is wet. Dip the rod in the dewar flask and take it out after a few seconds.

Measure where the ice forms on the rod. Make sure the rod is clean and you use distilled water so you don't contaminate the He.

Answer 2

The most simple way I know of is to connect several one thousand ohm, 1/8 or 1/4 watt, carbon-composition resistors in series, in a straight line. Lower this assembly into the Dewar. Pass a weak current (a few milliamperes) from a constant-current power supply through the chain and observe the voltage drop across it. The resistors submerged in liquid helium will have large differences in resistance compared to the resistors in the helium gas above the liquid. Calibrate the resistor chain by measuring the resistance of the chain at various fill levels and use this data to determine the approximate depth of the liquid.

There are more exotic, more accurate and more expensive commercial transducers, such as ultrasonic or radio-frequency depth transducers, that will work if you need to know the level accurately.

The resistor-string method described can be used to construct a device that will automatically re-fill the Dewar when the liquid gets too low or sound an alarm to alert the user to manually re-fill.

If you just need to know when the liquid level drops below a certain point, a single resistor at the proper depth will provided that information. There is a huge change in resistance (and the rate at which heat is dissipated by the current flowing through it) when a carbon composition resistor is submerged in liquid helium versus when it is not.

Answer 3

The guy about the thin metal rod is right if your talking about an open dewar. If its a pressure dewar than you really cant measure liquid just the pressure that its creating

Answer 4

The usual way is to use a special dip stick. The stick is a thin metal tube. At the top it widens to about the size of a quarter, and a sheet of rubber is stretched over the open end.

The boiling liquid at the bottom of the dipstick makes the rubber vibrate. As you lift it when you hit the surface the vibration fequency changes dramatically.