This simple, yet effective dipstick gives an accurate indication of liquid helium depth. It is made from stainless steel with a fine rubber diaphragm, held in place by an O-Ring over the cup shaped upper end of the dipstick.
The thermal gradient set up in the tube leads to oscillations, which are felt by the vibration of the membrane. The frequency of the oscillation with the lower end in liquid helium is noticeably lower than that when it is in cold gas, allowing the liquid level to be determined easily (in most cases). The dipstick should not be left in the cryostat when it is not in use because it introduces a large amount of heat.