Level Indicators

There are a number of ways to determine the level in a dewar of liquid nitrogen.  It cannot be done visually because of the fog that is always present above the liquid.

Manual - A simple wooden dipstick.  Listen for the sizzle.  Liquid will not be visible on the stick upon removal.

Pressure tube - A stainless steel tube is placed with the open end near the bottom of the dewar.  The top end is connected to a pressure sensor. The bottom of the tube is notched.  A resistor is soldered/braised to it and situated inside the tube with a wire running up the inside through the top.  The purpose of the resistor is to create bubbles that keep the tube full of gas.  There is a switch for low and high power to the resistor so that the power can be turned up while filling the dewar for a more accurate reading.  The pressure sensor can be connected to an automated system.  This is what Alcor uses.

Capacitance sensor - A metal tube is open at both ends and the change in capacitance of the tube is measured with complex electronic circuits.  These are commercially purchased systems.

Temperature probe - The probe must have a special heated tip.  It's not actually measuring temperature, because the temperature in the liquid is exactly the same as the temperature of the gas above the liquid.  Instead is measures thermal conductivity, which is much higher in the liquid.  This is accurate for low and high level indicators, but does not give any information about incremental levels.

Float - A float is mechanically connected to a meter placed outside the dewar.

Scale - The entire dewar is placed on a digital scale.  The scale is connected to an automated system that converts the weight to a level indication. A reset feature is included which must be used anytime something is added or removed from the dewar which would change its empty weight.  The level must be manually verified at that point.  This is more practical for smaller dewars.