How to Measure Liquid Nitrogen Levels
You should measure the level of liquid nitrogen in your tank at least weekly. The level should not be allowed to drop below 5 inches. Lower levels of liquid nitrogen (LN2) could allow some of the product you have stored to thaw. Levels below 2 inches are critical.
It's easy and quick to check your LN2 level. Just carefully lower the measuring device that came with your tank to the bottom of the tank and allow it to cool for a few seconds.
Then remove the device from the tank and a white frost will form allowing you to easily read the amount of LN2 remaining in the tank. If the humidity is low you might have to wave the measuring device around for a few seconds.