LN2 in the cold trap dewar is necessary for fast vacuum recovery. It is very useful, especially when you need to change grids and/or do a cryo session. However, if you are working with a negative stain or plastic section - a dry grid, the scope can still run without LN2 cold trap. Unless you are running automatic, long overnight session, you should always use it.
Generally, when you first put LN2 dewar to its stand, you want to be SLOW, or the strong evaporation will make LN2 spilling.
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