How can I best determine whether my liquid nitrogen dewar tank (for storing frozen cells) is failing

The rate of loss of nitrogen seems high, but of course the weather is warmer. I do not see leakage, ice, or anything. The upper part of the tank feels very cold (e.g., if I place my hand on the part of the tank that slopes up towards the neck). I switched to a different cover (I had a second cover that hasn't been used in years). But so far I am not sure this is making a difference. I am realizing that the tank has been being overfilled. I didn't fully understand your article, but I am worried about O-rings or something along those lines.

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