Q.What does pressure drop in cargo tanks mean and how to calculate it?

pressure drop within a tank can cause damage due to greater atmospheric pressure on the outside.

Cargo tank is normally tested to a positive pressure of 2500 mm of Water Column and negative pressure of -750 mm of Water Column.

How to convert mm of Water Column to bar ?

1000 mm of Water Column = 0.0980665 bar

For practical purposes we can take 1000 mm of water column = 0.1 bar