This is still site unseen for me so bear with me......

When charging an expansion tank (non-diaphragm) I always thought all you have to do is drain it completely and refill it so there is a cushion of air? A tech at my work says he has to pressurize it with a bike pump. I have never heard of doing this but wanted to check here before I put a stop to this. I told him that for a diaphragm tank but he said you can do it with a non-diaphragm as well???

Its on a closed loop water tower if that matters at all.

The tank has no rating plate or anything on it.