When to refill CO2 tank?

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When the pressure starts to drop that means there is no more liquid CO2 in the tank, only compressed gas.

Counterintuitively when the pressure in the tank drops there will be an increase in the gas flow through the needle valve and an increased bubble count.

Depending on how good your regulator is you may see this increased bubble count after the pressure has dropped a little or only after it has dropped a lot.

In any case I would refill it when you notice the pressure starting to drop.