What is the pre-charge pressure of the vessel in an accumulator?

The pre-charge pressure is the air pressure stored within the bladder of the vessel. The air in the vessel is compressed by the water pressure, which pushes the extra volume of water out of the vessel. AccuBoost accumulators require that the pre-charge pressure is 1.5 bar less than the pressure of the incoming mains.

In AccuBoost accumulator installations, the minimum pressure of the mains must be 2.0 bar so that the vessel can be set to 1.5 bar below (0.5 bar). If the vessel is set to 0 bar then the positive impact of the accumulator is negated.