the "drawdown" (ΔV ), which is the amount of water the tank will eject as the tank pressure goes from Phi to Plo, which are established by the pressure switch: The reason for the charging pressure can now be seen: The larger the charging pressure, the larger the drawdown. However, a charging pressure above Plo will not allow the pump to turn on when the water pressure is below Plo, so it is kept a bit below Plo. Another important parameter is the drawdown factor (fΔV), which is the ratio of the drawdown to the total tank volume: This factor is independent of the tank size so that the drawdown can be calculated for any tank, given its total volume, atmospheric pressure, charging pressure, and the limiting pressures established by the pressure switch. See also Pressurizer (nuclear power) References Bibliography External links Plumbing Pressure vessels