Before revision: Assuming that the temperature is equal for both cases, the above equations can be solved for the water pressure/volume relationship in the tank: Tanks are generally specified by their total volume Vt and 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.
After revision: Drawdown is more simply expressed in absolute pressures:Assuming that the temperature is equal for both cases, the above equations can be solved for the water pressure/volume relationship in the tank: Tanks are generally specified by their total volume Vt and 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.