The amount of drawdown capacity is determined by a formula known as Boyle's Law, and February's “Tech Topics” covers this formula and how to apply it to real-life conditions.
Last month, we said that the term drawdown refers to the amount of useable water that can be drawn from a hydro-pneumatic tank from the time the pressure switch cuts out, turning the pump off, until the pressure switch cuts back in, turning on the pump. The amount of drawdown capacity is determined by a formula known as Boyle's Law. Simply put, Boyle's Law states that as the volume of the air cushion in a tank decreases, the pressure of that air cushion increases.