For the initial conditions, you must select either "gas law" or "constant area approximation" for the "Tank calculation model" attribute of the hydropneumatic tank. The constant area approximation selection exposes the "Volume (effective)," "HGL on," and "HGL off" fields. The gas law selection exposes the "Atmospheric pressure" field. These fields are primarily there to support the WaterCAD and WaterGEMS products, which can directly open a HAMMER model. They are only used to track the change in HGL/volume for EPS simulations, which typically aren't used in HAMMER. A transient analysis typically begins with a steady state simulation, which only considers the "HGL (Initial)" and "volume of gas (initial)". This is because a steady state simulation is a snapshot in time, so the head/volume are not changing. So in most cases, it does not matter which tank calculation method you choose. You will likely want to select "gas law" for simplicity, but additional information on both approaches is provided below.

• Constant area approximation: This method approximates a hydropneumatic tank by using a tall, thin tank whose water surface elevation approximates the HGL in a hydropneumatic tank. The HGL on and HGL off fields represent the maximum and minimum hydraulic grade lines within the hydropneumatic tank (i.e. when an associated booster pump would turn on or off). An approximate diameter is computed based on the effective volume of the hydropneumatic tank so that the tank cross sectional area multiplied by the distance between HGL on and HGL off gives the same volume as the hydropneumatic tank.
• Gas Law: This method uses the ideal gas law, PV=nRT, to compute new hydraulic grades as liquid volume changes in the EPS simulation (nRT is assumed to be constant). The initial liquid volume is subtracted from the total tank volume to find the gas volume. The physical "elevation" is subtracted from the initial HGL to find the gauge pressure. The atmospheric pressure is added to the gauge pressure to get absolute pressure, which is used in the ideal gas law equation.

Both methods typically yield similar results within the "effective" control range, but the gas law is technically more accurate.