In a typical application, a differential pressure transmitter is installed near the bottom of the tank, where it measures the pressure differential between the weight of the liquid in the tank and atmospheric pressure. As the liquid level varies, so does the pressure differential and the corresponding signal from the transmitter. Atmospheric pressure variation affects both sides of the transmitter equally and therefore produces no net effect.

To measure discharge of liquid from the tank, a turbine flowmeter is normally used at the tank outlet. As liquid is discharged, the flowmeter produces a pulse output proportional to the flow rate. The firmware uses both pressure and output flow readings to calculate tank volume changes.

The method of calculating volume changes takes into account the change in tank level as measured by the level transmitter, the total volume pumped as measured by the flow meter, and the total volume manually removed as entered by the operator, all measured over a 24-hour period from the specified contract hour.

The firmware is only contained in the ROCPAC memory module; it is not available in the FlashPAC module.