A gauge reading is a level, not a volume
Manual gauging per API MPMS Chapter 3.1A is the reference method: a steel tape with a bob weight lowered through the hatch, read against a reference point. Innage measures liquid depth from the datum plate; ullage measures the empty space from the reference gauge point down to the liquid surface, used when the product is too volatile to tape. It also covers gauging free water at the tank bottom, where the water cut starts. The tape gives a level in inches. The capacity table turns it into a volume.
Automatic tank gauges take the same measurement on a loop
API MPMS Chapter 3.1B covers automatic tank gauges, or ATGs, for stationary aboveground tanks. An ATG measures innage or ullage continuously, without opening the hatch. It covers intrusive and non-intrusive designs, custody transfer or inventory control, and installation, commissioning, calibration, and verification. Data transmission is covered too. A gauge that is never verified is a guess with a serial number. The standard demands verification against a manual reference measurement. The payoff is a reading pulled from the office. The price is a gauge that can drift, silently.
Level becomes volume, then standard volume
The capacity table comes from tank calibration per MPMS Chapter 2. It converts level into observed barrels at tank temperature. Crude expands and contracts with heat, so observed volume is not the volume that gets booked. API MPMS Chapter 12 standardizes net standard volume calculations, including correction factors and rounding rules, so two calculators reach the same number. Temperature is Chapter 7, sampling Chapter 8, density and API gravity Chapter 9, sediment and water Chapter 10. Water cut is deducted from the gross volume. Book water as crude and the buyer pays for barrels that are not there.
Reconciliation is where the numbers meet. The morning gauge says the tank held so many barrels. The run ticket says the truck took so many at midday. The battery meter says another number still. They will not match to the barrel, only within the accuracy of the gauging method and the water cut. When the gap grows, check the capacity table, the gauge reference point, the temperature record, and the free water level. That discipline carries into crude hauling and custody transfer measurement and produced water hauling economics. Gauges that report into remote well monitoring and SCADA close the loop faster. If the gauge, the ticket, and the meter live in different places, start with an operations audit.