From where are ullages measured on tankers?

Ullages on tankers are measured from an above-deck datum, such as the top of the ullage pipe. This position is standardized, as it provides a consistent reference point for measuring the amount of liquid cargo in the tanks. Using the top of the ullage pipe helps ensure that measurements are not influenced by varying liquid levels or tank shapes, which can differ from one tank to another.

Measuring from the tank ceiling, tank top, or fire point would not provide the necessary accuracy and consistency that the top of the ullage pipe offers. The tank ceiling can vary based on the tank design, the tank top could change with loading and unloading, and the fire point is not a relevant measure when it comes to determining liquid levels accurately. Therefore, using the ullage pipe's top ensures that the crew has a reliable and universally accepted point to assess the ullage across the vessel.