A person lowers a float down into a storage vessel using a flexible measuring tape, until the tape
goes slack due to the float coming to rest on the material surface. At that point, the person notes
the length indicated on the tape (reading off the lip of the vessel access hole). This distance is
called the ullage, being the distance from the top of the vessel to the surface of the process material.
Fillage of the vessel may be determined by subtracting this “ullage” measurement from the known
height of the vessel.