Hydrostatic level transmitters, or pressure level transmitters, use the pressure exerted by a resting body of fluid in a container to determine how much of it is in the container. Basically, the more force the liquid exerts on the container, and thus the sensor, the more liquid there is present in the container.
This is one of the most common types of fill level detection technologies. However, to ensure accuracy, the density of the liquid being measured must remain consistent. As the specific gravity of the liquid increases, the pressure it exerts on the transmitter per cubic inch goes up too.
So, these types of level transmitters are sometimes less useful for applications where a liquid might have variable density because of impurities or other factors.