Level Transmitters

They aren't always installed below the level, but it makes sense to do so.

The pressure transducer measures the hydrostatic pressure of the fluid in the tank, which is of course related directly to the height or level of fluid.

In a washing machine for instance the pressure sensor that measures how much water has been added into the drum, is mounted above the water level to keep water out of the sensor. The measurement is still okay though as the air trapped in the pipe feeding the sensor, doesn't alter the pressure/water level measurement.

John.

Let's look at the practical side of things:

Here's a drawing of a differential pressure (dp) application where the transmitter is located at the middle or between the high and low tapping points. We're using impulse piping here (some call them "sensing lines").

Let's assume that both lines are filled with fluid from the tank. As long as the level in the tank is above the lower tap, we're okay. If the level drops below that, there's trouble. The fill fluid in the line drains out.

When the level goes back above the lower tap, no fluid goes into the impulse line, causing the transmitter to give an erroneous reading.

There is a multi-sensor hydrostatic probe called MTG (Multi-function Tank Gauge) that installs directly within a tank or cylinder that provides not only level, but mass, multi-point spot temperature, average product temperature, multi-strata density, average product density, free and entrained water, vapor pressure, vapor temperature, Mass flow rate, volume flow rate, and optional leak detection, atmospheric temperature, and atmospheric pressure.

The MTG is installed within the tank, usually within the gauge well for one to one compairison to the reference measurement. The MTG probe is bottom referenced, for density and mass measurements. However, for Level measurement; since it is a multiple sensor hydrostatic gauge, level is provided by the transducer accuracy and span between the level and the last crossed transducer, using the temperature at the last crossed transducer and the density of the strata just below the crossed transducer. Custody transfer accuarcy is provided.

The MTG is a single probe, single tank entry, single I.S. electrical connection for both sinal and power, and is self calibrating.