 Welcome to this WISC Online Module on Displacement Level Sensors. One way of measuring the level of a liquid is to use a mechanical linkage that has a float that rides on the surface. The mechanical linkage can be connected to a scale which shows the measurement level or to an electronic power source which produces a voltage proportional to the depth of the liquid. The disadvantage of this measurement device is that it cannot be used to measure the top layer of the heavier of two different liquids in a tank. The bulb will float on the top of the lighter liquid. When two liquids that don't mix together are in the same container, it is called liquid-liquid interface. They include the following fluids, oil and water, specific chemicals, and slurries. To make a liquid-liquid interface measurement, a displacement level sensor is used. This sensor is also capable of measuring the level of one liquid. The sensor consists of a probe suspended from a spring scale. The probe, called a displacer, is weighted so that it actually sinks in the measured fluid. However, the displacer has buoyancy, meaning that the liquid rises over a portion of it. This process is based on Archimedes' principle. A body immersed in a liquid is buoyed up by a force equal to the weight of the displaced liquid. The displacer displaces the same amount of liquid, even when the depth of the liquid changes. This is similar to a boat, which displaces the same amount of water regardless of the water's depth. As the level of the liquid in the tank changes, the displacer moves by the same amount. The connecting rod slides into or extends from the enclosure on which the scale is mounted. In a liquid-liquid interface application, the lighter liquid cannot support most of the weight of the displacer. Therefore, the heavier liquid, which is being measured, causes the displacer to float. Instead of using an analog scale or a digital readout, a transmitter can be used to produce electrical signals for remote readings or to control the loop. One place where a liquid-liquid interface is used is a fuel storage tank where water condensation occurs. Congratulations! You have completed this learning object, Displacement Level Sensors.