Typically, liquid level measuring devices, such as those made by AST, use the column height times the liquid density, which is pressure, to measure relatively large liquid levels. If the geometry of the container and the density are uniform, this approach gives good level measurements, but normally in feet or meters, not inches or centimeters. There are some applications, typically overflow measurements, requiring measurement of a few inches of water level. Typically using an LVDT with a float, we can measure liquid levels from a few inches (2-3 cm) up to 20 inches (50 cm) or so. There are other ways to use an LVDT, but this is by direct float and we have getting a lot of requests for this approach from potable water suppliers where they want to know when their storage tanks are getting full or close to full.

They want them for liquid level applications where overflow pumps have to come on to start pumping out excess water. In some cases, the main water pumps must be up and running before they open the valves to admit stored water. There are a number of applications for this approach. We have had requests for this type of overflow measurement in waste water treatment plants. The question often posed is: "Why an LVDT rather than some other technologies?" Typically an LVDT is being requested because they have foam or detritus on the surface of the liquid, which can create errors for ultrasonic sensors. Some radar-based level sensors can be tricked by this surface material, and many of the radar sensors don't have the resolution to do the small increments required for small levels.

One of the big advantages of an LVDT in general is the ability to separate the core from the coil assembly in a pressurized or hostile environment and use the coil in a unpressurized area. Typically, the LVDT used in the small level measurements has a hermetically sealed coil assembly, so it is unlikely to be affected by splash or immersion in the liquid or from weather. Because their housing is stainless steel and the floats are all stainless steel, LVDT based water level measuring systems don't have many parts and there isn't a lot that can go wrong, unless the media contains glue or adhesives.

This level measuring system uses direct or "unrestrained" floats, depending strictly on the buoyancy of a float. The size and buoyancy of the float limits the range of level that can be conveniently measured. There is another method of using a float with LVDTs, which use a long skinny float which develops a buoyant force as a function of the liquid level. This force is applied to a low rate spring coupled to an LVDT that measures its compression. Thus the measurement is buoyant force as opposed to measuring the level of the float itself. The float doesn't move very much but the buoyancy is the key. This approach can only give useful measurements as long as the float is not submerged, which is determined by the geometry of the float itself. So this liquid level sensing method is also useful only for relatively small ranges of level measurement.

Another reason for using an LVDT in small level applications is the possible requirement for a sensor that is classified for operation in a hazardous location. For example, in a waste water plant, the presence of flammable sewer gases might mandate a hazloc-rated sensor.

In a food processing plant that uses food ripening gases like ethylene, the flammability and explosive hazard would certainly require a hazloc classified system. Macro Sensors makes several lines of hazloc rated LVDTs that can be used in a liquid level measuring system.

In some applications, it is possible to use an LVDT-and-float-based liquid level sensing in combination with the more common column height pressure sensors to offer a hybrid system for liquid level measurement. The low pressure sensor is used for tank capacity and other quantative measurements, while the LVDT-based system is used to control overfilling. This combination of level sensors available from a single source is a good marketing tool.