Measuring liquid levels with LVDTs

LVDTs sometimes work better than pressure sensors for gauging tank volumes. A common way of measuring the level of liquid in a tank places a low-pressure sensor at the bottom. The pressure readout is the product of the liquid column height times the density of the liquid. As long as liquid density remains constant, the measured pressure is directly proportional to the liquid level. This method works with all low-vapor-pressure, low-viscosity liquids and, of course, water. The difficulty lies in inserting the pressure sensor at the right location in the tank to make the measurement. The sensing of lowdensity liquids or of small level changes may require special, lowpressure transducers that are expensive and hard to find. The use of a float attached to a nearly friction-free linear-variable-displacement transducer (LVDT) offers a reasonable replacement. LVDTs can measure liquid-level changes from a few inches to several feet. In the typical LVDT level sensor, a stainless-steel float coupled to a nonmagnetic stainless-steel rod is attached to the high-permeability armature core of the LVDT. The 4 to 20-mA loop-powered LVDT position transmitter senses the position of the core and, thus, the level of the float. Sensitivity to the change in level depends on the length of LVDT. The most common units measure level changes from 0 to 2, 12, or 20 in. (50, 300, or 500 mm) with the shorter lengths providing greatest sensitivity. As water level changes, the float moves up or down, bringing the LVDT core with it. A threaded stainless-steel rod protrudes from the other end of the core. It carries two jam nuts to adjust the position sensor output at the desired low water level. The nuts also prevent the core from falling out of the LVDT housing if the liquid level drops too low. A nonmagnetic clamp block attaches