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
Products & Services
Linear Variable Differential Transformers (LVDT)
Linear variable differential transformer (LVDT) linear position sensors have a series of inductors in a hollow cylindrical shaft and a solid cylindrical core. These devices produce an electrical output proportional to the position of the core
Liquid Level Sensors
Liquid level sensors are used to detect liquid levels or interfaces between liquids such as oil and water or liquids and solids.
Liquid Level Switches
Liquid level switches are used to detect specific liquid levels or interfaces between liquids. These level measurements are indicated via an electrical switching action.
Level sensors are used to detect liquid or powder levels, or interfaces between liquids. There are two basic level-measurement types: continuous and point or multi-point.
Level gauges are industrial meters used to determined the level of a liquid in a fixed storage or process tank
Topics of Interest
Changes in temperature can cause subtle errors in LVDT readings. Designers rarely consider how changes in temperature might affect linear-variable differential transformers (LVDTs). Yet a temperature...
Section 3-1: Positional Sensors
Linear Variable Differential Transformers
The linear variable differential transformer (LVDT) is an accurate and reliable method for measuring linear distance.
LVDTs may not be new. But frictionless measurement and nearly infinite resolution never go out of style. A cutaway LVDT shows its primary and secondary windings. The core material is usually a 50/50...
This application note covers the Fluid-Trac® 3-wire liquid level sensor used as a replacement for a standard 33-240 Ohm resistive float sender, reed-switch tube sensor, or capacitive tube sender...
This application note covers the Fluid-Trac® 2-wire water level sensor used as a replacement for a standard 33-240 Ohm resistive float water level sender, reed-switch tube sensors, or capacitive...