Measuring liquid levels with LVDTs

Feb. 23, 2006
LVDTs sometimes work better than pressure sensors for gauging tank volumes.

Edward E. Herceg
Manager
Macro Sensors
Pennsauken, N.J.

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 the stainless-steel body of the LVDT to a flat hook. The hook hangs the level-sensor assembly over the tank edge.

Core position is sensed using magnetic induction. In fact, the electronics of the LVDT are hermetically sealed inside its stainless-steel housing. A Teflon liner inside the bore minimizes any friction to core motion. There are no springs to fatigue or parts to wear out. And, LVDT transmitters come in agency-listed versions for use in Class 1, Div. 1 or 2, or Zone 2 hazardous locations.

This diagram shows the LVDT level sensor attached to the side of a tank using a flat-hook assembly. The high-permeability core follows the position of the stainless-steel float. The LVDT electronics sense the core position using magnetic induction. Output is a 4 to 20-mA instrumentation current loop corresponding to the position of the core in the LVDT body.


MAKE CONTACT
Macro Sensors, (856) 662-8000,
macrosensors.com

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