LVDT Linear Position Transmitters Serve as Ultra-precise Level Sensors

Sensors Insights by Karmjit Sidhu

LVDT linear position transmitters serve as critical components in control feedback devices used for gauging tank levels to prevent overflow conditions. Storage tanks are used in a variety of industries to hold liquids, compressed gas, oil, water, and other medium for short or long-term storage. For example, in the shale industry, water and chemicals are held in storage tanks to be used in the drilling process.

Depending on liquid media, storage tanks can vary in height from 1 to 100 feet. To prevent spillage when media fills the tank, a control feedback device gauges the level and slows down the flow when media reaches the final 4 to 8 inches of the tank to prevent overflow.

Because of its ability to measure small displacement changes from a few millimeters to 1 meter deep and operate in harsh environments, the LVDT linear position transmitter is the ideal level sensor for measuring liquid level changes and providing feedback to the gauging control system. Radar level and ultrasonic technologies cannot provide the same level of precise measurement output, especially if foaming occurs during tank fill or reflective particles are present in the liquid.

In this application, LVDT transmitters are configured with a float located at the top of the tank. The float enables the LVDT level sensor to travel in small distances to precisely measure liquid as it fills up near the top of the storage tank. The float is coupled to a nonmagnetic stainless-steel rod attached to the high-permeability, armature core of the LVDT (see diagram 1).

Diagram 1: 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.
Diagram 1: 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.

The linear position transmitter senses the position of the core and, therefore, the level of the float. Sensitivity to the change in level depends on LVDT length. While LVDT level sensors are used to measure level changes from 0 to 2, 12, or 20 in. (50, 300, or 500 mm), shorter lengths offer the greatest sensitivity.

In operation, as water level changes, the float moves up or down, raising the LVDT core along with it. A threaded stainless-steel rod, protruding from the other end of the core, carries two jam nuts that 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. Electronics of the LVDT are hermetically sealed inside a stainless-steel housing for protection against water and other environmental elements. A Teflon liner inside the bore minimizes any friction to core motion. As there are no springs to fatigue or parts to wear out, the LVDT is nearly friction free to offer infinite resolution and long-term operation.

In storage tanks containing harsh and high temperature media such as acids, brine, chlorinated water, and/or liquid hydrogen or nitrogen, new microelectronics enable LVDT electronics to be partitioned and remotely located in a secure housing away from harsh conditions. New construction materials also enable the LVDT transmitters to measure liquid levels in radioactive heavy-water tanks, high-temperature liquids, corrosive liquid levels, sealed tanks, and in a variety of extreme environments. For example, all nuclear resistant materials can replace stainless steel for radioactive media, nickel alloys for radiation resistance, and Teflon for chemicals.

LVDT transmitters also are available in agency-listed versions for use in Class 1, Div. 1 or 2, or Zone 2 hazardous locations. As a result, the LVDT position transmitter is gaining more usage in the chemical industry, petroleum refining, nuclear, power generation, process control, and oil field industries for various tank-level applications.

Signal processing electronics of the LVDT transmitter convert liquid level changes into pre-calibrated 4- to 20-mA output signals for easy integration with PLCs, digital indicators, host computer-based data processing, and QC data collection systems. Using position feedback from the LVDT Transmitter, the gauging control system can either stop or restricts product flow or alert an operator that the tank is close to capacity. Most storage tanks are required to have some form of overfill protection as spillage can not only lead to waste but cause damage to the environment and expensive clean ups. One example of a viable component is seen in fig. 1.

The HSIR 750 Series ¾-inch (19-mm) diameter, loop-powered 4-20 mA LVDT position transmitters from Macro Sensors are typically used for tank gauging applications. Constructed entirely of TIG-welded stainless steel and hermetically sealed against hostile environments to IEC standard IP-68, They can be installed in hostile environments while the loop wiring connects to remote indicators and/or controllers located in environments safe from hostile conditions.

The HSIR 750 Series ¾-inch (19-mm) diameter, loop-powered 4-20 mA LVDT position transmitters from Macro Sensors are typically used for tank gauging applications. Constructed entirely of TIG-welded stainless steel and hermetically sealed against hostile environments to IEC standard IP-68, They can be installed in hostile environments while the loop wiring connects to remote indicators and/or controllers located in environments safe from hostile conditions.

About the Author
Karmjit S. Sihdu is the Vice President of Business Development at Macro Sensors. He has over 25 years experience in pressure and position sensing sales and engineering and is currently completing his PhD in Material Science. Karmjit can be reached via email at [email protected].

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