Level sensing technologies

There are many different ways to measure the level of products in industrial storage and process vessels. One of the most commonly used devices is the differential pressure transmitter (dp). A dp device actually measures the height of material in the vessel and its density. These two variables multiplied, result in the amount of pressure exerted on the diaphragm, which then can be transl...

06/01/1999


There are many different ways to measure the level of products in industrial storage and process vessels. One of the most commonly used devices is the differential pressure transmitter (dp). A dp device actually measures the height of material in the vessel and its density. These two variables multiplied, result in the amount of pressure exerted on the diaphragm, which then can be translated into an indication of level. Dps are relatively economical and easy to install. This 'comfortable' technology is fairly accurate and dependable when used to measure the level of clean liquids. However, density compensation is required for accurate measurements. New installations require additional piping and isolation valves that add to initial installation cost.


One of the simplest devices for measuring level is the float . Floats are classified by the type of position sensor (reed switch, cable-and-pot, magnetostrictive, and sonic or radar). Advantages to using floats are unlimited tank height, excellent accuracy (depending on the float type), and comparatively low cost. However, they are intrusive sensors. Additionally, these mechanical devices are subject to wear, corrosion, mechanical failure, and 'getting stuck.' Floats are subject to material buildup which can effect their weight and therefore, accuracy.


Application can narrow choice

Related to the float principle of level measurement is the displacer . Displacer technology is based on Archimede's principal. Although they have fewer moving parts than typical float devices, actual mechanical motion is limited. Displacers are frequently placed in external 'cages,' which can affect accuracy if the vessel/cage level is misaligned. Long-span displacement devices may be very expensive.

Sonic instruments determine level by measuring the length of time it takes for a sound pulse to return to a piezoelectric transducer after bouncing off the process material. For maximum accuracy, the transmitter must be mounted at the top of the vessel and positioned so the internal structure of the vessel will not interfere with the signal path. Sonic devices are noncontact and minimally intrusive. Dust, solvent vapors, foam, surface turbulence, and ambient noise effect accuracy. Elevated process temperatures can limit application.


Radar-based devices beam microwaves at the process material's surface. A portion of that energy is reflected back and detected by the sensor. Time for the signal's return determines the level. Technologies in use include:

  • Frequency Modulated Continuous Wave-FMCW is very accurate, ignores vapors, and is immune to changes in physical characteristics (except dielectric constant) of process materials. Applications include 'still,' but not turbulent, fluids. Cost is quite high in comparison to other technologies ($5,000-$10,000 per point).

  • Pulsed Time of Flight-PTOF is lower powered and lower priced. Due to its lower power, its performance can be limited by the presence of vessel obstructions, agitation, foam, elevated pressure, and low dielectric materials (Dielectric constant less than 2).

  • Time Domain Reflectometry (TDR)-Unlike FMCW and PTOF, TDR is an intrusive measurement that uses a rod or flexible cable to 'channel' the microwave pulse. It can measure normal (low K on top) interface levels in immiscible fluids. It is low cost, can measure long spans, and provides good performance in lower dielectric materials.


Some methods intrude

Radio frequency (RF), based on capacitance or admittance, can handle a wide range of process conditions. Process temperature and pressure are limited only by the material system of the sensing element. Level transmitters of this type sense the change of electrical impedance that occurs with the change of level on the sensor. RF devices ignore material buildup on sensor and work with all types of process material. It is an intrusive technology.



Author Information
Teresa Parris, marketing communications manager, and John Roede, application engineer, Drexelbrook Engineering Co., a supplier of level sensing devices.




The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers. Vote now (if qualified)!
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Maximize ROI with integrated control system approach; Microcontrollers vs. PLCs; Power quality; Accelerate and rewire IIoT; Traits for excellent engineers
HMI effectiveness; Distributed I/O; Engineers' Choice Award finalists; System Integrator advice; Inside Machines
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. As we know, PLCs aren’t the only option for making decisions in a control loop, but they are likely why you’re here.
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers. Vote now (if qualified)!
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Maximize ROI with integrated control system approach; Microcontrollers vs. PLCs; Power quality; Accelerate and rewire IIoT; Traits for excellent engineers
HMI effectiveness; Distributed I/O; Engineers' Choice Award finalists; System Integrator advice; Inside Machines
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. As we know, PLCs aren’t the only option for making decisions in a control loop, but they are likely why you’re here.
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers. Vote now (if qualified)!
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Maximize ROI with integrated control system approach; Microcontrollers vs. PLCs; Power quality; Accelerate and rewire IIoT; Traits for excellent engineers
HMI effectiveness; Distributed I/O; Engineers' Choice Award finalists; System Integrator advice; Inside Machines
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. As we know, PLCs aren’t the only option for making decisions in a control loop, but they are likely why you’re here.
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me