Level Measurement and Instruments

The Measurement of Level involves a Comprehensive Knowledge of many Level Measurement Techniques and the Associated Level Instrumentation. This page which is just full of free Information on Level Instrumentation is a Fantastic Technical Resource.

Go to Specific Subject: General Level Measurement Theory | Level Measurement Standards | Level Measurement Applications | Level Measurement by Industry | Accoustic Volume Measurement | Capacitance Level Measurement | Displacer Level Measurement | Float/Magnetic Level Measurement | Radar Level Measurement | Resistance Tape Technology Level Measurement | Ultrasonic Level Measurement | Tapping Point Blockage Tools | Thermal Dispersion Level Measurement | Vibrating Level Switches | Boiler Level Control | Bubble Tube Level Measurement | Level Gauge Glass | Hydrostatic (Head Pressure) Level Measurement | Interface Level Measurement | Laser Level Measurement | Nucleonic Level Measurement | Penetrating Pulse Technology Level Measurement | Level Measurement using Profile Concepts | Radio Frequency Level Measurement | Level Sight Glasses | Maintenance of Level Systems | Standards for Tank Level Gauging


General Level Measurement Theory

The following Level Measurement Technical Papers are provided by Powerflo Solutions Pty Ltd and Magnetrol.

  • Technical Handbook from Magnetrol - This comprehensive publication provides important conversions, chemical properties, equivalents and pipe data.

  • A Guide to Level Instrumentation for Power Generating Plants - This guide serves as an introduction to level sensing and control products for Power Gen applications.

  • Comparing Displacer Transmitters with DP Transmitters - There have been many technologies over the years that have helped the process industry with level measurement. From the early days of simple mechanical float-operated level switches, the process instrumentation industry has been innovating new technologies to make its customers’ lives simpler. One of these older technologies commonly used in the process control industry is the differential pressure (DP) level transmitter, which first was introduced in the 1950s. It measures the hydrostatic head pressure of a liquid in a vessel and converts this to a level measurement, based on an input specific gravity/density of the liquid. A newer technology that is also dependent on specific gravity is the displacer level transmitter. There are significant differences between these two products, which affect the installation, maintenance and accuracy of the level transmitter chosen. These are discussed in this white paper.

  • Understanding Tank Bridle Level Measurement - A bridle is a vertical pipe connected to the side of a storage tank or process vessel, typically with side/side or side/bottom connections. Because the fluid inside the bridle will rise and fall equally with the level of fluid inside the tank or vessel, the bridle has been adapted for level measurement on a broad scale.

  • Magnetrol Level White Papers - Magnetrol have a swag of white papers covering their range of level instrumentation. Just select the Level icon.

Other Links
Level Measurement Information Notes - Inx Inc

Level Calibration Notes - Inx Inc

A Practical Overview of Level Measurement Technologies - Martin Bahner - There are multiple technologies available on the market to measure level. Each and every technology works, when applied properly. This presentation discusses the strengths and weaknesses of RF Admittance, Capacitance, Ultrasonic, Radar, Nuclear, Differential Pressure, and Bubblers level measurement technologies - from Gilson Engineering Sales.

Level Overview - a comparison of technologies - (supplied by Able Instruments and Controls Limited UK)

The Principles of Level Measurement - Gabor Vass, Princo Instruments, Inc and sensorsmag.com

Level Measurement selection Criteria - from omega.com

Water Level Accuracy and Correcting for Errors due to Gravitational Acceleration and Liquid Density - Ronny D. Harris, Ph.D -from In-Situ Inc

Improving Differential Pressure Diaphragm Seal System Performance and Installed Cost - Best practice diaphragm seal installation to compensate errors caused by temperature variations - from Emerson Process Management.

Choices in Automated Level Detection-Part 1 and Part 2 - This article provides a useful overview of level instrumentation techniques and technologies - from the excellent processonline.com.au.


Level Measurement Standards

Safety Standards of Level Control Devices - Malfunctioning level controls allegedly contributed to the 1986 Chernobyl meltdown and the 2005 Buncefield depot explosion north of London, to name just two of the more notorious incidents. For decades, the industrial firewall against safety incidents as they relate to level controls has been governmental and professional association standards that require manufacturers to make their products according to safety guidelines. The International Standards Association, however, lists some 180,000 varieties of international standards. The key health and safety standards that can affect level control devices and applications fall into three categories: (1) Instrument and Component Standards, (2) Safety Integrity Levels, and (3) Hygienic Standards - from Powerflo Solutions Pty Ltd and Magnetrol.


Level Measurement Applications

The following Level Measurement Technical Papers are provided by Powerflo Solutions Pty Ltd and Magnetrol:

  • Magnetrol have a Comprehensive Library of Level Applications which detail possible level measurement solutions. 

  • How well do you Understand Safety Integrity Level (SIL)? - This brochure targets safety applications and installations like Emergency Shutdown Systems, however it is worth noting that more than 90% of all applications are not safety-related. Some engineers are now using the SIL data as an indicator for reliability, i.e., the better the numbers, the more reliable the instrument.

  • The Impact of Instrument-Induced Errors on Feedwater Heater Heat Rate - A key way to reduce a power plant’s heat rate - and fuel costs - is to ensure accurate level control within its feedwater heaters. Although many physical anomalies can degrade feedwater heater performance, many older power plants use outdated level technologies. Older level technologies simply cannot achieve a performance level sufficient to manage controllable losses due to instrument-induced errors.

  • The Importance of Application Level Controls for Steam Generation Systems in Coal-Fired Power Plants - This blog discusses how level control technology can help you achieve accurate level measurement for critical applications within steam generation systems, including the condenser hotwell, condensate storage tank and deaerator.

  • Level and Flow Solutions for Gas Compression Skids - Modular skid systems are increasing in popularity in the process industry as a way for owner/operators, OEMs, and plant engineers to fabricate their unit operations. This post discusses level and flow instrumentation for gas compression skid systems.


Level Measurement by Industry

Magnetrol have a Comprehensive Interactive List of Level Solutions by Industry.


Accoustic Volume Measurement

Acoustic Volume Mapping - An ideal solution for bulk solids and powders - acoustic volume mapping can help you make informed inventory control and usage decisions. This system measures the volume of bulk solids and powders in any size or shape of storage vessel. It accurately measures bulk solids and powders in any type of container, silo or open bin - regardless of the type of material or product characteristics - to provide continuously reliable volume and inventory information Also it measures challenging applications that were previously not possible, including buildup loads and random irregularities that can occur over time - Powerflo Solutions Pty Ltd and Magnetrol.


Capacitance Level Measurement

Capacitance Level Control - The end of an Era? Is it time to replace your troublesome RF Capacitance/Admittance Liquid Level Transmitters? - Boyce Carsella - RF Capacitance/Admittance is one of the most flexible level measurement technologies ever developed. RF devices can be used in almost any type of media, and the probes are actually mechanical devices that serve an electronic function in the system. The probes can be made of almost any material and, therefore, they are very flexible devices. They can handle very high temperatures and pressures and corrosive media. Why then do some people refer to capacitance level, as “the level technology some people love to hate”? It seems that with this flexibility comes extreme application sensitivity as well. It's not difficult to find people who have had a capacitance application that did not work properly - from Powerflo Solutions Pty Ltd and Magnetrol.

Other Links
RF/Capacitance Level Instrumentation - from omega.com

Capacitance Level Measurement Technology - David W. Spitzer - Capacitance level measurement sensors are probes that are partially covered by material in the vessel. Rising level tends to cover more of a probe inserted from the top of the vessel - from Spitzer and Boyes.


Displacer Level Measurement

The following Displacer Level Measurement Technical Papers are provided thanks to Powerflo Solutions Pty Ltd and Magnetrol:

  • Electronic Displacer Transmitter Liquid Level Measurement Basics and Displacer Level Measurement, Displacer Controller Technology - A good introduction from Magnetrol.

  • Displacer switch operation - This is based upon simple buoyancy, whereby a spring is loaded with weighted displacers which are heavier than the liquid.

  • Comparing Displacer Transmitters with DP Transmitters - There have been many technologies over the years that have helped the process industry with level measurement. From the early days of simple mechanical float-operated level switches, the process instrumentation industry has been innovating new technologies to make its customers’ lives simpler. One of these older technologies commonly used in the process control industry is the differential pressure (DP) level transmitter, which first was introduced in the 1950s. It measures the hydrostatic head pressure of a liquid in a vessel and converts this to a level measurement, based on an input specific gravity/density of the liquid. A newer technology that is also dependent on specific gravity is the displacer level transmitter. There are significant differences between these two products, which affect the installation, maintenance and accuracy of the level transmitter chosen. These are discussed in this white paper.

  • Dont take the Risk - Is your Bulk Liquid Storage Tank Protected? - Do you have bulk liquid storage tanks containing petroleum products, ammonia, caustic chemicals, acids, ethanol, bitumen, pharmaceuticals, beverages, tallow where overflow could occur. This article describes how an high integrity, independent level switch, preferably of different technology to that used for the tank gauging system, is required to monitor any failure of the auto tank gauging system and shut down product transfers - thanks to Powerflo Solutions Pty Ltd.


Float/Magnetic Level Measurement

The Following technical information are provided by Powerflo Solutions Pty Ltd and Magnetrol:

  • The Magnetic Level Indicator: A Technology Overview - Demands for increased safety and improved efficiency in processing facilities have made the magnetic level indicator an indispensable level control device. With the ability to perform reliably under extreme process conditions, as well as offer dual-technology redundancy for safety critical applications, magnetic level indicators, or MLIs, such as those manufactured by Orion® Instruments, can make a smart alternative for a wide range of level measurement and control needs. This blog provides a closer look at the applications and operating principle of MLI technology.

  • Selecting the Right Magnetic Level Indicator - Companies in the process industry need the ability to visually monitor liquid levels in vessels (boilers, storage tanks, processing units, etc.). Traditionally, armored glass sight gauges have been used. However, many companies want an alternative to sight gauges to avoid problems such as breakage, leaks, or bursting at high pressures and extreme temperatures. In addition, the visibility of the sight glass can be poor and often affected by moisture, corrosion, or oxidation.Magnetic level indicators (MLIs) do not have the shortcomings of glass sight gauges and are suitable for a wide variety of installations.

  • Float - Details of float measurement techniques explained.

  • Atlas MLI vs. Gage Glasses.

  • Evolution of Magnetic Level Indicators.

  • Replacing DP Transmitters with Magnetic Level Indicator.

  • MLI Branch Connection Comparison - Full-Bore / Full Penetration vs. Extruded Tee - This application note provides a comparison of Orion’s full-bore/full penetration (conventional) branch connections and extruded tee branch connections as used on Magnetic Level Indicators.

  • External Caged Floats - the basics.

  • Float Level Switches - the basics.

  • Magnetic Level Indicators - the basics.

  • Advantages of the Magnetic Level Indicator - A magnetic level indicator is often used in applications where a sight glass (or glass sight gauge) is either ill-suited based on process variables or is under performing based on plant requirements. These can include enhanced safety for personnel; environmentally risky situations including media leakage or fugitive emissions; need for maintenance reduction; or need for high visibility from a distance.

Other Links
Typical Specific Gravities - from the Engineering Toolbox
Resistive Magnetic Level for LPG tanks - Thanks to Bintech


Radar Level Measurement

The Following technical information are provided by Powerflo Solutions Pty Ltd and Magnetrol:

  • Choosing the Right Guided Wave Radar Product - Kevin Martyn.

  • Guided Wave Radar vs. Differential Pressure Transmitters for Liquid Level Measurement - Over the years, however, newer level measurement devices have emerged and are consistently capturing market share from older technologies which utilize mechanical and pressure-based measurement-including DP transmitters. Technologies such as Non-Contact Radar, RF Capacitance, Ultrasonic, Magnetostrictive, and GuidedWave Radar employ the latest microprocessor-based digital electronics. By incorporating internal diagnostics, these devices have improved the control, analysis, communication, and overall reliability of fluid level management systems - from www.controlglobal.com.

  • Eclipse® GWR vs. RF Capacitance - Boyce Carsella -Is it time to replace your troublesome RF Capacitance/Admittance Liquid Level Transmitters?

  • Non-Contact Radar Measurement - An examination of the various types - Kevin Martyn.

  • Radar/Magnetic - This unit incorporates Magnetrol’s advanced Eclipse guided wave radar technology with the time proven and time tested magneticlevel indicator.

  • Comparison of Guided Wave Radar against other level Measurement Technologies - This is a neat comparison tool.

  • Guided Wave Radar - The basics.

  • Pulse Burst Radar - The basics.

  • Replacing Old Technology - Revitalising level measurement accuracy at a Soviet-era plant.

  • Sanitary Liquid Level Measurement - Drug-makers are now choosing solid-state, non-mechanical devices that deliver higher accuracy, require less frequent calibration, offer enhanced validation schemes, and are installed in such a way that an instrument failure won’t compromise process uptime or derail a batch in process.

  • Gauge Your Level Instrumentation - Keith Larson - Technology advances and downtime avoidance steer drug makers toward top-mount, non-intrusive methods for continuous liquid level measurement.

  • Level Indicators Reach New Heights - Exactly how much is in there? It’s an easy question if you can see into a tank, but what if you’re basically blind, or your tanks must be sealed? Sure, traditional solutions may work for awhile, but they’re historically hard to maintain and sometimes lack accuracy.

  • Radar Level Gauges - Frequently Asked Questions - Bob Botwinski.

  • Pulsar Radar Level Up to the Challenge at Chemical Plant - This facility offered numerous challenging level measurement applications, including overfill and spill protection requirements. This is a large 7.0 MB download.

  • Eclipse®705 receives SIL3 Certificate from Exida - Magnetrol International, Incorporated has announced that exida, an accredited global functional safety certification company, has certified the product reliability and the engineering change processes for the Eclipse® Model 705 Guided Wave Radar Transmitter as Safety Integrity Level (SIL) 3 capable per IEC 61508. SIL certification is obtained through analysis based on quantitative data and tests indicating the length of time between failures and expected performance in the field. A Failure Mode Effect and Diagnostic Analysis (FMEDA) confirmed that the Magnetrol® Eclipse Model 705 has demonstrated a solid field use history, includes sound engineering processes, and is designed with capable self-diagnostics. Download the IEC61508 Functional Safety Assessment here.

  • 6 Key Features to Look for When Considering a Guided Wave Radar Technology Solution - All process industries today require the ability to safely and accurately measure level in critical applications. This task, however, can be complicated by a variety of factors, including process media that are very low dielectric or contain corrosive vapors, foam, steam, build up, agitation, bubbling or boiling. Guided wave radar (GWR) technology, which is the operating principle used by the Eclipse® Model 706 transmitter from Magnetrol®, provides true level and interface control for applications across many process industries, including petroleum refining, electric power generation, chemical manufacturing, water and wastewater, pulp and paper, pharmaceutical processing and food and beverage. To ensure that you implement an optimal solution for your company, it is important to learn about some of the innovative features that the most reliable GWR solutions offer. The remainder of this post will discuss 6 features that you should look for when you are considering GWR technology as a possible solution for level measurement.

  • The Impact of Signal-to-Noise Ratio on Guided Wave Radar Transmitter Performance - In recent years, much has been said in the industry about the importance of the amplitude (size) of the guided wave radar (GWR) transmit pulse. While the size of the transmitted radar pulse is certainly important, it is a fact that pulse amplitude alone will not always yield accurate level measurement under all process conditions. A far more important parameter in reliable level measurement in difficult applications is the signal-to-noise ratio (SNR), which essentially describes the difference between the desired signal and the unwanted noise. If the amplitude of the noise approaches that of the level signal, loss of accuracy or linearity is the first observed effect due to distortion of the level signal as it passes through and interacts with the noise. Worse yet, if SNR is bad enough, the adverse signal interaction can actually result in a loss of the level signal. While it would be desirable to eliminate all the unwanted impedance discontinuities, it is simply not possible. The good news is that today’s most advanced GWR solutions address this critical design issue.

  • Investigation of New Level Technologies in Single Use, Disposable Systems - David Ladoski and Dan Klees - This article presents guided wave radar level measurement as an acceptable, less expensive alternate to load cell systems.

  • Reliable Foam Measurement Within Liquid Process Media is a Challenging Application - Process media susceptible to foaming are particularly challenging to accurate liquid level measurement. Foam’s lower density, as compared to a foam-free liquid, will absorb or deflect a substantial portion of the return signal, diminishing the all-important reflectivity required by non-contact measurement technologies.

  • Accurate Measurement of Water in Feedwater Heaters - Boyce Carsella, Jr - The accurate measurement of liquid levels in power plant operations is key to efficient operation. Although water is a liquid that can be easily measured by numerous measurement technologies, detection in applications like feedwater heaters, for example, takes on a range of complexity that stresses even the most robust devices.

  • Guided Wave Radar Instrumentation in Steam Drum and Feedwater Heater Applications - Many operators have questions and concerns about level instrumentation and how it can improve the efficiency and safety of their plants. Magnetrol® expert Donald Hite recently answered questions about guided wave radar as part of a power industry-focused webinar. Read his answers in this blog post.

  • Guided Wave Radar Provides Accurate, Cost-Effective Level Measurement for Single Use Systems - Guided wave radar (GWR) level measurement provides a cost-effective control solution for single use bioprocessing systems. Get the recent technical article that investigates the use of GWR technology in a wide range of single use system environments.

  • Six Reasons Why Guided Wave Radar Technology is Preferred Over Differential Pressure Level Control - Discover why differential pressure technology has lost ground to guided wave radar transmitters as the level control solution preferred by process industries.

  • Reliable Foam Measurement within Liquid Process Media is a Challenging Application - Foam Measurement and Liquid Level Instrumentation: A Magnetrol Applications Study Process media susceptible to foaming are particularly challenging to accurate liquid level measurement.

  • Level Measurement Techniques: Minimising Guided Wave Radar Probe Buildup - Learn simple, yet effective, level measurement techniques for minimizing guided wave radar probe buildup. Helpful information for natural gas, condensate and crude processing level control applications.

Other Links
Radar Level Measurement Best Practice - Sarah Parker, Applications Manager, Emerson Process Management, Rosemount division - The emergence of radar has been an important advance in the level measurement field. Radar represents a cost effective, accurate solution that is immune to density and other process fluid changes as well as most vapour space conditions - from Emerson Process Management and the Read-Out Instrumentation Signpost.

Non-Contact Radar Level Measurement - David W. Spitzer - Some basic technical information.

Innovation in Level Measurement - End-of-Probe Algorithm - From automation.com.

Guided Wave Radar Transmitters:Meeting the Challenge for Level-Detection Under Harsh Conditions - From automation.com.

Proper Employment of Guided Wave Radar in Steam Loops - Keith Riley and Ravi Jethra - This white paper discusses how guided wave radar can be used to measure level in steam applications such as feedwater tanks, high pressure preheaters, hotwells and drums from www.automation.com. and Endress+Hauser.


Resistance Tape Technology Level Measurement

Principle of Operation - Resistance Tape Level Measurement is remarkably different from other level sensors; in principle, appearance, and performance. The sensor consists of a gold plated, stainless steel base strip, partially insulated from a gold plated wire that is wound around the base strip to form a helix. This helix is sheathed in either Mylar or HastelloyTM to insulate the sensor's inner elements from the liquid to be measured. Sliding the sheathed helix into a protective channel that leaves the front of the sensor exposed then completes the sensor. The sensor is generally contained within a still pipe suspended from the top of the tank. However, other mounting options are available - from Jowa.


Ultrasonic Level Measurement

The following technical information are provided by Powerflo Solutions Pty Ltd and Magnetrol:

Principal of Operation :

  • Ultrasonic Contact - The basics.

  • Ultrasonic Non-Contact - The basics.

  • Look for Advanced Performance and Safety Benefits When Choosing an Ultrasonic Level Switch - Contact ultrasonic level switch technology was first applied to process control in the 1960s - and continues to provide accurate and reliable liquid level measurement in virtually every process industry today. Specific design features of ultrasonic level switches are intended to enhance safety in process environments.

  • Flexibility, Powerful Performance from Magnetrol Ultrasonic - Magnetrol’s new Echotel Model 355 Transmitter Can Handle Applications That Were Unthinkable Just a Few Years Ago - As advances in digital signal processing and low-power operation circuits make their way into the design of field instruments, ultrasonic level transmitters have gotten smarter and more effective. Thanks to www.controlglobal.com.

  • Level Sensing with Ultrasonic Contact Technology - Reliable level instrumentation is required to improve process efficiency and ensure the safety of your facility and your employees. Whether you’re specifying liquid level controls for a paper mill or a facility that processes natural gas, chemicals, petrochemicals, power or water and wastewater, finding high-quality instrumentation that can accurately measure level in your clean liquid applications is a mission-critical requirement. This blog discusses ultrasonic contact technology to provide an overview of how the technology works, application considerations and a comparison to other level control technology, specifically tuning forks.

  • Application Considerations When Specifying Ultrasonic Contact Technology for Liquid Level Sensing - Ultrasonic contact level measurement technology plays a key role in the safe and effective measurement of liquids across all process industries. To ensure that you implement an optimal solution for your facility, it’s important to learn about the different applications where an ultrasonic gap switch can be used - and the advantages and limitations of using this technology.

Other Links

Sound Advice - The measurement of level and flow is crucial to effective and efficient waste water treatment. Ultrasonic technology promises that and some more. From AIA.

No Contact Means Ultra Level Measurement - In level-measuring applications where it’s undesirable to have contact of the measuring instrument with the liquid in the process, a sonic or ultrasonic device may be an option. These types of level-measurement instruments really measure the distance from one point in the vessel (usually a reference point) to the level interface with another fluid. The general operating principle of both sonic and ultrasonic devices is similar - from ISA and InTech.


Tapping Point Blockage Tools

Tapping Point Blockage - Automatic Cleaning tool, see the advantages of this advanced technology on the level applications page.

Thermal Dispersion Level Measurement

Thermal Dispersion - The basics, thanks to Powerflo Solutions Pty Ltd and Magnetrol.

Liquid Level/Interface Monitoring in Flocculant/Sludge Control - Steven Craig - Precisely measuring and controlling the mixture of wastewater effluent, flocculants and sludge is essential to efficient water treatment. The level/interface instruments that support the process liquid measurements must be capable of distinguishing between liquids with varying properties in order to detect the levels where the different liquids interface in the pond or tank - from Fluid Components International (FCI).


Vibrating Level Switches

Vibrating Switches - The basics - thanks to Powerflo Solutions Pty Ltd and Magnetrol.


Boiler Level Control

Boiler Drum Level Transmitter Calibration - Steam Drum Level is both a critical and difficult measurement to make. Control of the water level in the drum must be precise. Thanks to Emerson Process Management.

The Boiler Drum Level Measurement Guide - Engineers must be ever vigilant to insure the integrity of the equipment and designs they employ when dealing with critical process applications. Some processes have been studied quite thoroughly and necessary control requirements are already well defined. Hundreds of thousands of boilers have been placed in service and the elements needed for proper drum level measurement are well understood. Years of experience have gone into determining satisfactory designs. Unfortunately, a failure to compile and distribute this information has resulted in many engineers going “back to the drawing board” for every new project. Isn’t this wealth of experience available somewhere. New products and methods are being constantly introduced. But how do these new products combine with existing equipment? Are there regulations which these new methods must meet? Is this really a new product, or a research experiment? This excellent 28 page guide is intended to help in the engineering effort. A sampling of “Tried and True” designs enhanced with the latest developments in equipment and methodology are presented. Hopefully it will provide safer operation of the boiler, save time for the design engineer, and simplify the selection of components that are required to efficiently and safely monitor and control the boiler drum level - Thanks to The Clark-Reliance Corporation.

Boiler Drum Level Control - Drum level control is critical to good boiler operation, as well as safe boiler operation - Jerry Gilman -The drum level must be controlled to the limits specified by the boiler manufacturer. If the drum level does not stay within these limits, there may be water carryover. If the level exceeds the limits, boiler water carryover into the superheater or the turbine may cause damage resulting in extensive maintenance costs or outages of either the turbine or the boiler. If the level is low, overheating of the water wall tubes may cause tube ruptures and serious accidents, resulting in expensive repairs, downtime, and injury or death to personnel. A rupture or crack most commonly occurs where the tubes connect to the drum. Damage may be a result of numerous or repeated low drum level conditions where the water level is below the tube entry into the drum - from ISA and InTech.

Boiler Drum Level Measurement and Control - David C. Farthing - Improved efficiency can have many connotations everything from fuel savings, improved equipment operation and useful life span, to labour and manpower savings. This paper will focus on the effects of boiler drum level and feedwater control as a means of energy savings by thermal/mass balancing the boiler. This paper will also discuss the effects of steam drum pressure and feedwater temperature on the overall efficiency of the boiler - from Federal Corporation.

Cascade, Feed Forward and Boiler Level Control - Allen D. Houtz - One common application of cascade control combined with feed forward control is in level control systems for boiler steam drums. The control strategies now used in modern industrial boiler systems had their beginnings on shipboard steam propulsion boilers. When boilers operated at low pressure, it was reasonably inexpensive to make the steam drum large. In a large drum, liquid level moves relatively slowly in response to disturbances (it has a long time constant). Therefore, manual or automatic adjustment of the feedwater valve in response to liquid level variations was an effective control strategy. However as boiler operating pressures have increased over the years, the cost of building and installing large steam drums forced the reduction of the drum size for a given steam production capacity. The consequence of smaller drum size is an attendant reduction in process time constants, or the speed with which important process variables can change. Smaller time constants mean upsets must be addressed more quickly, and this has led to the development of increasingly sophisticated control strategies - from the controlguru.

All in One Glass - Alternate approach to gage-glass maintenance leads to more accurate drum level measurement - Dale P. Evely - Southern Company, a producer of electricity, fiber optics, and wireless communications, developed an alternate approach to address gage glass maintenance issues as well as measurement uncertainties associated with water-column-type measurements - from ISA and InTech.

Boiling Water is easy as One, Two, Three, Four - according to David W. Spitzer. There’s more to boiler level control than measuring level and adjusting a feedwater valve. Improved measurements and inverse response are just a few of the influences on operating boilers in a reliable manner. Thanks to ControlGlobal.com.


Bubble Tube Level Measurement

Bubble Tube Installations for Liquid Level, Density, and Interface Level Measurements - The bubble tube principle of hydrostatic measurement is a convenient, low-cost method of measuring liquid level, density, or interface level in an open tank. It is particularly applicable for those installations where:

  • Process liquid could crystallize in transmitter lines.
  • Process temperature exceeds temperature limit of flange-mounted transmitter.
  • Process tank does not have side connections for flange-mounted transmitter.
  • Process liquid is corrosive and cannot have direct contact with transmitter.

from Invensys.

The Simple Bubble Tube - With all the new-fangled multifunction instrumentation available nowadays, it is easy to forget about the old tried and true measurement techniques. I believe the old adage, “the simplest solution is usually the best solution.” For example, I like the simplicity and elegance of a bubble tube. A bubble tube is a tube (typically ¼ inch to ¾ inch tubing or pipe) that is inserted into a tank a fixed distance from the bottom. The liquid is pushed out of the bubble tube with air or nitrogen, which is metered through a purge meter. The resulting backpressure is proportional to liquid level or density and is usually measured with a differential pressure transmitter. Bubble tubes can be used to measure liquid level, interface level, and density in open tanks- from Dex Automation.

Bubble-Tube Level System - In a Bubble-Tube Liquid Level System, the level is measured in a vented vessel by measuring the pressure required to force a gas into the liquid at a point beneath the surface. This method allows for level measurement without liquid entering the piping or the instrument. A pressure regulator and constant-flow regulator combine to establish a consistent flow of clean air or gas to a bubble pipe immersed a fixed distance in the tank. The flow is regulated to a very low level, building up pressure in the end of the bubble pipe. Thereafter, pressure is kept at this value by the escape of air bubbles through the liquid. Changes in the measured level cause the gas pressure to build or drop - from ControlAir Inc.

Purge Controls - A downpipe is the simplest type of liquid level sensor, often referred to as a “bubbler." This is merely a length of open ended pipe that extends downward into the tank. LiquiSeal™ or Purge Control differential pressure air flow regulators are highly refined versions of the basic bubbler air control.

More details on Bubbler Tubes can be found in the General Level Theory Section above.


Level Gauge Glass

All in One Glass - Alternate approach to gage-glass maintenances leads to more accurate drum level measurements - Dale P. Evely - Southern Company, a producer of electricity, fiber optics, and wireless communications, developed an alternate approach to address gage glass maintenance issues as well as measurement uncertainties associated with water-column-type measurements - from ISA and InTech.

Glass Liquid Level Gauges - There are numerous types of level detection devices that incorporate transducers, transmitters, sensors, or indicator instrumentation to monitor and regulate industrial systems. Most level gages rely on the principles of pressure differentials, conductivity, or capacitance and their operations can involve a range of different techniques, such as optical, electromagnetic, microwave, and ultrasonic detection methods. Liquid level gauges are designed for relatively straightforward level regulation and usually provide direct indications through visual, magnetic, or transduction properties. They typically consist of a measuring chamber connected to the vessel being monitored, with gage levels matching the changing levels in the vessel. There are a variety of different liquid level gage designs and each one features distinct operational characteristics and performance requirements. For example, a glass gage has a transparent design that allows for visual readings of the process level, while a magnetic level gage consists of an opaque metal measuring chamber. A floating device equipped with a permanent magnet is suspended upon the fluid in the chamber and it moves an indicator or a transducer through magnetic coupling to produce a level reading. The design differences between these types of liquid level gages determines their effectiveness in various applications, as well as the operating parameters for individual gage units - from www.thomasnet.com.


Hydrostatic (Head Pressure) Level Measurement

Level Measurement - Transmitters with Seals - Transmitters with remote seals allow the transmitter to be removed from direct contact with the process fluid - from Emerson Process Management.

Multivariable Approach to Liquid Level - Density Compensated Leveling Eliminates nearly all Error - from ISA and InTech.


Interface Level Measurement

Liquid Interface Level Measurement - Five leading interface measurement technologies in use today are discussed in this technical bulletin. Grouped by their operating technologies, these include Buoyancy (Floats and Displacers), RF Capacitance, Thermal Dispersion, Radar, and Redundant Technologies (those combining two measurement technologies in one instrument) - from Powerflo Solutions Pty Ltd and Magnetrol.

Technologies for Liquid Interface Level Measurement - Liquid level interfaces are used in a variety of processing applications. The need for interface level measurement arises whenever immiscible liquids-those incapable of mixing-reside within the same vessel. This blog post discusses available technologies for liquid interface level measurement - from Powerflo Solutions Pty Ltd and Magnetrol.

Measuring Level Interfaces - Measuring the Interface between Two Liquids in a Tank presents Unique Challenges - Gene Henry - A common measurement in the oil & gas, chemical, and petrochemical industries is detecting the interface level between two liquids in the same tank or vessel, such as oil and water. The dissimilar density or specific gravity of the two liquids means the lower density liquid will float on top of the higher density liquid. In some cases, the two liquids will entirely separate, resulting in a “clear” interface that will be easier to detect. In other cases, an emulsion or “rag” layer will exist between the two liquids. Other interface situations include multiple interfaces between more than two products, or the interface between a liquid and a solid. In some cases, it may be necessary to measure the thickness of the upper layer - from the ISA, InTech and Endress+Hauser.


Laser Level Measurement

Comprehensive Page on Laser Level - Thanks to ZEDFLO.


Nucleonic Level Measurement

Radiation Based Level Measurement - from www.omega.com.


Penetrating Pulse Technology Level Measurement

Principals of Penetrating Pulse Technology - From HiTech-A safe, economical, non-contact, non-invasive, “through the wall” digital technology for accurate level measurement - from HiTECH


Level Measurement using Profile Concepts

Profile Concepts -These are relatively new very accurate techniques with particular relevence to the oil and gas industry.
Technology Associated with Profiler Level Systems


Radio Frequency Level Measurement

RF Level Controls - The basics-thanks to Powerflo Solutions Pty Ltd and Magnetrol.

RF Level Measurement in Lined Vessels with Grounded Shell - from www.omega.com.

Dielectric Constants from Delta Controls Corporation.



Level Sight Glasses

The following Level Sight Glass Engineering resources are from L.J. Star Inc.

  • Are You Gambling with Your Sightglass? - Selection Strategies for When Failure Is Not an Option - Andrew Obertanec -Sightglasses are sometimes called “the weakest link” in a processing system because of the fragility of the glass. Actually, that’s not necessarily true. A sightglass need not be a weak link in a processing system because glass, when uniquely designed, is not necessarily fragile.

  • Technical Information and Guidelines for the Specification, Installation and Maintenance of Sight Glasses in Chemical and Pharmaceutical Processing Plants - This illustrated "Chemical and Pharmaceutical Sight Glass Application Handbook" spans the full scope of the subject from descriptions of common glass formulations to design factors, to a step-by-step checklist for installation and maintenance.You will have to register to get a copy.

  • Critical Standards for Specifying Sight Glasses - Sight glasses are highly engineered products. Although brands look alike, differences in their specs have tremendous importance for worker safety, sanitary processes, and maintenance costs.

  • Metaglas Sightglass Design - Andy Obertanec - Mechanically Prestessed Gas/Steel Sight Windows Resist Pressure and Impact Failure - The most effective approach to the use of conventional sight glasses has proven to be the simplest…and the most difficult: proper design, proper installation and proper maintenance, or the use of the new mechanically prestressed window design where applicable. Given proper application and maintenance of conventional borosilicate glass windows and, especially, with the availability of new technology, sight glasses no longer need to be the weakest link in a process / fluid system.

  • Step-by-Step Guide to Sight Glass Selection - Sight glass components allow operators to safely observe processes inside tanks, pipes, reactors and vessels. When it comes to selecting a sight glass, a wide variety of factors should be considered in order to ensure optimal performance. The guidelines that follow give tips and step-by-step advice on how to best select the ideal sight glass for a specific application. Start by defining the process to be observed, with considerations given to temperature, pressure, impact, physical characteristics of the process media, flow, and whether or not the process has sanitary requirements. With these factors in mind, one can begin the process of selecting the type of sight glass that best suits the needs of the process.

  • Video - LED Lighting for Sight Glass Applications - This video compares LED Technology with Halogen and outlines the advantages / applications.

  • Sight Glass Selection and Maintenance Tips Webinar - Glass is a critical element in process observation equipment, and understanding the properties of glass is important for proper specification. This streaming video webinar covers the physical characteristics of glass, ways to maximize its strengths and minimize its weaknesses, plus sight glass safety and maintenance - You will have to register to access this Webinar.

  • Important Guidelines for Mounting and Use of Sight Glass Fittings, Toughened Glasses and Luminaires - These commissioning and servicing instructions are critical to ensure safety.

  • How to Spec Lighting for Sight Glass Applications - Chances are pipelines and vessels are too dark for level detection and to view important stages of a process through a sight glass. Flashlights cannot supply sufficient lighting, and may cause a glare on the sight glass, making visual inspection virtually impossible. If the view port is small, there may not even be enough room to combine viewing and lighting. Illumination may be supplied by adding lights (also called “luminaires”) on sight glasses. When deciding upon a proper light for an application, be sure to consider all of the properties of the light. The size, weight, voltage, wattage, materials of construction, mounting configuration, and light pattern are all very important factors.

  • Sight Glass and Sight Glass Lighting Tutorial Video Library - L.J.Star Inc’s collection of tutorial videos created to help you learn about sight glass applications such as flow indicators in chemical and pharmaceutical processes. Learn how sight glass lighting allows for even more effective monitoring. Also learn how thermal shock and pressure affect sight glasses, as well as how to properly select a sight glass for your application. Videos also cover sight glass safety as well as energy savings considerations of LED lighting.


Maintenance of Level Systems

Testing, Inspection and Maintenance of Your Overfill Prevention System - A key point of the updated API 2350 recommended practices includes a provision for proof testing the equipment involved in an overfill prevention system (OPS). The new guidelines prescribe that all OPS equipment required to terminate receipt must be tested annually, while the High-High sensor alarm must be tested semi-annually. Additionally, continuous level sensors should be tested quarterly and point level sensors, semi-annually - thanks to Powerflo Solutions Pty Ltd and Magnetrol.

A Comparison of Recommendations for Overfill Prevention - The Buncefield Report (MIIB) & API RP 2350 - Back in 2005, there was a dangerous accident that occurred at the Buncefield Oil Depot, which was the largest fire in Europe since World War II. This fire was caused by an overfill of an outdoor storage tank, causing a release of a flammable vapour that was ignited. The overfill safety system for Tank 912 in bund A failed to operate and shut off the supply of petrol to the tank. When API 2350 was released, it was based on the events of the Buncefield Oil Depot overfill back in 2005. Both API and the MIIB (Major Incident Investigation Board) released new revisions and reports respectively to their standards after reflecting on what went wrong at Buncefield. API RP 2350 was released in 2012 and helped establish good practices. The Buncefield final report was released in 2008 and helped lay out recommended practices for primary, secondary and tertiary containment of a potential overfill situation. These recommended practices covered a wide range of overfill prevention areas from having systematic assessments of SIL requirements to creating a culture where high performance and leadership are expected. The Buncefield reports and API 2350 cover very similar topics relating to overfill prevention - thanks to Powerflo Solutions Pty Ltd and Magnetrol.


Standards for Tank Level Gauging

The following links are from Powerflo Solutions Pty Ltd and Magnetrol:

  • Automated Tank Gauging Redundancy through Disparate Level Instrumentation Technologies - Anyone who specifies, operates or maintains instrumentation for critical applications knows that tank gauging redundancy is one of the most effective methods of reliable level detection. In fact, the updated API RP 2350 standards recommend redundant sensors for Category III - or unattended - tanks. Ideally, redundant sensors will be of disparate technologies to avoid multiple or simultaneous failures due to an intrinsic behavior or “weakness” of a given technology.

  • Storage Tank Categories: API RP 2350 Definitions - With the American Petroleum Institute’s API RP 2350 fourth edition guidelines having been updated earlier this year, tank storage facility operators are using the recommendations to safeguard against potential life- and environment-endangering overfill accidents. One aspect operators need to understand is tank categories, upon which overfill prevention methodologies are based. API 2350 categorizes storage tanks by the level of staffing present during receiving operations.

  • Calculating Levels of Concern Ensures a Successful Tank Overfill Protection System - Many tank inventory operators have begun adopting the recommended practices set out in the American Petroleum Institute’s API RP 2350 fourth edition - which features safety measures to prevent tank overfill hazards. A key recommendation of API RP 2350 is the development of Levels of Concern, or LOCs, for each tank in an inventory. Simply put, LOCs are calculated product levels in the tank, upon which all alarm and alert positions and response times are based. Careful calculation of tank LOCs ensures the success of an overfill prevention system that complies with the API recommended practices. The API 2350 guidelines define five LOCs, starting from the top of the tank.

  • Factors to Consider When Assessing Tank Overfill Risk - With increased focus on improved safety measures in the petroleum refining industry, the American Petroleum Institute has addressed tank overfill prevention with its recently published API 2350 4th edition update. It is a requirement of the recommended practices and the responsibility of the terminal owner/operator to develop a written risk assessment procedure. The procedure should incorporate a process for determining the probability of overfill release, evaluating the consequences of an overfill occurrence and identifying the means of reducing risk. The API RP 2350 guidelines do not specify how risk assessments should be conducted, due to the specific nature of site risk and stakeholder values. It recommends that, where possible, owner/operators and transporters conduct risk assessments jointly.

  • The Impact of Roof Configuration on Tank Overfill Protection: Level Instrumentation Considerations - The API RP 2350 4th Edition Update provides details about the type of storage tank, class of liquid, level of concern and attendance category that are affected by the new recommended practices. Additionally, it requires sensors on tanks with floating roof configurations to detect both the roof and the liquid level, should the liquid cover the roof. Beyond this recommendation, however, the API 2350 does not detail level instrumentation standards for external versus internal floating roofs - with or without an instrument well - or for fixed roofs. Yet, certain level instrumentation technologies work better than others, depending on the roof configuration.

  • New API 2350 Overfill Prevention Standards: What You Need to Know About the Fourth Edition Update - In the wake of the Buncefield Oil Depot and other significant tank overfill incidents in recent years, the American Petroleum Institute has revised its API 2350 recommended practices to address malfunctioning or insufficient tank level gauging. While not mandatory, the API 2350 standards are being used by many facilities to improve tank storage safety. This is a quick summary of what you need to know about the new guidelines.

Other Links

Standards for Tank Gauging - Some of the reference standards used for liquid level or mass tank gauging worldwide - thanks to Gauging Systems Inc.

The Art of Tank Gauging - This excellent 26 page technical reference from Enraf gives an excellent overview.

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