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Level Measurement and Instruments

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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 | Non Contact 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 thanks to our sponsors Powerflo Solutions Pty Ltd and Magnetrol.

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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

Level Measurement selection Criteria - from 

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 

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 thanks to our sponsors Powerflo Solutions Pty Ltd and Magnetrol

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 

RF Capacitance- An excellent overview can be found on page 4 onwards -  thanks to our sponsors Powerflo Solutions Pty Ltd and Magnetrol.

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.

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RF/Capacitance Level Instrumentation-from 

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 our sponsors Powerflo Solutions Pty Ltd and Magnetrol

Float/Magnetic Level Measurement 

The Following technical information is compliments of our sponsors Powerflo Solutions Pty Ltd and Magnetrol.

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Typical Specific Gravities - from the Engineering Toolbox

Resistive Magnetic Level for LPG tanks  - Thanks to Bintech

Radar Level Measurement 

The Following technical information is compliments of our sponsors Powerflo Solutions Pty Ltd and Magnetrol

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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 

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

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 and 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 is compliments of our sponsors Powerflo Solutions Pty Ltd and Magnetrol
Principal of Operation

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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. This tool has the potential to save big $$ in production downtime, utilities (purge) use and above all has the potential to improve safety.

Thermal Dispersion Level Measurement

Thermal Dispersion -The basics, thanks to our sponsors 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 our sponsors 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

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 .

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 our sponsor ZEDFLO.

Nucleonic Level Measurement

Radiation Based Level Measurement - from

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 our sponsors Powerflo Solutions Pty Ltd and Magnetrol

RF Level Measurement in Lined Vessels with Grounded Shell - from

Dielectric Constants from Delta Controls Corporation.

Level Sight Glasses

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

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 our sponsors 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 our sponsors Powerflo Solutions Pty Ltd and Magnetrol.

Standards for Tank Level Gauging

The following are from  our sponsors Powerflo Solutions Pty Ltd and Magnetrol

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.