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The Big 6 level measurement technologies

Where to use them and why
— Anyone who’s ever worn a tool belt knows that Level-measurement applications
MS Sreekanth
Global Product sometimes you have to use a tool for something it’s Knowing the amount of liquid contained in a tank is an
Manager, Level not designed to do. If you don’t happen to have a essential piece of data in almost every process and
Measurement
Technology hammer, a heavy wrench will work in a pinch. But of production operation. That information is primarily
course, the job will be done more effectively when you needed to fill one of five requirements:
ABB Measurement
& Analytics use the right tool. -- Inventory control: Ensure purchasing and stocking
the optimum volume of material
When it comes to measuring the level of materials in a -- Process efficiency: Maintain appropriate levels of
tank or other vessel, there are many tools you can materials to optimize production processes and
choose from. Some are clearly better suited to certain use storage options most efficiently.
applications, but in many cases the best level -- Safety: Avoid overfills/overflows to prevent injury,
measurement technology for the job can be hard to environmental issues and need for cleanups.
identify. Understanding the most-common types of -- Consistent supply: Have the required volume of
fluid-level-measurement technology will help you materials to meet production or customer
make a good choice and select the right measurement demands without interruption
tool for the job. -- Custody transfer: Meet commercial and legal
requirements for accurate measurement of
transferred materials.
Technology considerations
When choosing the potential technology for an
application, the first question to answer is “Are you
measuring solids or liquids?” There are some meter
types able to work with both types of materials, and
others uniquely applicable to one medium or the
other. We will focus only on liquid-measurement
application.

The second question is “Do you need continuous or

point-level measurement?” We will narrow the focus
for our exploration to only continuous-level
technologies.

Within the universe of continuous-level measurement

of liquids, there are six commonly-used meter types:
-- Differential pressure
-- Ultrasonic
-- Guided Wave Radar
-- Laser
-- Magnetic level gauges
-- Magnetostrictive

Let’s look at each of them, how they work, and what

applications they’re best suited to.

The Big 6 fluid-level measurements

Differential pressure density by compensating the calculated density based
This is probably the oldest, most trusted, and on a fourth variable. For liquid and wet-leg
commonly used, level-measurement method for applications, the meter includes a temperature probe,
liquids and liquefied gases in both open and closed decreasing the calculated density as the temperature
tanks, including pressurized tanks. The meter typically increases. For steam applications, the fourth variable
includes a stable body and a diaphragm with a sensor is determined based on static-pressure tables.
to measure the pressure exerted by a column (or head) Multivariable sensors may require multiple
of liquid in the vessel. The diaphragm is deflected by penetrations points in the tank.
the pressure differential, changing the electrical
property of the sensor and creating a proportional Advantages
electrical signal. The sensor could be one of many • Most versatile and widely used technology
different types. • Indirect, contact measurement
• Simple design with low purchase cost
Calculating the value of the signal requires a bit of • Configurable with a variety of options to suit
math that includes three variables: pressure, density application
(specific gravity) which must be entered as a • Externally installed or retrofitted to existing vessel
constant, and the measured product height or level.
Limitations
In open tanks, the meter calculates the differential • Measurement affected by changes in specific
between atmospheric pressure and fluid pressure. In gravity/density
closed tanks, it calculates the difference between the • Mounting constraints / limited flexibility compared
low-side, blanket pressure and the fluid pressure. In to other technologies
both cases, accurate level calculations depend on • Higher total cost of ownership considering periodic
knowing the fluid density. calibration and maintenance
• Require two vessel penetrations in closed tanks,
In applications where density changes, more creating leak potential
sophisticated meters must be used. These
“multivariable’ meters accommodate dynamic liquid
Ultrasonic There are two general types of ultrasonic meters. The
This technology relies on sound waves to determine integrated version encompasses both the sensor and
material level by gauging the distance between a top- the electronics in a single, compact instrument. There
of-tank transducer and the surface of the material, is also a remote version that separates the two
whether liquid, solid, or slurry. The transducer emits components, enabling remote level monitoring.
short ultrasonic impulses (sound waves) that
represent mechanical energy. The waves bounce off Advantages
the top of the material like an echo and return to the • Proven, widely used
transducer which calculates the “time of flight” to de- • Indirect measurement
termine the height of the material in the tank. The • Acoustic, non-contact with no moving parts
transducer emits a relatively narrow beam, so obstruc- • Simple and cost-effective
tions in the tank typically don’t impair its applicability • Unaffected by changes in product density,
or accuracy. composition, moisture content, electrical
conductivity, or dielectric constant
Like the differential pressure meter, ultrasonic • Narrow beam angle minimizes effect of obstructions
technology has been in use for decades and is another • Lower-cost alternative to radar / laser
old and proven technology. Due to inherent limitations • Not affected by material dust
of ultrasonic measurement, this technology is used
primarily in water/wastewater and aggregate Limitations
applications. • Dirt, irregular and sloped material surface affect
measurement accuracy
Unlike the differential pressure gauge, there is no • Affected by many interferences
bottom penetration in the tank, and therefore no risk • Doesn’t provide high repeatability
of leaks. The ultrasonic meter is also unaffected by • High pressure and/or temperature affects meter
changes in material density or specific gravity. accuracy
However, because sound waves can’t travel in a • Weak echo, and reduced accuracy, due to dispersion
vacuum, ultrasonic meters can’t be used in vacuum or and/or absorption of material (e.g. foam)
reduced-pressure vessels. And accuracy can be • Vapor and condensate can create false echoes
affected by the dispersion and absorption properties
of the liquid. Foam, for example, can create false
reflections.

Laser This is a universal measurement technology, used for

This is one of the latest advancements in level- both liquids and solids. It can also be used for
measurement technology. It’s really only in the last positioning applications and on conveyor belts. Its
five or six years that it’s become a mature, reliable versatility makes it applicable in many applications
technology. and industries. That versatility comes at cost, though.
Compared to ultrasonic, laser is significantly more
Like the ultrasonic meter, a laser is also mounted at expensive.
the top of the vessel pointing down. But instead of
emitting a sound wave, the laser emits a flash of light. Advantages
The light is reflected back from the surface of the • Indirect measurement
material to the meter where it is detected by an • Non-contact
optical receiver. The distance is calculated based on • Very high update rates are achievable; no “lock-in”
the time it takes for the light to travel to the surface issue sometimes experienced with ultrasonic meters
and back to the instrument. • Effective in both very narrow (down to 2”) and deep
tanks (330 ft. for liquids, 650 ft. for solids)
Compared to the sound pulse of an ultrasonic meter, • Not affected by material density
the laser beam is significantly narrower. With virtually • Works with solid or liquid materials
no beam spread (0.2 degrees), it’s possible to find an
appropriate mounting location that will ensure Limitations
consistent, reliable readings even in tanks with many • Costlier than ultrasonic
obstructions. The laser emitter can be mounted • Not suitable for interface measurement
anywhere on the tank lid, including along the tank • Not applicable for environments where dust or other
wall. Ultrasonic and radar systems work best when suspended material will interfere with laser beam
centered at the top of the tank. Laser technology also • Doesn’t work will with shiny materials or when foam
works well in narrow tanks, something else that gives is present
ultrasonic and radar meters trouble.
Guided wave radar (GWR) choice in these applications. However, instruments
With this technology, also called time-domain supported with appropriate algorithms can overcome
reflectometry (TDR), the meter is mounted on the top some of these challenges.
of the tank or chamber with a probe that usually
extends the full depth of the vessel. A low-energy Unlike laser meters, which simply assess the speed of
pulse of microwaves is sent down the probe. When the the beam return, GWR meters actually assess both the
pulse reaches the liquid level (the air/liquid interface), speed and the waveform of the reflected signal. The
a significant amount of the microwave energy is signal must therefore be properly tuned during
reflected back up the probe to the transmitter. As with commissioning to ensure accuracy.
most other meter types, the time delay between the
transmitted and received echo signal is used to Advantages
calculate the distance to the liquid surface. • Unaffected by changes in pressure, temperature,
density, conductivity, etc.
This reflective action depends on the liquid’s • Contact type
dielectric value. High-dielectric liquids reflect the • No moving parts
entire pulse, providing reliable, accurate • Unaffected by dense fog, dust or steam, and by high
measurement. Low-dielectric material doesn’t pressure or temperature
adequately reflect the pulse, resulting in inaccurate • No beam-angle issue: Works even with difficult tank
readings. Some advanced meter designs can geometry or interfering obstacles
overcome this issue. • Can be used with liquids, sludges, slurries, and some
solids
This lack of complete reflection can be capitalized on
to provide interface measurement in tanks containing Limitations
two liquids with different dielectric properties. That • Commissioning requires special expertise
makes it possible to measure both total level and the • Sensitive to build-up on the probe
interface with a single meter. • Challenging to measure interface with emulsion/rag
layer
Emulsion layers create an indistinct boundary
between atmosphere and liquid, making GWR a poor

Magnetic level gauge (MLG)

Aside from putting a graduated stick in the top of a Advantages
tank to measure fluid level, the sight glass is probably • Requires no power
the oldest and simplest of all level-measurement • Simple design and purchasing, with piping drawings
approaches. The major drawbacks to using a sight reduced to equipment specification schedules
glass are safety and maintenance concerns. The MLG • Often used in high temperature, high-pressure or
mimics the approach of a sight glass in a safer, more toxic/corrosive environments.
reliable way that provides a simple but powerful • Longer life compared to electronic measurement
approach to measure liquids. techniques
• Low total cost of ownership
An MLG system includes a sealed float enclosing a
magnetic ring housed in a non-magnetic float Limitations
chamber. The float moves up and down in the tank as • Affected by changes in specific gravity
the liquid level rises or falls. Outside the vessel, a • Movement of the in-tank float can be affected by
highly visible magnetic “shuttle” is contained in a high-viscosity or sticky liquids
separate, sealed glass tube. As the tank level increases • Completely customized solution, increasing
and the float rises, the shuttle moves in unison. This purchase cost
provides a continuous, highly visible indication of
liquid level.

Compared to a sight glass, this gauge has fewer

potential leak points and lower maintenance. There
are no tank penetrations required. While the basic
design provides only local, visual indications, a
transmitter can be added for remote monitoring.
Magnetostrictive (MR) layer. Compared to other technologies, this provides
This is a relatively unknown technology compared to superior interface measurement. The second float can
the others discussed above, but it offers some unique be designed specifically to the application providing
benefits, including the highest accuracy (1mm) and accuracy even with emulsions.
resolution. It also provides four measurements: level,
interface, temperature, and ullage. While the non-intrusive mount requires additional
components and engineering, the payoff is both
An MR gauge can be configured in two ways. higher safety and lower maintenance, making it the
-- A probe is directly inserted in the tank with a mag- most common and attractive solution.
netic floating ring that moves up and down on the
probe. (Intrusive) Advantages
-- A vertical, metal column or chamber is attached to • Only moving part is the very low maintenance float
the tank with connections at the top and bottom, • Unaffected by process conditions: foam, emulsion,
configured like a sight glass but without the ability mist, gas layering, dust, dielectric value,
to directly view the liquid in the column. Instead, a temperature, etc.
magnetic float moves up and down in the column. • No maintenance or calibration
The probe is positioned outside and parallel to the • Best technology for interface measurement
column. (Non-intrusive) • Lower total cost of ownership

In both designs, the meter generates low-energy Limitations

pulses of current through the probe. These pulses • Float movement affected by high-viscosity or sticky
create a magnetic field that interacts with the float. liquids
The resulting torsional waves travel through the • Length limited to 75ft (22m), less than other
sensor wire at a known velocity, enabling calculation technologies.
of the liquid level. • Floats designed for specific application

It’s possible to include multiple floats, enabling

measurement of both the upper level and interface

Making the right technology choice

This overview of level measurement provided you with
a good foundation for selecting the right technology
for your application. Still, it’s obvious that this can be
a complicated selection process. It’s typically prudent
to contact vendors offering these technologies to
gain further insight into their specific product
capabilities, measurement accuracies, pricing, etc. to
ensure you have all the facts as you make your
decisions.

Click here for a pre-recorded webinar on this topic.

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Selecting the right level technology

Magnetostrictive Magnetostrictive
Feature Guided wave radar Laser Ultrasonic
( non-intrusive ) ( direct mount )

Non-Intrusive/Non-contact Yes No No Non-contact Non-contact

Fast Response Time 10 updates /second 10 updates /second 10 updates /second 2 updates /second 2 updates /second
Accuracy ± 1.27 mm ± 1.27 mm ± 2 mm ± 11 mm ± 2 mm
Simple Start-up Yes Yes Know-how required Know-how required Yes
Simple Maintenance Yes Yes Yes Yes Yes
Adjustable insertion depth Non-invasive Yes, with compression fittings Special probe Non-contact Non-contact
Probe Exchangable/Cuttable No No Yes Non-contact Non-contact
Top or Bottom Mount Excellent Can be done Theoretically ! No No
Certified for use in SIL2/3 Yes Yes Yes Yes No
Integral RTD in Sensor Yes Yes No No No

Magnetostrictive Magnetostrictive
Vessel type Guided wave radar Laser Ultrasonic
( non-intrusive ) ( direct mount )

Storage Vessels Good Good Good Good Good

Process Vessels Good Good Good Consult Factory Consult Factory
Stilling Well/EC Excellent Consult Factory Excellent Good Consult Factory
Horizontal Cylinder Good Good Good Good Good
Small Vessels Excellent Consult Factory Consult Factory Excellent Good
Tall vessels Up to 50ft Up to 75 ft Up to 217ft Up to 330ft Up to 33 ft
Tall or unusual nozzles Consult Factory Good Use Coax or SW Excellent Consult Factory
Internal obstructions Excellent Consult Factory Consult Factory Consult Factory Consult Factory

Magnetostrictive Magnetostrictive
Process conditions Guided wave radar Laser Ultrasonic
( non-intrusive ) ( direct mount )

Turbulent Surface Good Consult Factory Good Good Consult Factory

Foam Excellent Excellent Good Consult Factory Consult Factory
Steam Good Good Good Consult Factory Consult Factory
Vapors Excellent Good Good Consult Factory Consult Factory
Flashing Service Excellent Consult Factory Consult Factory Consult Factory Consult Factory
Sparging Service Good Consult Factory Consult Factory Consult Factory Consult Factory
Slurries N/A Consult Factory Good Consult Factory Consult Factory
High Viscosity Consult Factory Consult Factory Consult Factory Good Good
Coating Service Light Coatings Okay Light Coatings Okay Light Coatings Okay Good Good
Interface Applications Excellent Excellent Good N/A N/A
Pressures Swings Good Good Good Good Good
Temperature Swings Good Good Good Good Consult Factory
Extreme Pressures Vacuum to 5000 Vacuum to 2400 Vacuum to 5000 Consult Factory Consult Factory
Extreme Temperatures Excellent Good Good Consult Factory Consult Factory
Specific Gravity Swings Consult Factory Consult Factory Excellent Excellent Good
Dielectric Swings Good Good Good Good Good
Vibration Good Good Good Good Consult Factory
With Agitators Consult Factory Consult Factory Good Good Consult Factory
Sump Consult Factory Good Good Excellent Excellent

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WP/Level_Big6-EN Rev A

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