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Flow Line Options has provided information on two of the most commonly used level measurement technologies: ultrasonic and guided wave radar (GWR).

With more than 20 different level measurement technologies on the market today, it is important to choose the right level transmitter for your process conditions.

Ultrasonic technology has been on the market for years and is still considered a trusted technology throughout the industry.

It is non-contacting and offers a cost-effective solution for most straight-walled tank applications.

Over the years, however, newer level measurement devices have emerged into the market and are quickly capturing market share from older technologies that use sound or echo-based measurement – including ultrasonic transmitters.

Technologies such as GWR are price comparable and have proven to be a much more reliable solution in even the most irritating conditions.

GWR is suitable for both liquid and solid applications and operates independent of process conditions.

In today’s market, GWR is among the most versatile technologies being used for level measurement.

Additionally, unlike ultrasonics, which tend to be installation sensitive, GWR offers a solution that will work in all applications.

Ultrasonic is a non-contact level measurement method that uses sound waves to determine the process material being measured.

Ultrasonic transmitters operate by sending a sound wave, generated from a piezo electric transducer, to the media being measured.

The device measures the length of time it takes for the reflected sound wave to return to the transducer.

A successful measurement depends on reflection from the process material in a straight line back to the transducer.

However, there are various influences that affect the return signal.

Factors such as dust, heavy vapours, tank obstructions, surface turbulence, foam and even surface angles can affect the returning signal.

That is why the conditions that determine the characteristics of sound must be considered when using ultrasonic measurement.

There are other problematic aspects of ultrasonic transmitters to consider.

In vacuum applications, sound must travel through a medium (usually air).

The absence of air molecules prevents the propagation of sound waves.

Regarding surface condition, sound waves must be sent and received in a straight line, and reflective surfaces must be flat (in other words, non-agitated/non-turbulent condition).

Foam and other debris collected on the surface of the liquid absorbs the sound waves and impedes their return sound travel to the sensor.

Ultrasonic units are typically plastic with a maximum temperature of (60C) 140F – varying process temperatures may generate inaccurate readings.

Ultrasonic devices are not intended for extreme pressure limits – maximum working pressures should not exceed 30psig (2 bar).

Ultrasonic devices should be mounted in a predictable environment – vapour, condensing humidity, and other contaminates that change the speed of sound through air greatly affect the accuracy of the return signal.

The most popular benefit of through-air measurement principles such as ultrasonic, radar, or laser measurement is the fact that the measuring signal never comes in contact with the product being measured.

But if you think about it, this ‘fact’ is not entirely accurate.

Take ultrasonics for example: when sound energy leaves the transducer, it travels through air at 1,125ft/s until it reaches its target (in other words, the liquid surface).

Similar to all other ‘non-contacting’ type of level measurement, at some point the measuring signal must come in contact with the liquid surface before it begins its return trip back to the sensor.

This not only explains why the air quality between the sensor and liquid surface can be problematic, but also why the quality of the liquid surface needs to be accounted for.

Every disturbance it picks up on its way down and back will disturb the actual level measurement information in the signal.

It is important to understand that ultrasonic transmitters will provide a sensible solution, when properly applied.

Remember, the ultrasonic transmitter is just as good as the echo it receives.

GWR is a contacting level measurement method that uses a probe to guide high-frequency electromagnetic waves as they travel down from a transmitter to the media being measured.

GWR is based upon the principle of Time Domain Reflectometry (TDR), which is an electrical measurement technique that has been used for several decades in various industrial measurement applications; among its first fields of application was the location of cable damage.

In level measurement, however, TDR has only been used for a little over a decade.

With TDR, a low-energy electromagnetic pulse is guided along a probe.

When the pulse reaches the surface of the medium being measured, the pulse energy is reflected up the probe to the circuitry, which then calculates the fluid level from the time difference between the pulse sent and the pulse reflected.

The sensor can output the analysed level as a continuous measurement reading through its analogue output, or it can convert the values into freely positionable switching output signals.

The advantages of GWR in the level industry are endless.

Unlike older technologies, GWR offers measurement readings that are independent of chemical or physical properties found in the contact media.

Additionally, GWR performs equally well in liquids and solids.

GWR is suitable for a variety of level measurement applications.

In unstable process conditions, changes in viscosity, density, or acidity do not affect accuracy.

Boiling surfaces, dust, foam and vapour do not affect device performance.

GWR performs well under extreme temperatures up to 315C (600F) and is capable of withstanding pressures up to 580psig (40 bar).

It can be used with fine powders and sticky fluids, including vacuum tanks with used cooking oil, paint, latex, animal fat and soy bean oil, saw dust, carbon black, titanium tetrachloride, salt, and grain.

One of the most common misconceptions of GWR is the effects of product build-up on the probe.

One would think that if you have a mass of product stuck to the probe, or a coating of product throughout the entire probe length, that the signal would misidentify the true liquid surface.

This is not the case with advanced GWR technology.

The radar signal of GWR has a very large detection area around the probe covering 360deg of area over several feet of coverage.

When this pulse energy comes in contact with a mass of product on the probe, the signal is returned and analysed to see if it is the true liquid level.

Since the liquid level always has a larger signal return than the smaller mass that is sticking on the probe, the liquid surface is easily identifiable.

The advanced algorithms developed over the last decade have made this contacting form of level measurement the ideal solution for even the stickiest of fluid applications.

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