'Time of Flight' technology for level measurement

Andy Smith - Level Product Specialist at Endress+Hauser UK explains the different 'Time of Flight' techniques now available for measuring level in liquid and powder storage tanks and silos

Driven by the ever-changing requirements of industry and the regular introduction of new European standards and legislation, manufacturers of process instrumentation constantly redevelop products that have only been on the market for a few years.

Previously, product life cycles may have been in the region of 10 to 12 years, but now 3 to 5 years are not uncommon.

For the end user the benefits are huge - modern instrumentation uses the latest microprocessor technology and the needs of the different market sectors can be addressed quickly and effectively.

Today, instrumentation must be fit for purpose, modular and cost-effective! Process level measurement technology has changed greatly in recent years.

In the mid-1950s, capacitance systems were commonplace and although still available and widely used today, other techniques are proving more suitable and reliable in specific applications.

By far the biggest level measurement developments have been seen in products using the Time of Flight principle.

The Time of Flight family consists of three techniques - ultrasonic, radar and Time Domain Reflectometry (guided radar).

The same principle is adopted in all cases, although there are advantages and drawbacks with each technique.

In simple terms an energy pulse is transmitted towards the product surface either travelling through the gas layer above the product or guided along a steel rope or rod.

A percentage of the energy is reflected by the product surface and returns to the sensor, which now acts as a receiver.

The travel time between transmission and reception is measured, divided by two and multiplied by the speed of propagation to give the distance from the sensor to the product surface.

The vessel contents can then be calculated from this distance measurement.

Whilst the principle is straightforward, correctly applying Time of Flight products needs careful thought and a degree of application know-how.

Almost all problems experienced are due to either poor installation or incorrect choice of instrument.

The benefits of non-contact devices, such as radar or ultrasonic, are clear.

Abrasive or corrosive products can be measured without the sensor coming into contact with the process.

The measurement does however require energy to be reflected from the product so the quality of the surface (ie the process conditions) need to be considered.

Surface disturbances cause a reduction in the amount of reflected energy and hence a decrease in the signal to noise ratio.

In liquids the effects of agitators can be difficult to predict and give rise to two conditions.

As an obstruction, they may be responsible for reflecting energy and could be misinterpreted as the actual product level.

Highly advanced statistical filtering and complex algorithms have been developed to overcome this scenario and in general, the effects are ignored.

However, disturbance to the product surface presents the greatest challenge when working with agitators and the amount of disturbance is determined not only by the size, shape and speed of the agitator, but also the product viscosity and vessel dimensions.

However, the overall effect on performance may simply be a reduction in measuring range, and by selecting a different antenna this may be overcome completely.

When applying non-contact devices in solids applications, consideration must be given to the angle of the reflective surface.

As product is conveyed into the silo, angles of repose are created and may be responsible for loss of reflected energy.

Again, selection of the most appropriate sensor and correct installation can minimise these effects.

Pneumatic conveying can cause dusty and turbulent conditions that may affect reliability, although the latest sensor technology copes with all but the most arduous of conditions.

The presence of foam in liquid process applications is difficult to quantify and different types of foam have different effects on the accuracy and reliability of the measurement.

Light, non-conductive foam will generally be penetrated by either ultrasonic or radar signals and a reliable measurement of the liquid surface can be made.

If the foam is dense or conductive, the energy may be reflected from the surface, giving rise to content measurement (foam and liquid) instead of purely liquid level.

Under certain conditions the foam may absorb the energy completely, rendering non-contact devices useless.

So far we have mainly discussed the effects of process conditions on non-contact Time of Flight devices.

Many of the challenges highlighted above can be overcome by using a Time Domain Reflectometry system, more commonly known as guided radar.

The principle remains the same, but the energy pulse no longer relies on a carrier medium for transportation.

The pulse is launched on to a steel rod or rope and guided towards the product surface.

When a change of impedance is detected, energy is reflected back along the rod or cable to the receiver where it is detected, timed and converted to a distance measurement as before.

The reflective properties of the product are therefore no longer important, as the change of impedance alone is responsible for reflecting the energy.

When applied in solids applications, angles of repose create no problems and the signal is no longer attenuated due to dusty atmospheres.

However, attention must be paid to the amount of pull force acting on the cable, especially as the silo empties.

Recent developments in cable and coupling technology have reduced these forces to a minimum and so rarely cause problems.

The new Endress+Hauser Levelflex M guided radar device benefits from these advances and it is possible to replace the rope or rod should it become worn or a longer measuring range is required.

This is not possible with some other devices on the market.

When applied in liquid applications, the guided radar device generally measures level independently of surface foam, a limitation with non-contact devices.

The low frequency ensures the foam is penetrated and only the liquid level is responsible for reflecting the energy.

This section would not be complete without considering the properties of the actual product being measured and its effect on the different techniques.

The well-established ultrasonic technique relies on a step change in density in order to reflect energy.

Assuming the surface conditions are constant the signal to noise ratio would be the same when measuring for example either water or oil, as the difference in density between the gas layer above the liquid and the liquid itself is sufficiently great.

In comparison, radar energy is reflected when faced with a step change in electrical properties or dielectric constant but is unaffected by density changes.

If applying a radar gauge on a water level application with a dielectric constant of approximately 80, a high percentage of the energy would be reflected.

When used to measure oil level, dielectric approximately 2.0, the size of the reflected signal will be dramatically reduced because the difference in dielectric between the gas layer (1.0) and the oil (2.0) is small.

It can be seen from these examples that even with perfect reflective properties each technique brings its own benefits.

We have taken a look at the effects of process conditions on the reliability of each technique.

It can be seen that whilst for example foam provides no problems for a guided radar device, it may not be wise to fit a rod or rope version in to an agitated vessel.

Equally, a contacting device such as Time Domain Reflectometry may not be suitable for a chemically aggressive application on which only PTFE can be used.

In many cases a compromise must be sought.

The installation requirements also warrant careful consideration when applying Time of Flight devices.

Manufacturers provide detailed guidelines to ensure a reliable measurement and although these may seem a little strict the effects of poor installation can not be over emphasised.

Tall and narrow mounting nozzles are often found on process or storage vessels, but the wide variations in process connection and choice of antenna or sensor means these are generally overcome.

The requirements for each specific technique are simple, but failure to follow the guidelines can result in a perceived poor performance from the instrument in question.

For this reason companies such as Endress+Hauser provide in depth technical back up and work closely with end users to ensure all devices in the Time of Flight product basket are applied and installed correctly.

Attention to detail at the engineering stage can ensure that the units perform as expected.

The selection of not only the technique but also the sensor or antenna is only part of the requirements that need to be fulfilled in order to ensure a reliable measurement.

Incorrect commissioning can also have a detrimental effect on the reliability of the system.

As previously stated, Time of Flight devices use complex algorithms which have been developed over many years to ensure measurement integrity and reliability.

Whilst the theory and operation of the algorithms is of interest, from a commissioning point of view the end user requires a simple straightforward routine which ensures correct and accurate set up.

Various menu type structures have been used in the past but most required 'vendor know-how' in order to successfully commission the unit.

Recent developments have simplified the commissioning process greatly and now ask only a handful of straightforward questions in a step-by-step format.

The 'M' series from Endress+Hauser has been developed with these points in mind and makes correct commissioning a simple exercise requiring no specialist knowledge.

For the more technically minded, a software package is provided which enables both the standard and advanced functions to be carried out.

With all Time of Flight devices the ability to view the envelope curve and see what the instrument sees is a great benefit.

On the more advanced systems this is now available on the local display negating the need for a laptop in the hazardous area.

The need to document the measuring point and verify its integrity is becoming a standard requirement and the latest software packages allow this to be done in a simple common format.

By comparing the signal with an 'ideal echo curve', the integrity and reliability of the measurement can be confirmed at all levels.

In conclusion, it is clear that Time of Flight technology has taken over as the 'measurement of choice'.

It can also be demonstrated that whilst no single technique can provide a reliable measurement in all applications and under all conditions, it is fair to say that in general terms either ultrasonic, radar or Time Domain Reflectometry will suit your requirements.

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