Why ultrasonic systems matter for wastewater

Early ultrasonic systems were based on analogue electronics. They were tricky to set up and unreliable in all but the simplest applications. The biggest challenge was ‘false echoes’, where signals from hard elements surrounding the measuring sound, such as stanchions, struts or stirrers, interfered with and overwhelmed the ‘true echo’.

Ultrasonic measurement devices have evolved to be more than level sensors – from asset management and predictive maintenance to total expenditure (TOTEX) and event duration management – today’s industry is demanding more.

These days, average ultrasonic systems are more than just sensors. They are small pump station controllers.

Ultrasonic systems operate by exciting a piezo-electric crystal to emit a pulse of ultrasound. The sound reflects off objects, and the return ‘echoes’ re-excite the crystal. The time taken for the signal to return is related to the distance of the reflecting object. Advancements in technology allow echoes from fixed objects within the sound path to be disregarded, and the true echo can be identified. There are few applications where this technology won’t work.

The non-contact nature of this technology means no moving parts, so no maintenance. Almost all wet wells are fitted with non-contact ultrasonic devices. Level measurement isn’t the only function. They are also used for pump control, differential, and volume measurements.

When a technology has been around for as long as ultrasonic systems, some myths and rumours are bound to be bubbling under the surface. One of the latest is that radar measurement is superior to ultrasonic measurement. While radar does have its place and advantages in certain situations, the advancement of digital signal processing, low voltage and high acoustic power output in ultrasonic transducers has meant that nearly 95 per cent of all applications can be solved with an ultrasonic system. An ultrasonic with high acoustic power can give a reliable signal return from a rough and foamy surface.

What are the benefits of ultrasonic systems?

The technology is a well-proven, well-understood measurement technique and is consistently and routinely used throughout industries all over the globe. Ultrasonic measurement is reliable and gives accurate readings every time. Aside from its consistency, a number of standard control routines provide users with a good level of control.

Today’s market is diverse, with customers demanding more specific solutions to their measurement requirements. Businesses are constantly reminded to reduce their power consumption and have their carbon footprint at the forefront of their minds.

There are low-power, loop- or battery-powered operating ultrasonic systems that have been developed to provide a solution to those issues. These systems provide answers to monitoring levels in remote locations and reducing power consumption on site. Advances in power management technology mean that the battery life of these systems is measured in years – something that has been unachievable in non-contacting systems.

A non-contacting system does an upstanding job, deriving distance by firing a signal and listening for the return echo. But now, with millions of dollars of research and development, tens of thousands of installations and technological advancements, ultrasonic measurement remains the foundation of process control and measurement.

What is radar?

Non-contacting radar technology comes in two types: pulsed and Frequency Modulated Continuous Wave (FMCW). Both technologies emit radio frequency energy and measure the time it takes for a signal to return from a target with a higher dielectric constant than air.

The key difference between the two types of radar measurement is that pulsed radar emits a series of radio frequency pulses and measures the time it takes for the signal to return from the target to the emitter. A challenge when at the speed of light is that the signal will return in a fraction of a microsecond. At the same time, FMCW measures times of flight but transmits continuously, constantly, varying the signal frequency. The frequency of the returning signal is compared to the signal emitted at that moment using a mathematical technique called Fast Fourier Transform (FFT). The difference between the two corresponds to the time the signal has taken to return. FMCW is said to be the more accurate of the two because of its narrower beam angle and, in most cases, a stronger signal.

How are radar and ultrasonic systems different?

There is no difference in control and measurement functionality between radar and ultrasonic systems. The measurement type is the only major factor in determining which technology to use. Users can start by assuming that ultrasonic measurement will solve their problem, even in the cluttered, busy wet wells we see in everyday sewage treatment applications.

Radar echo strength is also related to the dielectric constant of the reflecting object. If something is being measured with a low dielectric constant and obstructions with a high dielectric constant, there will be some measurement problems. Ultrasonic is only concerned with the object’s surface texture for its ability to reflect sound rather than from what the thing is made.

When should radar technology be considered?

1. Longer range open-channel flow MCERT applications

Monitoring Certification (MCERT) schemes are independent schemes designed to provide a framework for businesses to meet quality requirements. Under class 1 certification, the first three most accurate devices listed are ultrasonic, with a 0.04 per cent combined accuracy, compared to radar on the same scheme having a class 2 certification with a combined accuracy of 0.22 per cent. However, radar does have its advantages in those applications, more than a few meters of the measurement range.

2. High-temperature applications

Where the surface of the substance being measured is hot, it can create a temperature gradient above the surface. This will affect the speed of sound and produce an inconsistent ultrasonic signal, effectively reducing measurement accuracy.

3. Acoustic noise interface

Electrical noise interference can be ignored using low voltage but high acoustic power ultrasonic measurement. However, sometimes acoustic noise can interfere with the signal. Using a radar sensor for these applications can eliminate this rare occurrence.

4. Foamy applications

Radar measurement will produce more stable results than an ultrasonic sensor with limited acoustic power on foamy applications. This is because the foam interrupts the signal of the ultrasonic transducer. You can use a sensor with high acoustic control. However, one thing both technologies have in common is that they can’t see through the foam to the liquid surface.

5. Dosing plants and intermediate bulk containers (IBCs)

One feature of radar is that it can read through the container wall. This is useful in chemical dosing plants where chemicals are supplied in plastic IBC tanks. The low dielectric constant of plastic means that users can accurately measure usage and stock levels without introducing a new process connection to the container.

6. Digesters

One of the long-standing issues with an ultrasonic system is that it has struggled with the inability to measure reliably within the methane-rich, elevated temperature and pressurised environment of a sludge digester. With businesses making an effort to be environmentally friendly with biogas generation, radar measurement is seen as a way to measure levels within the digesters by following a standard set of communications and protocols that communicate with the rest of the site.

Whatever a user is measuring or trying to achieve. They can rest assured that ultrasonic measurements will achieve what they want. However, radar will solve the problem for the five per cent of applications outlined above. One thing that will be essential to the outcome of a measurement is that users must ensure they choose a retrofittable controller with both technologies.

If the application suddenly changes, or the conditions of the process change, and they need to swap one technology for the other, they need to ensure they have a control system that enables them to carry out that function.

Having a control system with the flexibility for both technologies ensures that these decisions can be made quickly, without having to retrain engineers or having the expense of installing a new control system.

Also, servicing with on-site maintenance is made simpler. The user will only need one set of control spares meaning only one set of instructions to learn.

For more information, visit pulsarmeasurement.com