Pulsar Process Measurement Ltd

State of the Ultrasonic Art

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Source: Pulsar Process Measurement Ltd

Non-contacting ultrasonic equipment has, for many years, been the default method to measure level and flow within the water and waste water industries. The combination of reliability and low maintenance, coupled with the sophistication of the most recent systems have meant that thousands upon thousands of applications worldwide call upon this most well proven of technologies.

Ultrasonic technology has been used for level and flow measurement since the 1970’s. Most ultrasonic systems operate by ‘time of flight’, a piezo-electric crystal being electrically excited to emit a sound pulse, and the time taken for that pulse to bounce off the target and return being measured and converted into a distance. Early systems were based on analogue electronics and, while ingenious, were very tricky to set up and unreliable in all but the simplest applications. The challenge, then as now, is that of ‘false echoes’, where signals from hard elements in the vessel such as stanchions, struts or stirrers interfere with and can overwhelm the ‘true’ echo.

The huge advance came with the advent of digital signal processing in the 1980’s. As that developed, measurement in more and more applications became possible, including the often cluttered, tight, deep and hostile environments in wastewater pumping stations. The benefits of non-contacting measurement in wastewater are obvious, but the advent of the later generation the latest generations of digital signal processing, along with low voltage, high acoustic power output transducers has meant that ever more difficult applications can be reliably measured. Now, almost all wet wells are fitted with non-contacting ultrasonic devices, and they are used throughout the industry in pump control, differential measurement, and open channel flow measurements to the MCERTS standard where accuracy is critical. For example, Pulsar’s FlowCERT system has been independently tested to 0.044% combined accuracy.

Inevitably, when a technology has been around for a long time, and has been used in such numbers, certain myths have sprung up. We are still told that spider webs interfere in some way with the ultrasonic signal (they don’t, that one’s never been true!), that ultrasonics won’t measure foam, that wind affects the signal. Some of these go back to the bad old days of analogue systems and modern digital devices will lock onto the correct level and follow it, regardless of how strong the competing signals may be from other parts of the measurement. Pulsar recently measured a 60m deep reservoir using an outdoor mounted transducer, and there are many hundreds of applications in cluttered and foamy wet wells. Digital systems such as Pulsar’s also ignore cross talk from other ultrasonic systems and are unaffected by interference from VSDs. A great example recently has been Pulsar’s measurement of a fumed silica tank. An amazing powdered material that forms everything from the gel in batteries to the anti-coagulant in custard, it has a bulk density of 30 g/litre and a surface area that can be hundreds of square metres per gram! The vagaries of the application meant that a new radar failed, but by putting in an ‘over-powered’ transducer it was possible to get enough of a signal for the ultrasonic system to work, providing vital stock control information.

So, we have a well-proven, well-understood technology in use throughout the industry. Armed with a measurement that can be relied on, a huge number of standard control routines have been developed for pumping station control. Many of the functions that would previously have been the province of a bespoke PLC system are now handled by the ultrasonic system. A ‘standard’ controller such as Pulsar’s Ultra 3 and Ultra 5 now provide an amazing level of control - all manner of pump routines depending on the duty required, the ability to run pumps on or exercise them, vary on points to reduce build-up and so on. A raft of different data logs are built in, and the same units can be used for open channel flow measurement and differential control, depending on model.

Today, the market is becoming more diverse and customers are demanding specific solutions to their requirements. Both at the ‘simple’ and more integrated ends of the market, non-contacting ultrasonics are filling important niches. Low power, loop or battery operated systems such as Pulsar’s dBi intelligent transducers are providing an ideal answer to the issue of monitoring in remote situations such as CSOs, and have now been fitted in thousands of applications. Advances in power management technology means that battery life is measured in years, something that has been impossible to achieve in non-contacting systems until very recently.

At the other end of the scale, complete Pumping Station Controllers such as Pulsar’s Ultimate Controller are rapidly gaining ground. Until recently, the ultrasonic pump controller has stood alone. Now, there is a drive toward integration, taking the PLC out of the equation completely. Control with compliance, performance monitoring with carbon footprint reduction. That the best of them are also reducing installation and maintenance costs along with the number of man-hours required to manage each site is just an extra bonus!

A number of devices that have gone some way along this road by bringing together the PLC and RTU functions into a single box. The final part of the puzzle has been the incorporation of the main level measurement device into the pumping station controller. Pulsar’s Ultimate Controller offers cost saving through management of high tariff electricity periods, Asset Management, Predictive Maintenance and support with Compliance with discharge consents including specific features such as attempting to restart a tripped pump to save a fitter’s journey. Now that reliable measurement is available along with the other elements, wastewater companies have access to economical, effective control systems that deliver functions that would have been prohibitively expensive to program and maintain in the field using even a small cohort of PLC installations.

At its heart, a non-contacting ultrasonic system does a straightforward job, deriving a distance by firing a signal and listening for the echo. But now, decades on and with millions of pounds of R&D supporting tens of thousands of installations, ultrasonic measurement remains the foundation stone of this critical part of wastewater control and management.

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