The battle of the non-contacting measurement methods has ‘raged’, at least for those inside the industry, for more than a decade. Only now, though, with the advent of interchangeable transducers, are users able to enjoy the best of both worlds.
Within the water industry, ultrasonic systems have ruled the roost world-wide, as increasing sophistication has made sure that ultrasonics are rock solid reliable in terms of measurement confidence, and the latest systems have been built specifically around the sophisticated control, security and communications requirements of the water and waste water industries. Asset Management, predictive maintenance, TOTEX, EDM and more have influenced and informed developments in the technology, so that a modern ultrasonic controller is much more a pumping station management system than a level measurement device. Hundreds of thousands of sophisticated ultrasonic measurement and control devices provide vital enabling technology to control pumping stations – from the arctic circle to the baking heat of Western Australia, without any backup, in critical control applications managing water and waste water networks and providing the key measurements and control signals to drive key installations.
At Pulsar, we estimate that more than 95% of the measurement challenges we are presented with are readily measured using ultrasonic systems. However, there are certain measurements where RADAR units score well. For reasons mentioned below, ultrasonic measurement can struggle in digesters, because of the methane-rich atmosphere. In a pressurised atmosphere the ultrasonic signal can fail, and probably most regularly, an ultrasonic system can be unstable on some types of foamy surfaces.
Outside the water/waste industries, the picture is more nuanced, with RADAR systems offering significant advantages in a number of situations. RADAR is unaffected by the medium it travels through and immune to pressure or temperature changes, whereas ultrasonic systems are at their best in situations where the measurement conditions are consistent. The accuracy of the ultrasonic measurement depends on the accuracy of the knowledge of the speed of sound within the application. For example if there are layers in the medium, which can happen in petrochemical applications, the ultrasound signal is affected along with accuracy. Some atmospheres attenuate an ultrasonic signal too, for example high Carbon dioxide concentrations in breweries where measurement may not be possible over more than very short distances.
How else do RADAR and Ultrasonic Systems Differ?
It could be said that there are three, rather than two non-contacting mechanisms here. Ultrasonic measurement operates by a transducer emitting and receiving an ultrasound pulse, which bounces from the target, each process exciting a piezo-electric crystal array. The pulse, naturally, reflects from every hard surface within the area of the sound pulse, normally referred to as the ‘beam angle’ and is analysed either within the transducer itself or in the controller. That’s an important point, and one I’ll return to shortly. The competing echoes are disregarded and the ‘true echo’ identified. Pulsar originated their DATEM system of echo discrimination back in the 1990’s and have continued to develop and refine the algorithms so that now, there are very few applications where an ultrasonic system won’t work, unless it is a result of the atmosphere rather than, for example, a cluttered wet well.
Non-contacting RADAR comes in two types, pulsed and FMCW, or Frequency Modulated Continuous Wave. Both work by emitting radio frequency energy and measuring the time it takes for a signal to return from a target with a significantly higher dielectric constant than air. The key difference is that the 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, the signal will return in a fraction of a microsecond. FMCW also measures ‘time of flight’ but transmits continuously, constantly varying the frequency of the signal. The frequency of the returning signal is compared to the signal being emitted at that moment using a mathematical technique called Fast Fourier Transform (FFT), the difference between the two corresponding to the time the signal has taken to return. FMCW is the more accurate of the two techniques, with a narrower beam angle and, typically, a stronger signal.
So, what’s new?
What Pulsar have achieved is to create a smart RADAR transducer family, the dBR16 and dBR8, which contain the initial signal processing software is within the transducer itself rather than in an associated controller. It still needs the controller to finalise the calculations, it is still a two-part system, but what that means is that the family of controllers that provide the range of water-industry specific functionality and have been developed around ultrasonic technology operate just as well with RADAR measurement as they do with ultrasonic. So, for example, a Pulsar Ultra 3 controller includes a substantial set of control functions designed around the requirements of a small pumping station, removing the need for detailed programming of the site PLC. If the conditions of the station change, if foam levels, for example, become an issue, then it is a straightforward swap-out to replace an ultrasonic ‘head’ with a RADAR transducer. Seamless and straightforward, with no loss of service. Not only that, but dBR RADAR is backwards compatible with existing Pulsar controllers, so a dBR RADAR transducer can be retro-fitted to an existing installation where conditions have changed.
How to assess applications for RADAR or Ultrasonic measurement
Just to emphasise the point, there is now no difference in control and measurement functionality between RADAR and ultrasonic systems. Pulsar controllers will operate exactly the same way, in control and communication terms, whether they are fitted with a dB10 ultrasonic transducer or a dBR8 RADAR head. A Pulsar Ultimate Controller will still offer the comprehensive set of control tools, still operates as a PLC, RTU and pump controller in one economic, easily programmed unit, still supplies valuable and water-industry specific functions including tariff management, time to spill alarms, pump trip/reset management to avoid unnecessary vehicle movements and site attendance, and still logs data and communicates via WITS whichever measurement is used.
Therefore, the ONLY assessment you need to make when deciding between RADAR and ultrasonic is the measurement itself. First off, when making the assessment, you can start from an assumption: it’s probably ultrasonic. The vast majority of the measurements we make and the applications we see are easily within the limits of ultrasonic measurement, even the cluttered, busy wet wells and foamy surfaces that we see every day in sewage treatment applications. There are, however, some situations where you should consider RADAR:
Although Pulsar ultrasonic systems are the most accurate you can get on short-range Open Channel Flow MCERTS applications, on longer range – more than a few metres – applications, RADAR has the edge. We normally quote an ultrasonic application at ±0.25% of measurement range, so for a 6m measurement a typical accuracy would be ±15mm, whereas a similar RADAR measurement would offer accuracy around ±2mm.
Any application where the liquid or solid surface is hot can create a temperature gradient above the surface, which affects the speed of sound and creates an inconsistent ultrasonic signal, reducing accuracy.
Electrical or acoustic noise
RADAR is unaffected by electrical noise, for example from a nearby inverter, or excessive acoustic noise in the well. It’s very difficult to assess this prior to installation, making Pulsar’s approach a simple retrofit option.
RADAR measurement will tend to give a more stable result on a foamy surface than ultrasonic.
A really interesting application of RADAR is in chemical dosing plant, where chemicals are supplied in plastic IBCs. Because plastic has a low dielectric constant compared to a liquid surface, RADAR can ‘see’ through the container wall to the liquid surface, meaning that you can accurately measure usage and re-order points without having to introduce a process connection to the container. The ±2mm accuracy really helps here too.
A long-term frustration for ultrasonic measurements has been the inability to measure reliably within the methane-rich, elevated temperature and pressurised environment in a sludge digester. With the increase in emphasis on bio-gas generation, RADAR measurement offers an easy and well-understood way to measure levels in the digesters with a standard set of communications and control tools connected to the rest of the site.
With the advent of the new interchangeability, RADAR and ultrasonic are able to be considered as simply two approaches to the same problem. Project specification has been made easier, because every application can be considered with a single set of control tools and only the change in measurement transducer to decide, and even then, those decisions can be made retrospectively without the associated delay, expense and inconvenience of having to program a new controller which may or may not be compatible with the rest of the site tools. Service and on-site maintenance are made much simpler – only one set of control spares, only one set of instructions to learn.