The term ‘state of the art’ is used in many industries. The European Standardization Committee considered it so important that it gave the term a proper definition in the EN 45020 standard in 2004. There it states: ‘State of the art’: developed stage of technical capability at a given time as regards products, processes and services, based on the relevant consolidated findings of science, technology and experience.
If one applies the above definition exactly, one can say that radar level measurement technology has been the state of the art for quite some time already. Radar gauges have been used for more than 15 years in the chemical and petrochemical industries and have performed excellently. Radar technology has also found its way into the bulk solids industry, establishing itself as a universal measuring method for a wide variety of solid bulk materials and process conditions.
In the water and wastewater sector, radar sensors have been used up to now only where the properties of microwaves offered significant advantages for a particular application. In most cases, ultrasonic sensors have continued to be the method of choice for non-contact measurement of levels or flow measurement in open channels.
According to Vega, radar, can however, offer considerable advantage over ultrasonics. For instance, measurement with radar is less affected by the influences of temperature, wind, fog or rain, and, due to the high sensitivity, most radar sensors can still detect water levels even under a thin layer of foam.
Level measurement in the pumping station
If there is insufficient natural slope, the wastewater is lifted to a higher level in pump stations and conducted to the treatment plant in several steps. Ultrasonic sensors, which detect the level of the wastewater contactlessly, are used in addition to pressure transducers to control the pumps. The advantage of radar technology in this application is its maintenance-free, reliable operation under all process conditions.
Even in confined spaces and shafts with heavy build-up on the walls, the excellent focusing of the radar signals delivers reliable measurement data. Installed pipelines and large pump housings or ladders have no influence on the measuring results. Even highly turbulent water or foaming detergent residues cannot impair the pump control function.
Rainwater overflow basin and quantity measurement
Storm water basins serve as a buffer to keep the treatment plant from becoming overloaded during heavy rains. If the volume of the basin is not sufficient to accommodate the huge quantities of water coming from heavy precipitation, the water is discharged via an overflow.
The amount of water released in this ‘blast’ has to be measured and documented.
Radar sensors can measure levels that get quite close to the antenna system: no minimum distance, as with ultrasonic sensors, must be maintained. This allows the contents of the basin to be measured almost all the way to the ceiling. Through the use of a simple metal reflector such as a mirror, the radar signal can be deflected by 90°. The sensor can then be installed horizontally, reducing the overall height of the sensor and extending the measuring range even further.
The high accuracy (2 mm) of a radar sensor ensures not only a reliable measurement of the level, but also an accurate documentation of the overflow quantity. The radar sensor is well protected against flooding by its high protection rating of IP 68 (2 bar) and is completely maintenance free.
Flow measurement in open channels
Waste water quantities are often measured by means of the level in flumes or overflow weirs. A flume-specific linearisation table stored in the sensor as a calculation formula allows the flow rate to be transmitted directly as the output signal.
The one outstanding strength of radar technology in these applications is its high accuracy, which remains unaffected even under changing environmental conditions. With ultrasonic instruments, the influence of temperature on the propagation speed of sound waves has to be compensated for. A sensor in the sound transducer measures the ambient temperature and compensates for the run-time error in the signal processing.
This works quite well under stable, unvarying conditions, but if the temperature changes rapidly or the sun heats the mostly black sound transducer, the temperature effect is not compensated for properly. Especially with the very small measuring ranges of flow measurement, this can lead to significant measurement errors.
Many users are unaware of this influence, because the measurement accuracy of ultrasonic devices is always specified under reference conditions – but these rarely exist in practice. Solar radiation, temperature fluctuations, wind, rain and fog all cause significant errors, depending on the application.
Level measurements in rivers and canals
The water levels in rivers, lakes and canals provide important information for navigation and serve as the basis for protective measures against flooding. Radar technology offers significant advantages here, too. The small, light sensors can be mounted on bridges, locks, or compact booms; complex and expensive adaptations to buildings are not necessary.
Thanks to the low power consumption, devices can be powered by batteries and operated via wireless modem – they provide reliable level data and thus help keep flood damage to a minimum.
Radar technology optimised for wastewater applications
To make it possible to use the advantages of radar technology in the water industry, the sensors have been adapted to the required boundary conditions: temperature range -40-80°C, maximum pressure 2 bar, housing of highly resistant plastic, permanently attached cable with high protection rating IP68 (2 bar). Expensive process fittings of stainless steel were deliberately avoided in order to reduce costs.
The result is a radar sensor that is optimised for the water industry, including the pricing structure. The Vegapuls WL 61 radar sensor from Vega offers substantial advantages in all the typical applications of this industrial sector. In spite of environmental conditions such as extreme temperature fluctuation, intense sunlight, rain or fog, the sensor delivers exact level data.
The available mounting aids allow users to mount the sensor easily and without additional costs. With its 4-20 mA current output, the sensor can be easily integrated into existing systems; adjustment and operation is carried out digitally via a superimposed HART signal.
The radar sensor was developed in compliance with the new European Standard EN 302729 for ‘The Use of Radar Sensors for Level Measurement Outdoors’ and fulfils the latest directives, which were passed in 2011.
Thanks to these new developments, radar technology has become not just an alternative to ultrasonic devices, but, as in the case of Vegapuls WL 61, is actually redefining the ‘state of the art’ in the wastewater industry and setting new standards for the future.
For more information contact John Groom, Vega Instruments SA, +27 (0)11 795 3249, [email protected], www.vega.com
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