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The benefits of low cost radar technology for level measurement in open water level measurement

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Until recently, level measurement in the water processing and monitoring industries have relied mainly on hydrostatic pressure transmitters and ultrasonic gauges. Thanks to a new generation radar sensor - this is set for change. In many other process industries, radar level technology has become a front-line solution as it delivers more accurate and reliable measurement due to less influence from the process or surrounding environment.

Although radar measurement technology is already well-established across many process industries, many water monitoring bodies have less experience with this technology. This is due in part to the cost of radar instruments, which in the past were considerably higher than others like ultrasonic and hydrostatic, as well as this technology still carrying the image of being complicated.

These preconceptions are now changing. Jürgen Skowaisa - Product Manager for radar and ultrasonic at VEGA, considers radar level measurement technology to be a better alternative for the tide and river gauging, primarily because of its robustness in the face of external atmospheric influences and it is non-contact. Fog, wind or rain can impair ultrasonic sensors, as well as temperature from solar radiation. The velocity changes of ultrasonic signals are related to temperature changes (1.6% per 10 Kelvin degrees). These changes are normally compensated for by a temperature sensor in the transducer. "As long as it's cloudy, you get reliable values", explains Skowaisa. "But during longer periods of direct sun, temperatures - on the usually black transducers - increase and the sensor temperature becomes much higher than the ambient temperature."  One solution would be installing an additional temperature sensor, but this would mean extra costs and damage risk.  Also, surface conditions, such as waves or some foam, also affect measuring precision, often the solution is to install ultrasonic sensors in a stilling tube for protection from weather, but then blockage from silt, debris or crustaceans also becomes an issue, causing slower drainage or filling, as well as false signals causing a misread. Compared to ultrasonic, radar measurement technology offers considerable advantages; it operates completely independent of temperature and weather effects.  Compared to hydrostatic sensors the non-contact technology offers many advantages, including simpler installation and calibration, do drift, damage, corrosion or build up issues.

Winds of change for level measurement technology
For this reason, VEGA is launching VEGAPULS WL61on the market, which is a new radar sensor especially developed for the area of water/sewage.  "It boasts high precision, is very easy to use and with regard to cost is a real alternative to ultrasonic instruments" explains Jurgen further, "The price differential between ultrasonic and radar measurement technology used to be very high. Today, radar measurement technology is only slightly more expensive, so it is actually the better solution these for these applications." 

Some basic information about VEGAPULS WL61: It is a 2-wire, loop-powered device with HART (optional ATEX approval) and an accuracy of +/- 2 mm, A key feature is the 80 mm plastic encapsulated antenna providing excellent focusing, its measuring range extends up to 15 metres  (other units up to 75m are available). The instrument has submersible protection rating of IP 66/68 (1 bar), made possible by the moulded cable entry and watertight, robust housing. A variety of different mounting and communication options make the sensor easy to integrate into existing  infrastructures. In the development process, a special emphasis was placed on designing a straightforward, user-friendly operation and handling. Application parameters are thus optimized for the intended areas of use. "Users in this industry do not need parameters for application in large tanks, storage silos or for solvents as the measured medium.” said Jurgen, “so  the new sensor has optimized applications such as open channels, flow-rate and other similar applications typical of this sector.  Linearization curves can be stored in the sensor itself, so this allows, for example, direct indication of the flow in open channels.” He added, “The measurement data can be transmitted either directly to a compatible existing control unit or to a SCADA system (4 … 20 mA, Profibus PA or Foundation Fieldbus), So this makes it a true all rounder” he concluded. 

Ahead of the forecast
When it comes to approvals, the user is on the safe side with the new VEGAPULS WL61 as well as ATEX, all existing VEGA radar sensors have been approved according to the prevailing regulations for operation out in the open using microwaves. However, the necessary tests were not always fully defined in the past. To initiate a European guideline that describes the necessary testing scenarios precisely and creates legal certainty for all stakeholders, leading measurement technology companies, including VEGA, got together. The result is a new European norm with accompanying guidelines in which the test specifications applying to radar level sensors in the future are exactly defined. The new EN 302729 is to be ratified in the 1st quarter of 2011. VEGAPULS WL61 is the first measuring instrument on the market that is already in compliance with this norm. "An important consequence of the new test specifications is that future radar instruments will operate at a frequency of 25 GHz instead of the current 26 GHz", reports Skowaisa. But this seemingly tiny modification has had a significant effect on the electronics and antenna, so system designs had to be correspondingly revised. 

Reliable data in demanding applications: The following examples show the advantages the new sensor has in daily use. 

Precision measurement: In flow rate measurement in open channels, accuracy is key due to the exponential nature of level to flow calculations. A measurement error of anything up to 10 % can easily arise with ultrasound sensors, for example; the differential of sun on the transducer to the ambient air temperature. According to Jurgen, most users do not even realize that this ultrasonic technology can deliver inaccurate measuring results. "Users simply trust that the values stated in the manual are correct, but in reality, these only apply under ideal measuring conditions."  Regulators charge for the registered sewage quantities on the basis of these values. Radar technology gives them far more accurate and reliable measurement data, as it is completely independent of weather effects. 

Storm water overflows: Storm water is collected in on-site catchment and storm water overflow basins to relieve the sewer network and sewage plants during heavy rains. This ‘buffer’ allows it to be forwarded in a manageable way to the sewage plant after the event. SWO chambers are often sited underground with little headroom. In this application, the user benefits greatly from the small mounting height of the sensor.  "Radar technology has almost no blocking distance, which means measurement can continue even with very high water levels, where ultrasonic sensors have to be mounted well above the maximum level to allow for their blocking distance, otherwise they can mis-read or lose the level." Jurgen mentions another advantage, "Even flooding is no problem, because these radar sensors have IP68 protection."

Open waterways: Where needed, VEGAPULS WL 61 can simply be suspended from a cable down into the shaft to reduce interfering reflections. Apart from sewerage and storm water, another interesting application for radar sensors is the level measurement of waterways, drainage channels, rivers and estuaries. Flood risk is increasing – as the result of more concentrated extremely heavy rainfall in some areas, rising sea levels in others. The non-contact radar sensors, with their immunity to sun, wind, rain and fog etc. really comes in useful. They can be easily mounted ‘in the open’ without still wells or sounding tubes, under bridges, or retrofit to existing tube installations. VEGAPULS WL 61 can thus monitor and provide an early warning of impending high water.

The Outlook
With its contradictions of scarcity and deluge, the requirement for more monitoring of sea, river and water levels will continue to grow in the future, and it will always place new challenges on measurement technology. "Future market and environmental requirements can be quickly met with such a capable core technology", concludes Skowaisa, “I am convinced radar instruments will play a major role in achieving this."




Author: Jürgen Skowaisa, Product manager Radar VEGA Germany
Doug Anderson, Marketing Mananger VEGA UK


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