Versatility key in level measurement technology

1st February 2013

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The latest level measurement technologies can cope with dust, noise, strong air currents and corrosive atmospheres - while being robust, accurate and low-maintenance. Sean Ottewell reports.

VEGAPULS SR 68, VEGA's new radar sensor for continuous measurement of bulk solids, is especially suitable for level measurement in silos and bunkers with heights up to 30 metres.

The company is well known for its innovative level measurement technology and pragmatic solutions that offer the user many practical benefits. When the wish for a reliable level sensor for measuring ranges up to 30 metres arose, the company reacted quickly. Its tried and true radar technology was adapted once again to the requirements of users - a new sensor optimally covering applications in medium-height silos was the result.

With its 30 metre measuring range, VEGAPULS SR 68 is ideally suited for a wide variety of bulk solids applications and offers the user all the advantages of radar technology at a keen price. Uninfluenced by dust, noise and strong air currents during filling, the sensor reliably detects the level and contributes to profitable plant operation. Since even the weakest radar reflections from the surface of the bulk solid material can be detected, the sensor is employed in a broad range of applications. From plastic granulate to grain to coal - with VEGAPULS SR 68, reliable level measurement of any medium is guaranteed and fast product changes can be carried out easily without a fresh adjustment (Fig.1).

The flexible sensor concept allows individual adaptation to the respective requirements of the measuring point. Antenna covers are available for dust protection and rinsing connections for keeping the sensors free of sticky bulk material. Different antenna systems with process separators of plastic and ceramic make application possible in environments over 250°C and give the user the necessary measuring certainty for smooth operation of production systems.

In another development, VEGA says that its new VEGAPLUS WL 61 is the first radar level sensor developed especially for the water/sewage sector. Compared to classic measuring techniques, such as ultrasonic, this modern technology delivers far more reliable measurement data and has proven successful in everyday operation, says the company.

The new radar sensor was conceived especially for use in water management and is a real alternative to the commonly used ultrasonic sensors, also with regard to price.

Radar technology offers considerable advantages over ultrasonics because it operates without being affected by temperature, wind, fog or rain. And the much higher accuracy of radar sensors makes a big difference in applications such as flow-rate measurement in open channels, where the measuring ranges are sometimes very small.

The completely encapsulated cable and robust housing of VEGAPULS WL 61 provide a high protection rating of IP 66/68 (1 bar). Thanks to the different mounting options the sensor can be easily integrated into existing infrastructure.

VEGAPULS WL 61 is used primarily in applications where users demand high precision and reliable function and previous solutions have not fulfilled all requirements.

One application, somewhat apart from classic level measurement, is level measurement of rivers. The small, light radar sensor, mounted on a jib, records the level of the water contactlessly. Compared with elaborate gauging pipes, the costs of installation and maintenance are very low. And unlike non-contact measurement with ultrasonic devices, radar is not influenced by changing temperatures, solar radiation or strong gusts of wind. It measures the river level with a precision of +/-2mm and provides early warning of impending floodwaters.

Simple mounting, fast setup and low maintenance requirements make VEGAPULS WL 61 the ideal sensor for many different applications - from level or gauge to flow-rate measurement, says VEGA.

New from SIEMENS is is the SITRANS LR560 2-wire, 78GHz frequency modulated continuous wave (FMCW) radar level transmitter for continuous monitoring of solids in silos to a range of 100metres. SITRANS LR560's plug and play performance is ideal for most solids applications, including those with extreme dust and high temperatures to 200°C. A novel design allows safe and simple programming using the intrinsically safe handheld programmer without having to open the instrument's lid.

The transmitter includes an optional graphical local display interface (LDI) that improves setup and operation using an intuitive quick start wizard, and echo profile display for diagnostic support. Startup is easy using the quick start wizard with only a few parameters required for basic operation.

In terms of overall benefits: the rugged stainless steel design is ideal for industrial applications; the 78GHz high frequency provides a very narrow beam, virtually no mounting nozzle noise, and optimal reflection from sloped solids; an optional aimer can direct a beam to an area of interest, such as draw point of cone; the lens antenna is highly resistant to product build up; an air purge connection is included for self-cleaning of extremely sticky solids; and the LDI allows local programming and diagnostics

The LR560's key applications are with cement powder, plastic powder/pellets, grain, coal, wood powder, and fly ash.

OMEGA's contribution to the level measurement market includes numerous models and technologies. For example, its non-contact ultrasonic level sensor LVU41 comes with power and programming options, RS-232 or RS-485 communications capability, pushbutton calibration, and temperature-compensated signalling. They are highly accurate over their entire range of 0.3 to 18.3 metres and incorporate filtering that virtually eliminates false echoes from peripheral obstructions. In conjunction with a programmable logic controller or an OMEGA CNi Series controller, they can be used for point level measurement.

The company's LVRD500 series are an affordable solution in applications requiring non-contact measurement of liquid levels. They are ideal where vapour, dust, or a foaming surface prevents ultrasonic measurement. Their radar sensor uses microwave pulse technology to track target liquids from the antenna tip to the bottom of a tank. This advanced 'echo marker' signal processing provides a reliable continuous pulse shape unaffected by environmental conditions. RS-232 and RS-485 models are available, too.

Then there is OMEGA's LVD800 series of rotary paddle switches. These use patented magnetic technology that makes fail-safe bin, hopper, silo, and tank monitoring a reality. A self-diagnosing microcontroller constantly monitors rotation of the shaft and motor mounting plate, reacting immediately to material presence and to mechanical or electrical failure. Magnetic sensing avoids the fouling problem inherent to optical systems. The twist-on cover does away with bolts, there are two conduit entrances, and most models have a status light. High-temperature models are rated to 399°C.

The company's LV3000/4000 Series probes give reliable continuous level measurements in difficult applications. Appropriate for liquids, pastes, and some solids - whether conductive or non-conductive - they have no moving parts and are easy to install. After rectifying and filtering incoming power, generating a radio frequency signal, and calculating changes in current, the electronic circuitry produces a 4-20 mA 2-wire output signal proportional to the process level. Convenient zero and span adjustments let the user factor in variables such as media type, vessel dimensions, rod length, and installation position. If the stainless steel probe will be used near a conductive vessel wall, a PTFE-insulated model will prevent short circuiting.

New from Pepperl + Fuchs is the LGC hydrostatic level control for level measuring and temperature control (optional) of freshwater drinking water and wastewater. The measuring system consists of a LGC level probe and a SMART transmitter power supply.

Central to the control is a ceramic measuring cell (dry measuring cell). The pressure acts directly on the rugged ceramic membrane of the LGC level probe and causes it to move by a maximum of about 0.005mm. The effects of air pressure on the liquid surface are transferred via a pressure compensating tube through the extension cable to the rear of the ceramic membrane and compensated. Pressure-dependent changes in capacitance caused by membrane movement are measured at the electrodes of the ceramic carrier. The electronics then convert the movement into a pressure-proportional signal.

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