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Increasing demands in the process industry are driving the search for more precise and reliable level measurement. Alan Hunt, of ABB Measurement & Analytics, looks at how these demands are being met by advances in level measurement technologies

robably the most well-known industrial level measuring device is the sight glass, which for a long time served as the method by which operators assessed process conditions. As a manual approach to measurement, however, sight glasses have always presented a number of limitations. The material used for transparency can suffer catastrophic failure, causing environmental impact, hazardous conditions for personnel and/or fire and explosion. Their seals are prone to leaks whilst build-up, if present, can obscure the visible level. Because of these drawbacks, conventional sight glasses have been replaced with more advanced technologies. Other level detection devices include


those based on specific gravity, such as a simple float. Hydrostatic head measurements have also been widely used to infer level.

The trend today is to replace mechanical and pressure-based measurement tools with systems that measure the distance to the fluid surface by a timing measurement, commonly termed as ‘time of flight’. Put simply, this technique operates by measuring the distance between the liquid level and a reference point at a sensor or transmitter near the top of the vessel. Magnetostrictive, ultrasonic, laser and guided-wave radar transmitters are among the most versatile technologies of this type available. Such systems use the sharp change of a given physical parameter at the process fluid surface, such as density, dielectric constant and sonic or light reflection, to identify the level. Magnetostrictive level transmitters – The advantages of using a magnet containing a float to determine liquid level are already established. Instead of mechanical links, magnetostrictive transmitters use the speed of a torsional wave along a wire to find the float and report its position. In a magnetostrictive system, the float carries a series of permanent magnets. A sensor wire is connected to a piezoceramic sensor at the transmitter and a tension fixture is attached to the opposite end of the sensor tube. The tube either runs through a hole

in the centre of the float or is adjacent to the float outside of a non-magnetic float chamber.

To locate the float, the transmitter sends a short current pulse down the sensor wire, creating a magnetic field along its entire length and simultaneously activating a timing circuit. The time interval between the start of the current pulse and the wave's arrival at the piezoceramic sensor is then measured. Using this information, the float's location can be precisely determined and presented as a level signal by the transmitter. Key advantages are that the signal speed

is known and constant with process variables such as temperature and pressure, and the signal is not affected by foam, beam divergence, or false echoes. Another benefit is that the only moving part is the float that rides up and down with the fluid's surface. Ultrasonic level transmitters –

Ultrasonic level sensors measure the distance between the transducer and the surface using the time required for an ultrasound pulse to travel from a transducer to the liquid surface and back. While the sensor temperature is compensated for – assuming that the sensor is at the same temperature as the air in the headspace – this technology is limited to atmospheric pressure measurements in air or nitrogen. Laser level transmitters –Designed for

bulk solids, slurries, and opaque liquids such as dirty sumps, milk, and liquid styrene, lasers use the speed of light to find the level. Lasers are very precise, even in vapour and foam, are suitable for use in vessels with obstructions and can measure distances up to 1,500 ft. For high temperature or

high pressure applications, such as in reactor vessels, lasers must be used in conjunction with specialised sight windows to isolate the transmitter from the process. These glass windows must pass the laser beam with


In addition to ensuring the accuracy of processes, tank level measurements are used to minimise environmental impact and avoid hazardous conditions arising

The latest level measurement technology makes use of the latest electronic techniques and incorporates embedded microprocessor-based digital computers for control, analysis and communication functions

minimal diffusion and attenuation and must contain the process conditions. Radar level transmitters –Through-air radar systems beam microwaves downward from either a horn or a rod antenna at the top of a vessel. The signal reflects off the liquid surface back to the antenna, and a timing circuit calculates the distance to the liquid level by measuring the round-trip time. A drawback of through-air radar systems is that the radar waves suffer from the same beam divergence that afflicts ultrasonic transmitters. Internal piping, deposits on the antenna, and multiple reflections from tank build-up and obstructions can all cause erroneous readings. To overcome these problems, complex algorithms using fuzzy logic must be incorporated into the transmitter, which means set-up can be tedious. These problems can be overcome by using guided wave radar (GWR) systems. As the guide provides a more focused energy path, guided wave radar is 20 times more efficient than through-air radar. Different antenna configurations allow measurement down to dielectric constants of 1.4 and lower. Moreover, these systems can be installed vertically, or, in some cases horizontally, with the guide being bent up to 90° or angled to provide a clear measurement signal. GWR exhibits many of the advantages and few of the liabilities of ultrasound, laser, and open air radar systems. Radar's wave speed is largely unaffected by vapour space gas composition, temperature, or pressure. It works in a vacuum with no recalibration needed, and can measure through most foam layers. Confining the wave to follow a probe or cable eliminates beam-spread problems and false echoes from tank walls and structures. Refined digital electronics are making level sensors and other measurement devices increasingly accurate, reliable, easy to use and less costly to purchase and own than ever.



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