Reflections on level

Curiously, it's been our desire to keep in touch that has been the biggest factor in the increased popularity of radar level detection.

When radar level detectors were first developed, they tended to be large, heavy and consumed large amounts of power, especially seeing as higher sensitivity demanded a larger antenna and higher power consumption. This meant that their use was generally confined to shipping, particularly on oil tankers. What changed all this was the explosion in popularity of mobile phones.

The advent of mobile phone technology has been a major factor in the development of radar. Like radar, mobile phone technology uses microwaves, and the popularity of the phones has driven research and development into improving the quality of transmission and detection components, while also as reducing their size.

The latest generation of chips has made possible huge advances in signal processing, which in turn has increased the sensitivity of radar sensors; for example, Endress + Hauser's latest radar product, the Levelflex M sensor, is capable of measuring the level of plastic powders and granules, and can provide level readings even when silos are being filled, producing clouds of dust.

The effect of technology has been particularly noticeable over the past five years, where solid state, fieldbus-connected radar units have hit the market. The technology has gained in popularity across the board, and seems set to become the most popular method of level measurement in the near future.

Emerson, for example, has introduced what it claims to be the first two-wire loop-powered radar transmitters. Working on the guided wave principle, the transmitter head sends a microwave pulse down into the tank along a probe which runs from the instrument to the base of the tank. This prevents the signal from spreading out through the tank and improves the signal-to-noise ratio.

The instrument then receives signals back from the liquids and solid materials surrounding the probe and analyses them. When the probe is surrounded by materials of different dielectric constant, a portion of the pulse is reflected by any interfaces it encounters.

Turbulent times

The transmitter incorporates digital signal processing to decipher the reflected pulses, which according to Emerson give higher resolution and more reliable measurement than other types of transmitter.

The device, dubbed the Rosemount 3300 by Emerson, is most suitable on tanks containing turbulent or agitated liquid surfaces or where the vapour spaces may be full of dust or steam. It can also be used with vessels containing solids with contoured or sloping surfaces. The key to the instrument's usefulness is the Rosemount MultiVariable signal processing system, which allows the device to detect both the top liquid surface level and the lower interface level on tanks containing two immiscible liquids.

Distant echoes

Also using this system is the company's new non-contact measurement transmitter for process applications on liquids and slurries, the Rosemount 5600. Capable of measuring levels up to 20m away, the transmitter uses frequency-modulated continuous wave (FMCW) echo tracking, a technique which uses linear sweeps of signal with constant amplitude across the surface. These transmitters are equipped with software allowing them to distinguish between false echo sources and true surface echoes.

There are some applications where the limits of the technology are stretched. For example, they can have problems when the reflectivity of the surface is poor, leading to inaccuracies when the liquid surface is near the antenna. This can be overcome by using transmitters which work at high frequencies.

For example, Vega Controls installed a 26GHz Vega Puls transmitter in a polymerisation tank at Wacker Chemie in Burghausen. This replaced an earlier lower frequency (5.8GHz) model. The units had the same size flanges, so the installation was simple. The set-up was more complex, involving storing the interfering reflections from the tank stirrer and then running the system with the tank filled with water. But once the process had been started, the system was found to work perfectly, with accurate readings even when the product surface was close to the radar antenna.

Instrumentation giant Siemens Process Instruments is also present in the radar sector with its Sitrans product line. It has found that the products can solve some seemingly intractable problems with a range of different products. The cement sector, for example, presents challenges to any process equipment - it uses aggressive materials which cause both corrosion and erosion, and instruments must be able to withstand (and in the case of level sensors, see through) ever-present clouds of dust. For example, Canadian Portland cement producer St Mary's Cement, based near Toronto, turned to Siemens radar equipment for a critical application which prevented its production vessel from running out of material.

Seeing through dust

The vessel is fed directly from a homogenising silo whose contents are critical to the operation of the plant. Operators control the level in the silo to optimise the quality of the final product, while the production department use the level readings to check inventory levels and plan output quantities. The readings are also used to schedule shutdown periods.

The problem was that the conditions inside the vessel are about as unpleasant as it's possible to be. The powdery material in the vessel creates clouds of dust whenever it is agitated and aerated, which ultrasonic sensors cannot penetrate, and yo-yo type sensors failed due to turbulence inside the vessel which broke the cables. However, a radar device - a four-wire Sitrans LR 400 - proved the answer to their problems, with its 24GHz FMCW signal penetrating the dust clouds and its signal processing capability suppressing false echoes. Moreover, it proved extremely easy to install - it was simply bolted on top of an existing manhole cover.

Siemens's portfolio also includes the waveguide type of instrument, several of which are currently in service at BP's lubricants refinery in Hamburg. The company uses the instruments on large tanks which store crude, white oil and other petrochemicals. The 50mm-wide waveguides on the instruments prevent the signals bouncing off the walls of the tanks' 400mm diameter stillpipes.

Radar products have also penetrated the food and drink sector, albeit with modifications necessary to meet hygiene requirements. Like all other food equipment, the sensors must be made of a neutral, easy to clean material. The Rotkäppchen company, which produces sparkling wine at Freyburg in south-west Saxony uses Vega's hygienic radar detectors, with PTFE sensors, to measure levels in its production tanks.