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Revisions are currently being made to European standards that will, in the future, allow process environments to adopt a radar device for level measurement in both open and closed applications.

Derek Moore, from Siemens Industry Automation and Drive Technologies, discusses the standards that are currently in place, those expected in the future and what those working in process environments can use for now.

Level radar devices may be the equipment of choice for many working in process environments, but European standards mean they can currently only be used within closed, not open, applications.

Until recently, level radar devices were not covered at all by a European standard and so fell under the guidance offered by EN 300 440 – the standard that commonly covers generic short-range devices.

Typical level radar devices available on the market are wide band (usually 1GHz or more) and as such they do not fit into available frequency bands (for example, the industrial, scientific and medical (ISM) band at 24GHz is only 250MHz wide).

Until recently, manufacturers have marketed their radar devices in Europe by demonstrating that when installed in a closed tank, emissions measured outside of the tank are below the allowed spurious emission limit of -41.3dBm/MHz.

EN 302 372, a more efficient standard for level radar devices in closed tanks, has recently been published.

This standard lays down clear requirements (including power and frequency of operation) for the use of level radar devices installed in closed tanks.

Use of this standard, and the frequency bands used by level radars installed in tanks, was harmonised across the European Union in November 2009, making radio approvals easy for closed-tank use in Europe.

The European Telecommunications Standards Institute (ETSI) is the body responsible for creating associated standards.

In a new initiative, ETSI has established a committee to investigate the creation of a harmonised standard for level radar intended for operation in open-air applications.

This work is at a very early stage and could take several years to complete.

A major concern for the regulators is the power emission level and wide bandwidth of level-radar devices.

All 25 – 26GHz level radars currently on the market do not meet the emission-level limits and band designation that allow use in open air in Europe.

Until the new initiative leads to an adopted open-air harmonised standard, operation of level radar devices in Europe will be restricted to closed tanks.

While the industry awaits a European standard that allows the use of radar devices in open-air applications, there are many other technologies that can be used for level measurement.

Non-contacting level measurement, including ultrasonic, laser and radar, can be applied in endless potential applications.

While each technology has its advantages and disadvantages, ultrasonic continues to be the technology of choice for most open-air or non-enclosed vessel applications, due to cost, performance and radio-approval standards.

Open-air applications for level measurement include any application where the silo or vessel has a large opening to the outside environment, or where there is no vessel, in the case of a pile of stones in an aggregate pit.

Ultrasonic technology offers cost-effective high-performance solutions because of the following application characteristics: measurement ranges are usually short, less than 10m (33ft); dust levels are not severe; and it has excellent reflection properties for low dielectric / high density targets.

Ultrasonics are perfect in these applications because: rugged, encapsulated sensors offer extreme vibration and shock resistance; the narrow beam of the transducer can be aimed towards a specific material level of interest; they are completely legal and can be used without restriction in open-air environments; a highly active sensor face keeps itself clean in the mildly dusty environment; and they have an excellent track record in thousands of applications over the past three decades.

A significant number of these open-air applications are found in the aggregate market, where high vibration and the potential for the occasional knock with a stone is the norm.

Encapsulated ultrasonic transducers are ideal for this environment: the sensor mounted at the application site contains the mechanical transducer components and the sensitive electronic components can be remotely located.

Radar technology has recently gained wide acceptance and experienced high growth rates on solid applications and has even become the preferred level-measurement technology in the cement industry for long-range dusty applications.

Radar devices are highly successful on very dusty applications, because the technology uses electromagnetic waves in the microwave spectrum, usually six, 10 or 25GHz.

These waves travel at the speed of light and are generally unaffected by vapour, pressure, temperature or dust.

Electromagnetic waves travel through the environment with almost no attenuation and are used for many other applications, including radio and television transmission, navigation, speed detection and cellular transmission, thus requiring careful regulation.

Until a European standard is created that allows level measurement via a radar device in both open and closed applications, ultrasonic technology is a reliable alternative to level measurement.

Siemens Automation and Drives

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