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Meeting level measurement challenges with radar

17 August 2019

Denny Nelson explains how the latest non-contacting radar transmitters can overcome food processing challenges using frequency modulated continuous wave technology.

There are various reasons why accurate and reliable level measurement is essential in the tanks, silos, hoppers and bins in which products are processed or stored. These include optimising inventory management and stock availability, and helping to ensure consistent product quality, all of which can affect profitability.

However, level measurement instrumentation must be able to withstand the challenging conditions present in typical food and beverage production applications. These include long measurement distances in tall silos, materials with a low dielectric constant, along with dust and uneven surfaces when measuring solids and powders. Devices must also comply with strict hygiene requirements to prevent product contamination.

A wide range of level measurement technologies are typically employed within modern food and beverage production. These include guided wave radar transmitters, differential pressure transmitters, vibrating fork level switches, capacitance point level switches, ultrasonic transmitters, acoustic transmitters, laser-based devices and load cells. These technologies must not only be able to meet the demands of each specific application, but must also have food grade parts and the appropriate certification for use in hygienic applications.

An innovative new level measurement technology for challenging food and beverage applications is non-contacting radar transmitters. These devices provide accurate continuous level measurements without touching the material surface, which results in low maintenance requirements, helping to ensure long-term reliability. In addition, their on-board diagnostics capability supports preventive maintenance. Non-contacting radar transmitters have a large measurement range capability and also provide the benefit of being unaffected by a wide range of process conditions, such as density, viscosity, conductivity, coating, corrosiveness, vapours, and changing pressure and temperature.

Modulation techniques
Non-contacting radar transmitters use one of two main modulation techniques – either pulse radar or frequency modulated continuous wave (FMCW) – to perform level measurements. The sensitivity of devices based on FMCW technology can be more than 30 times higher than that of pulse transmitters. This means that their signal strength is greater, enabling them to provide superior measurement accuracy and reliability, even when measuring materials with a low dielectric constant.

In solids applications in small to medium-sized silos, where there can be rapid changes in level, the latest FMCW transmitters can provide high accuracy by using a measurement algorithm that merges the peaks of an uneven surface. Large amounts of dust are created during fill cycles, which can pose problems for some level measurement technologies. Non-contacting radar transmitters usually handle dust well, but the signal can be blocked if there is a heavy layer of dust on the antenna. However, the latest FMCW devices can overcome this challenge with an integrated air purging system for cleaning the antenna.

Although FMCW technology provides advantages in terms of sensitivity and accuracy, its need for more processing power has traditionally been a drawback. For this reason, FMCW technology was typically deployed only within four-wire devices, thereby often requiring users to put additional cable infrastructure in place, which is costly and time-consuming. However, advanced two-wire FMCW transmitters are now available, enabling simple installation using existing cable infrastructure, and thus reducing costs and saving time. These transmitters overcome the problem of high processing power requirements for FMCW technology by replacing the traditional circuit board with radar-on-chip technology, enabling them to be less power-hungry and more energy-efficient.

Denny Nelson is a marketing engineer at Emerson Automation Solutions.

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