Level best: radar meets measurement needs
Accurate and reliable level measurement is vital in food and beverage manufacturing, but demanding process conditions can make this very difficult. Philip Holland, business development manager at Emerson, explains why non-contacting radar transmitters using frequency modulated continuous wave (FMCW) technology provide a robust level measurement solution.
Food and beverage manufacturers must be able to accurately and reliably measure the level of liquid and solid products contained within the various tanks, silos, hoppers and bins typically used in their storage and production processes. There are many reasons why precise and dependable level measurement is essential in production. These include ensuring consistent product quality, optimising inventory management, and providing protection against tank overfills and pumps running dry, all of which can affect profitability.
However, many food and beverage applications involve challenging process conditions in which level measurement instrumentation must be able to perform seamlessly. These conditions can include large temperature variations, foam, dust, turbulence, condensation and the presence of agitators in vessels.
Non-contacting radar transmitters
One of the most widely applied technologies that is suitable for the most challenging applications is non-contacting radar transmitters. These devices provide a highly accurate direct measurement of the distance to the surface of liquids, sludges, slurries and solids. A significant advantage of this technology is that its performance is unaffected by process conditions such as density, viscosity, conductivity, coating, corrosiveness, vapours, and changing pressure and temperature. Non-contacting radar transmitters also have no moving parts and do not come into contact with the product surface, which results in low maintenance requirements and promotes long-term performance and reliability. In addition, their advanced built-in diagnostics capability supports preventive maintenance; their installation and commissioning is very straightforward; and devices can be proof-tested remotely, thereby improving operational efficiency and increasing worker safety.
Non-contacting transmitters perform level measurement using one of two main techniques — either pulse or frequency modulated continuous wave (FMCW). The power/sensitivity of transmitters based on FMCW technology can be more than 30 times higher than in devices using the pulse technique. This maximises their signal strength and enables them to deliver greater measurement accuracy and reliability, even when measuring level in the most challenging environments. The additional sensitivity and measurement reliability provided by FMCW transmitters makes them an ideal solution for a broad range of challenges.
Until recently, a drawback of FMCW technology had been its need for more processing power, which has led to FMCW transmitters typically being deployed only within four-wire devices. This can require additional cable infrastructure, which is both costly and time-consuming. However, the latest FMCW transmitters are more energy-efficient and need only two wires for power and communication. This enables simple installation while still providing the same high amount of data and diagnostics that would normally require four-wire connections.
Many food and beverage environments have large variations in ambient temperature, which can impact pulse radar devices as their accuracy is based on reference conditions at 15°C. FMCW transmitters use a crystal oscillator to perform online adjustment of the transmitted frequency, thereby ensuring consistently high accuracy in dynamic ambient temperature conditions.
The effect of foam on radar measurement depends largely on the foam’s properties, such as its thickness, density and dielectric constant. Microwaves typically pass through dry foam and detect the product surface below, but with wet foam, microwaves are often reflected from the foam surface and thus it is the foam surface level that is measured. The latest FMCW transmitters overcome this problem with a double surface handling function that enables the product surface to be measured rather than the foam layer.
In solids and powders applications, 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. The latest devices overcome this with an integrated air purging system for cleaning the antenna.
Agitators, turbulence and condensation
Other process conditions that can impact measurement accuracy are agitators in tanks, turbulence and condensation. A good example of how the latest FMCW devices are able to overcome these challenges can be found at an application in Australia, where a manufacturer required accurate and reliable level measurement in a stainless steel mixing tank containing starch slurry.
The manufacturer had tried using various pulse radar transmitters, but none of them provided an accurate or reliable level measurement. The three-metre-high tank contained agitators for mixing, and these obstructions were causing false signal reflections, making it difficult for the pulse transmitters to detect the true surface level. This resulted in frequent false high-level alarms which interrupted production and a high risk of tank overfills. Further problems were caused by the turbulent liquid surface in the vessel. The turbulence caused the pulse transmitters to sometimes lose individual pulses, causing them to misregister and lock onto the next available pulse, which resulted in inaccurate level measurements. In addition, high amounts of condensation within the tank were causing the pulse transmitters to experience problems with loss of level measurement, again leading to alarms and inaccurate measurements. These inaccuracies were not only proving to be costly, but they also put the pumps in danger of running dry when the devices showed the level to be higher than it really was.
The manufacturer overcame all these challenges by installing an FMCW-based Rosemount 5408 non-contacting radar transmitter from Emerson. FMCW technology allows sophisticated software algorithms for masking and ignoring false signal reflections caused by tank obstructions such as agitators, so that measurement accuracy is unaffected. These algorithms are not possible with pulse transmitters. Measurement errors caused in pulse radars by lost individual pulses cannot occur in FMCW transmitters because of their different measurement technique, thereby increasing their accuracy and reliability. The condensation that affected the pulse transmitters was also not a problem for the FMCW technology, which is generally unaffected by condensation and low-pressure steam. It should be noted that the measurement accuracy of FMCW devices can sometimes be affected by heavy condensation, but in such instances, adding an air purge or selecting the process seal antenna can reduce the negative effects of heavy condensation.
Since being installed, the greater signal sensitivity provided by the Rosemount non-contacting radar transmitter has delivered the accurate and reliable continuous level measurements required by the manufacturer. This has prevented tank overflows and pumps running dry, thereby increasing safety. In addition, there has been no need for downtime to perform pump maintenance or clean-ups, which has increased output considerably.
To learn more about the benefits of non-contacting radar transmitters based on FMCW technology, visit Emerson.com/rosemount5408.
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