Modern sensors provide precise insights into production

Wireless technologies, ethernet and asset management systems - in recent years, a number of new technologies have entered the food production industry and substantially changed the world of process automation. Despite all that, one thing has remained the same: the central importance of sensors.

Measurement technology is more important than anything else when it comes to monitoring operations in the production process and supplying the control system with the necessary signals. Today’s ‘intelligent’ measuring instruments know more about the process than generally thought and even make it possible to react earlier to critical conditions at the production plant.

Sophisticated environment = high demands on the sensors

No matter what food is being produced, the measurement conditions are seldom ideal in practice. Dynamic process conditions represent a particular challenge, because each point of measurement is subject to changing operating conditions. In the vats, tanks and process equipment the raw materials used vary in density, consistency, conductivity and pH value. If extreme temperature fluctuations then come into play as well, as in the case of freeze drying, normal sensors quickly reach their limits.

As a general rule, producers have to choose between sensors that come into contact with the product they are measuring and those that don’t. And measuring instruments used for dry bulk goods have to meet requirements that are very different from those imposed by liquid media.

Beverages involve their own specific factors, ranging from flow properties in pipes to aspects of frothing and gas overlay. Moreover, the measurement technology must not only be precise and reliable, it also has to satisfy the stringent hygienic regulations in the food industry. As one might expect, chief among these requirements is a high protection rating.

In other words, the sensors must be optimised for CIP and SIP processes as well as external cleaning. That in turn places highest demands on housings, electronics and sensor circuitry.

Which technology for which application?

Temperature

Whether it be beverages, delicatessen food or baked goods, temperature is an important parameter in all sectors.

Temperature sensors in the food industry are almost always equipped with Pt100 sensors of various levels of precision. These sensors are based on the way in which changes in temperature cause changes in the electrical resistance of platinum. Because of their low response time, special thin-film sensors based on this technology are faster than simple platinum elements.

Level

In cases where filling level is measured, a distinction is made between continuous measurement and limit level detection.

For continuous measurement of bulk goods, the established technology uses radar or microwaves. Freely radiating devices are usually used, but sensors with rod or cable antennas are becoming increasingly popular also. Limit level detection is accomplished with both capacitive probes and vibronic level switches, with the latter detecting the rate of attenuation of a vibrating fork or rod. Fill level measurement in liquids is based on the measurement of hydrostatic pressure; this pressure results from the liquid column above the sensor and is directly proportional to the fill level. But liquid fill levels can also be measured with microwave systems, either freely radiating devices or systems that come into contact with the product. The less-expensive variants include capacitive sensors and ultrasound sensors.

Flow

A large number of diverse options are available once again when it comes to flow measurement.

Since most foods are conductive, they can be measured with the established magnetic-inductive sensors. If the product is not conductive, Coriolis sensors are used. In contrast to magnetic-inductive sensors, these measure not volume flow but mass flow. Another advantage of Coriolis devices is their robustness. The measuring tube can be made of stainless steel, Hastelloy, titanium or zirconium, which means that the benefits of this measurement principle can also be exploited when critical media are involved.

Measuring instruments with ultrasound sensors are less precise than the Coriolis technology. They make up for that by being more hygienic, however. They are attached to the outside of the pipe or tube (clamp-on method) and have no contact with the product: the sensor never becomes dirty, and there are no signs of wear and tear.

Multiparameter instruments - the all-rounders

Coriolis mass flowmeters and ultrasound volume measurement instruments are some of the innovative multiparameter technologies with the greatest growth potential in the food industry.

Coriolis mass flowmeters in particular are now regarded as state of the art and considered true all-rounders. They provide safe and highly accurate measurements of gases and liquids, and they permit the simultaneous measurement of multiple parameters, such as mass flow, density, temperature and concentration (degrees Brix, degrees Plato).

The fact that these instruments measure such a large number of parameters suggests that they might also be used for inline quality control.

Corrosion or deposits, which lead to changes in food, are already reliably detected and signalled with Coriolis mass flowmeters. Many devices are also capable of detecting when pipes are empty or only partially full. There are now flowmeters available that can detect gas bubbles starting at only 1.5 to 2% air content in the media being measured. That makes it possible for the plant control system to react much earlier to critical conditions, such as a dry running pump.

With modular designs, the user can always extend the functionality of his measurement equipment and adapt it more closely to his process by means of upgradeable software.

The latest developments in sensor technology will be on display at Anuga FoodTec. This international trade fair, covering the latest trends in food and drink technology, will take place from 27 to 30 March 2012, in Cologne, Germany. For more information visit www.anugafoodtec.com.