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Level regulation without blemish - Balluff SmartLevel sensors in chocolate paradise

20 January 2016

Capacitive sensors can detect levels of liquids, powders and granular materials through non-metallic container walls without the sensor coming into contact with the substance. 

However, conventional capacitive sensors also come with number of disadvantages. The sensitivity of the sensor must be adjusted so that the signal is triggered not by the container itself, but by the container wall together with the medium, which is time-consuming.

Furthermore accurate detection using a conventional capacitive sensor where the sensor, via a hole in the container wall,  is in contact with the medium  to detect the level of chocolate or ketchup for example can be problematic. 

These substances do not drain without leaving residue on the sensor  which  regularly leads to sensor false triggering,  making error-free measurement impossible. 

This was the challenge that renowned Swiss chocolate manufacturer Gysi faced. Gysi needed to equip the agitators of various tempering machines for heat treatment of chocolate when retrofitting new sensors for level regulation. 

The previous level detection system based on measuring the pressure difference regularly had to be checked and cleaned, incurring considerable effort and downtime. 

Therefore Gysi looked for a new solution. First attempts with a built-in sensor from the machine manufacturer, however, did not look promising. 

"We couldn't leave the application unattended," says Ulrich Streit, who is responsible for the technology at Gysi, "the container kept overflowing. That was caused by sensing errors right in the temperature range from 45 to 28 °C, in which the tempering machine operated. 

“All it took was a temperature change of a few degrees to change the permittivity of the medium in use enough that the sensor no longer switched correctly.

"Balluff provided us with comprehensive consultation and together we chose the right sensor based on the polarity of the medium to be measured – the chocolate – and then checked it in a test installation in real-world operation.” 

Gysi decided on a sensor with SMARTLevel technology from sensor specialist Balluff. 

Streit adds: “It worked right way, now we have a solution that works without any errors whatsoever, even in long-term operation.” 

Bryan Hall from Balluff UK explains: “Because our SMARTLevel sensors work well with strongly conductive media they open up new application fields while being significantly easier to handle, so they are capable of independently compensating for container walls and deposits, which enables error-free measurement without elaborate readjustments. 

“At the same time, they are also compatible with all sensors used for level measurement of media having a dipole character. This applies to immersion applications and level detection through non-metallic walls with a maximum thickness of 10 mm. As a result, even detection of chocolate through the 3-mm thick membrane of the plastic sleeve into which the sensor is screwed is an easy task. 

This is possible because SMARTLevel sensors operate at an oscillator frequency significantly higher than conventional capacitive sensors. In addition, the patented electronic processor unit gathers more information than is usually the case with capacitive level measurement. 

It evaluates not only the capacitance, but also the conductivity value of the medium and through this analysis  the new sensors have no trouble distinguishing between  deposits on the sensing surface of the sensor and the real liquid  level. This means that sensing errors with media that do not drain without leaving residue, such as chocolate, are largely prevented. 

Gysi now has six machines retrofitted with the new sensors and there are already plans to retrofit additional systems. "Since we do the conversion ourselves," Streit continues, "the expenses are low. To do it, the sensor is simply inserted into a sealed plastic sleeve and rotated into a separate metal container in the container wall such that it is flush with the wall." 

The sensor in the container wall detects the level of the chocolate directly through the end face of the plastic sleeve in the container wall. If the chocolate falls below a certain fill level, the sensor triggers and after 30 seconds liquid chocolate is refilled until the optimum fill level is reached. 

The SMARTLevel sensor is easy to adjust with a potentiometer, which has to be done only once after the installation. Furthermore, it operates without needing any maintenance whatsoever, cleaning procedures for the sensor are taken care of as part of the regular maintenance cycle.

Unlike conventional capacitive sensors, these fill-level indicators do not have to be readjusted, neither during operation, nor when changing the recipe. Thus the switch point between white and dark chocolate, for example, differs by only three millimetres.

"It is somewhat more expensive than a standard capacitive sensor," summarizes Ulrich Streit, "but when you find a perfect solution that works permanently, the price plays only a subordinate role. The device pays itself off within a very short time." 

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