In the food industry, the hygienic requirements are continually growing. This poses a problem for many standard sensors because they are only of limited suitability for use in product-contacting areas. This is where the advantages of ultrasonic sensors come into play: they detect and count objects without contact, regardless of color and transparency. For this purpose, the sensors cyclically emit short sound pulses more than 20 kHz (ultrasound) and measure the time until the arrival of the echo signal – like a bat. These values can be used to calculate the distance to the object or the fill level.
A provider of ultrasonic sensors for distance measurement is microsonic, whereby the original areas of application of the sensors are, for example, when measuring the level of paints and varnishes in the printing industry or when measuring the winding diameter of a paper roll. However, the advantages of ultrasonic sensors have also made them increasingly popular in the food industry: for example, the pms ultrasonic sensors that comply with the EHEDG guidelines (European Hygienic Engineering & Design Group) and are made of FDA-approved materials.
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Slightly sloping surfaces ensure hygiene
When looking at the sensor, the unusual shape of the 1.4404 stainless steel case stands out. None of the surfaces of the sensor is aligned horizontally, regardless of the mounting position. This ensures that cleaning and disinfecting liquids always drain. Even with a level measurement in a vessel where the sensor measures vertically downwards, the rear of the housing is chamfered by ≥3 ° so that cleaning liquids drip off even in this installation position.
Also, the smooth sensor housing has no joints or edges in which food residues or bacteria could accumulate. The ultrasonic transducer itself protects a PTFE film that repels aggressive cleaning and disinfecting agents. As a result, the sensor has a high resistance and is Ecolab certified. With four detection ranges, the ultrasonic sensors cover a measuring range of 20 mm to 1.3 m.
The EHEDG guidelines also require the hygienic installation of the sensor. In the search for a solution, micro sonic was at the company Lapp find. Melanie Harke, Head of Marketing at microsonic, remembers trying out some cable glands for sensor mounting, “but only the Skintop Hygienic from Lapp has met our requirements.” In the combination of sensor and screw connection, the torsional strength is essential: The sensor must be firmly fixed after installation and must not twist during maintenance or cleaning operations in its fastening, the cable gland – otherwise, this would falsify the measured values. With the Skintop cable gland, this requirement is met by the special design, the geometry of the individual parts and the choice of material.
No dead spaces due to seals
When choosing the elastomer, it needs know-how for suitable designs, shore hardness, material displacement and the achievable holding force, because the elastomer must be entirely and firmly around the cable or in the case of microscopic around the 12 mm sensor shaft – that prevents the unwanted pulling out and twisting of the cable. For this purpose, the elastomer seal is longer towards the cable; thereby additional volume is displaced along the inside of the cap nut when the nut is screwed. This increases the holding force and closes dead spaces between the elastomer sealing ring and stainless steel pipe or cable. Under the cap nut also hides a plastic basket with flexible lamellae. The inside chamfered cap nut pushes down these slats when screwing down and inside, where it loads the sealing ring, which allows targeted material displacement. In the stainless steel base, there are also small recesses, in the corresponding pins of the lamellar basket grab, so that even the basket can not twist.
Not only must the gaskets of the cable gland grab vigorously, but they should also guarantee the protected removal of the electrical connection line and prevent the ingress of liquids. Therefore, the Skintop Hygienic IP69 is sealed against water and particles. This was tested in the laboratory by spraying under high pressure. The cable glands are also compliant with the requirements of IP68, with cable and screw connection under water withstanding 10 bar, which corresponds to a water depth of 100 meters.
In addition to the seal inside the Skintop has two other seals: one where the cable gland rests on the housing and one under the cap nut. The seal to the house is designed according to the principles of Hygienic Design as a flat gasket – not as usual as an O-ring, which sits on or in a chamber of the housing. Even in this depression, food residues could settle, which even a high-pressure cleaner can no longer remove. Even otherwise, the cable gland offers no points of attack for dirt and germs:
- The sliding seal under the cap nut closes potential dead spaces.
- Instead of a hexagon on the neck and cap nut, there are only two flat sides on which the key is attached to avoid hygienically problematic corners and edges.