VEGA has re-engineered limit switch technology, more commonly known as point level sensors, and developed the VEGASWING 66, a sensor for extreme situations. Holger Sack, head of product management, explains more…

The term ‘limit switch’  appears rather infrequently in technical texts. Even my Google search was not very successful. Why?

Holger Sack: Today, the old, ‘tried and tested’ analogue limit switches with potentiometer adjustment etc. are gradually being replaced by digital technology. I think that’s why these devices are now more commonly called ‘point level sensors.’ We’ve found that point level measurement is indeed very closely related to level measurement.

That is likely to be the reason why ‘level’ is used in some descriptions as a synonym for ‘point level.’ To what extent are the technologies different?

Holger Sack: Level measurement is used to describe continuous measurement of a changing level, whereas point level is used to indicate a discrete condition, i.e. the existence of the level at a certain point. To clarify, ‘Level’ means the continuous measurement of contents from empty to full. ‘Point level’, on the other hand, means a discrete on/off signal given when a product has reached a significant level in a vessel.

Continuous or discrete – can the two areas of applications be clearly separated from one another?

Holger Sack: No, they can’t. You can find both level and point level sensors everywhere. Even combined in one application, depending on the customer requirement. Usually, when both sensor types are installed, it is to increase safety.

Does the simpler technology behind point level switches mean that they are cheaper than continuous level sensors?

Holger Sack: Roughly speaking, yes. Because, continuous level sensors are more complex in structure and sometimes also in terms of technology. But this need not always be the case. You see, with regard to the process, point level sensors have to meet the same requirements as continuous level sensors. Level switches just carry out a ‘simpler evaluation’ of level data.

In point level detection, a switching command starts or stops the filling equipment. How do you monitor the process, i.e. ensure that the sensing element and the electronics are working properly?

Holger Sack: On the one hand, the individual sensor has to be considered, and on the other, the entire measurement chain. In the modern sensors, microprocessor technology enables numerous functions that monitor the electronics as well as the sensing element during operation. A high percentage of faults in the electronics as well as in the sensing element, such as build-up or corrosion, can be detected and reported. Looking at the entire measurement chain, we see that information on tank contents is forwarded to the control system or special actuators through cables or bus systems. These systems are responsible for ensuring that valves, pumps, etc. operate at the right time. Until now, all devices in a system were analysed individually from safety-engineering standpoint. Today, engineers look at the entire measurement chain, that is, from the sensor to the transmission of measured values to the actuating components (valves, pumps, etc.). This ultimately ensures that the switch-off mechanism in its entirety works, preventing overfilling or dry running of pumps.

When it comes to safety, a lot has been done in recent years. Has level switch technology also been made safer?

Holger Sack: As I said earlier, the basic technology is very old, so there are still a  lot of old but proven technologies in service, such as floats or paddle switches. The capacitive measuring principle is also a very old, tried-and-true measuring method, albeit with a few limitations when compared to the vibration principle. Next to microwave/radar, vibration is currently the most universal measuring principle that we offer. We at VEGA have been focusing on electronic measuring systems for years, because they offer advantages in terms of maintenance and Life Cycle Costs. That’s why, although they are also a little more expensive to produce and to buy, the extra outlay pays off over a service life of 15 years or more.

Does this mean that your new vibrating level switch VEGASWING 66 is just old technology in a new guise?

Holger Sack: No, not in this regard. Here we are breaking new ground with our new, patented technology. This instrument can be used in temperature and pressure ranges where previously only a few technologies could be deployed, and certainly not the tuning fork measuring principle. The basis of this technology is a tuning fork that is electrically excited and made to vibrate over a few micrometres range. Until now, it was not possible to use vibration technology in temperatures above 250°C. With our VEGASWING 66, applications up to 450°C are now possible; not only that, it is also capable of temperatures as low as -195°C.

How did you make it possible to use the switch in the extreme temperatures and pressures found in processing?

Holger Sack: For one thing, we replaced the previously used piezoelectric drive with a special solenoid that we developed ourselves. This solenoid now drives the tuning fork and is able to withstand the high temperatures. Another point is, we now use ceramic materials and have designed the electrical connections made so secure that they operate reliably even at 450°C. And last but not least, we achieved the high pressure resistance through the mechanical stability of robust materials that withstand pressures up to 160 bar.

In point level detection, current problems are, for example, difficult product properties or foaming. To what extent do these factors influence the quality of the measuring results?

Holger Sack: On VEGASWING 66, for example, we can detect build-up, and we can also detect whether the tuning fork is corroded or broken. Build up changes the amplitude of the oscillation, which allows us to use the available processor technology to electronically evaluate this change and notify the customer of a problem.

Users nowadays are also calling for the simplest possible instrument handling. Do your level switches follow plics platform moto, ‘simpler is better’?

Holger Sack: Absolutely. VEGASWING 66 is designed according to the modular instrument platform plics. This means the customer can combine different components as required. But plics is more than that: it’s designed to make handling easier for the customer throughout the instrument’s life-cycle.

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