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Combining five into three

Fast route to selecting the right pressure transmitter
Combining five into three

The relaunch of the Vegabar pressure transmitter family is proving to be a revelation. This already compact family has been slimmed down even further and a whole series of technical details optimised. The ceramic measuring cells in particular have undergone significant improvements. What’s more, all instruments in the Vegabar 80 family are SIL-compliant.

Author Florian Burgert Product Management Pressure Transmitters, Vega

Vega Grieshaber has completely overhauled its pressure transmitter portfolio. The 80 series has been reduced to just three process pres-sure transmitters which cover all scenarios. The Vegabar 82 with its ceramic measuring cell is an all-rounder that meets 80 % of requirements. The Vegabar 83 has a metallic measuring cell and is the number one choice for high pressure applications. Finally, the Vegabar 81 – offered with a broad range of chemical seal options – is deployed whenever high temperature and chemical resistance are called for.
All instruments in the Vegabar 80 family consis-tently follow the plics principle. This standardised instrument platform is based on a universal housing, electronics and operating concept regardless of the measurement method. It facilitates day-to-day work throughout the entire life cycle of pressure and level instruments – from selection and ordering through installation and setup to maintenance and servicing.
The “family DTMs” already familiar from Vegaflex guided wave radar sensors provide extra convenience. Previously, a special driver was required for each and every instrument to integrate it into a measuring system. Now, only one DTM is needed for the whole instrument family and the Vegabar 80 to which it is connected is recognised automatically.
Prepared for any shock
Apart from their many advantages, ceramic measuring cells also have a few weaknesses such as their sensitivity to temperature shock and moisture. Thanks to the intensive further development of the Certec cell, Vega has succeeded in minimising, or in some cases completely eliminating, these problems.
The work on temperature shock compensation has resulted in a technological masterpiece. If sudden, extreme temperature changes occur, it can take several minutes for sensors to begin delivering reliable readings again. Users are often unaware that a sensor is experiencing temperature shock at the diaphragm and transmitting incorrect values. With Vega’s ceramic measuring cells, they can be absolutely sure of getting the correct measurement data. Even very fast temperature fluctuations no longer degrade the accuracy of a pressure measurement.
This is how it works: in addition to the usual temperature sensor on the rear of the Certec, there is also a second sensor in the glass joint directly behind the diaphragm. It is mounted in a part of the process with extreme technical demands. Due to its exposed position, even the slightest temperature change is detected, enabling any occurrence of thermal shock to be fully compensated by means of a sophisticated algorithm.
Another positive side-effect of having a second sensor so close to the process is that the temperature is measured with an accuracy of ± 2 K. Although earlier versions of the measuring cell were also able to output a temperature signal, the sensor was only suitable for storage tanks – which normally have a relatively stable temperature anyway – owing to its slow response time. The new instrument generation allows a separate temperature sensor to be dispensed with in many cases.
Protected against moisture
The new technology also significantly reduces the moisture sensitivity that is so typical of this kind of sensor. The problem with exposed electrodes in capacitive measuring systems is that the ingress of a conductive fluid like moisture changes the capacitance inside the measuring cell. The pressure reading changes as well because the instrument is not able to distinguish the reason for this capacitance change. With the new instrument generation, the entire internal surface of the measurement and reference capacitors is coated with a thin layer of glass. This passivation prevents any contact with the fluid, so changes to the dielectric constant apply to the measuring cell as a whole, in other words to both the measurement and the reference capacitor. The coefficient of these two values is compensated and the correct result shown.
Another special feature of Vega ceramics is the overload resistance factor, which can be as high as 200. Progress has also been made with regard to the temperature range. Whereas 120 °C used to be limit, the Certec cell can now withstand temperatures up to 130 °C. The additional 10 °C may not sound very much, but it makes a big difference to the food and pharmaceutical industries because, in many cases, customers no longer have to purchase special high temperature versions for their sterilisation processes.
The ceramic measuring cells now boast a very low measuring range right down to 25 mbar (previously 100 mbar) with no electronic turndown, giving the pressure instrumentation expert the lead in the market for gauge and absolute pressure transmitters. The measuring range has also increased significantly at the high end from 60 to 100 bar.
Absolutely tight
The ceramic measuring cell allows front-flush mounting because its radial seal is recessed and protected from the medium. This advantage is particularly evident in abrasive applications. Caking is likewise a thing of the past as the sensor simply cleans itself in the flowing medium.
This “second line of defence” is vital in phosgene applications, for example, in order to prevent the highly toxic medium from leaking into the terminal compartment and endangering people and the environment. The special electronics design allows Vega to offer a fully welded sensor module – with a ceramic measuring cell for measuring gauge pressure. Even if very small process fittings are used, very high measurement accuracy is still achieved.
The innovative software and hardware concept makes it possible to combine all instruments in the Vegabar 80 family – both the all-rounder and the high pressure and high temperature sensors – in an electronic differential pressure system. To obtain a differential measuring system, for instance, the customer merely has to take a standard Vegabar 82, which could well be in stock anyway, order an additional sensor, select the “slave” electronics version and connect the two together. Easy selection, identical adjustment and operation and simplified stocking are the benefits.
The fact that there are no oil-filled capillary tubes which need to be insulated in order to rule out environmental influences, such as temperature changes or severe vibration, as well as the resulting measurement errors is among the other advantages of the electronic differential pressure system.
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