Chemical Technology July 2015

Petrochemicals COVER STORY

recovery depends largely on how long it takes the sensor to reach equilibriumwith the process temperature to stabilise. However, VEGA’s in- novative new design seeks to improve this with a temperature circuit mounted directly onto the rear of the diaphragm, which also means that temperature measurement can now be trans- mitted as an additional process measurement, reducing connections and costs. Summary The CERTEC ® dry ceramic measuring cells used in the VEGABAR series 80 avoid the risk of pro- cess and product contamination with fill fluid, but they also bring other advantages. The mate- rial is harder than steel with excellent overload,

or months and, even if dried out, the sensor is never the same. It can even occur many years after a new plant is commissioned and suddenly the sensors can start to mis- behave. Time and production is lost removing, exchanging, recalibrating and replacing sensors. Special Gore-Tex ® style membranes and filters are mainly used to keep this at bay, but it is still not always successful in the long run – humid air finds a way inside. To counter this, VEGA has introduced a new, extra pro- tective measure. Using an insulating coating on the inside surfaces of the cell, the sensitive gold measuring elements are protected against moisture and the microscopic droplets causing the short circuits and drift, thus delivering long-term reliability, even in the most humid of environments. Temperature performance Higher specification ceramic cells can handle direct process temperatures up to 150 °C, not only the ceramic itself, but also the electronic components. Temperature measure- ment is extremely important for any pressure sensor and especially for ceramic designs. The compensation for the coefficient of expansion is crucial – as the temperature changes, so the materials expand, with direct effects on the minute deflection of the diaphragm. On most measur- ing cells, temperature is monitored behind on the main sensor body, so there is inevitably a lag behind the process temperature. This lag, and particularly the reaction of the ceramic to sudden temperature changes, means they will have an incorrect reading for a period of time, especially on applications directly against the process (eg, flush dia- phragm). The time period depends on the speed and size of temperature change and the mounting configuration, as the

Latest electronic differential pres- sure systems with ceramic cells. They are hard wearing, remove the need for capillaries and deliver well under very high temperatures.

vacuum and pulsation resistance and it has the ability to resist the harshest abrasion. They lend themselves well to flush mounting to the process, but not all designs are the same and careful choice of elastomer and design to suit your process is important. Although temperature shock can be an issue for ceramic cells, the latest designs in the VEGABAR 80 technology have innovative temperature compensation systems, whichmain- tain stable measurement accuracy and reliability whatever temperature swings the process delivers. The new electronic DP systems remove the need for costly capillaries and have improved response and accuracy, even with temperature gradients. If you are looking for stable, accurate, reliable pressure measurement with minimal recalibration and maintenance, then dry ceramic cell technology from VEGA is worth consideration over more ‘traditional’ oil filled designs. z

5

Chemical Technology • July 2015