Chemical Technology July 2015

Petrochemicals

Chemical Technology • July 2015

COVER STORY

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

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,

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.

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.