Chemical Technology July 2015

Chemical Technology • July 2015

Real measurement benefits

from VEGA’s ceramic pressure transmitters

s well as improved performance, excellent accuracy,

linearity and process hygiene, dry ceramic cells are

generally far more robust than traditional oil-filled

pressure systems. One of the main issues is that oil-filled

transmitter membranes/diaphragms on a pressure sensor

are necessarily very delicate by design in order to transmit

the pressure, which means they can be easily damaged or

compromised. Careful consideration must also be given to

the type of fill-oil used for the application. There are many

variants, all designed to minimise any contamination should

they rupture, but of course, most end users would rather

not risk this occurrence at all! Often, to accommodate

the various types of oil fills that are needed on a typical

plant, multiple oil filled types need to be carried as spares.

The VEGABAR series 80 with CERTEC

dry ceramic sen-

sors can be all-rounders across site, as well as being the

optimumchoice for each application. They can operatemuch

longer in process conditions where traditional filled pressure

cells will require regular recalibration, or even replacement

on a routine basis.

What are ceramic pressure sensors and

are they all the same?

The ceramic substrate material is made from a highly

compressed powder with a binding material. The ceramic

itself is extremely durable and hard, based on aluminium

oxide, a substance used for many applications in industry.

But not all ceramics are structurally the same; the finer and

purer ceramic materials produce the highest performance.

The best materials are sapphire ceramic based. A dense

crystal design provides excellent mechanical strength,

corrosion resistance and reliable long term stability. In

these materials, the characteristic surface finish is also

very smooth at <0,7

m Ra, also making it suitable for use

in the most demanding of hygienic applications, including

those regulated by FDA and other chemical requirements.

Chemical resistance

Ceramic as a substance is, of course, resistant to many

chemicals with the finer, higher purity ceramics offering the

best all round resistance of all, although some care has to

be taken with some alkalis and acids. A competent supplier

will offer comprehensive resistance lists and advice on this.

In general, with the right elastomer seal, eg, Kalrez

, they

can be fully process compatible with some fairly aggres-

sive and corrosive media. Some combinations can even

have all ceramic/polymer based mountings, threads and

flanges providing all non-metallic wetted parts, for excellent

resistance to aggressive process environments. These op-

tions mean ceramic cells can save costs compared to large

flanged, oil filled chemical and capillary seals using special

and often expensive coatings or alloys.

Condensation resistance

The majority of measurements are ‘gauge’ pressure, which

means they have to be referenced and breathe to the at-

mosphere. A gauge pressure dry cell will always have the

ability to take in moisture from the environment around it.

The air will inevitably have moisture in it and, in humid areas

(which encourage moisture formation on any temperature

differentiated surface - even inside the sensor), microscopic

droplets can even form on the sensitive electronics of any

measuring cell, thus causing micro short circuits resulting

in drift or an offset. This can materialise in days, weeks

VEGA’s dry, oil-free ceramic cell technology

CERTEC

is a real alternative for a wide

range of measurements, offering major

benefits to users.