Key points
When process protection and reliability are paramount for pressure applications, there is a wide choice of sensor materials and styles for pressure transmitters. For flush mounted installations, oil filled pressure cells are frequently the first choice, sometimes they are integrated into the sensor and most often they are used in conjunction with capillary’s and ‘chemical seals’.
They are adaptable, well engineered and plentiful in supply. However, using a dry, oil free ceramic cell is now a real alternative for a wide range of measurements, offering major benefits to users. The ceramic substrate technology is now widely used by a wide number of pressure sensor manufacturers.
As 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 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!
To accommodate the various types of oil fills that are needed on a typical plant and processes, often, multiple oil filled types need to be carried as spares. In many cases, one type of ceramic sensor can be an ‘all rounder’ across site, but still be the optimum choice for each application.
They have shown can operate much longer in process conditions where traditional filled pressure cells will require regular recalibration, or even replacement on a routine basis.
What are Ceramic Pressure Transmitters, are they all the same?
The ceramic substrate material is a basically a sintered product, 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 – because its very uniform, 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 pharma’ requirements.
Some part ceramic designs use piezo-resistive circuits glued to a ceramic wafer. This article looks at the ‘all ceramic’ measuring cell, which is formed of a main ceramic body ‘block’ and a thinner front process diaphragm. The front diaphragm is glued/welded to the main body around its edge, gold plated electrodes measure the tiny deflection of the diaphragm as the pressure changes, most often using a capacitive technique, again some use the piezo-resistive system. (see Fig 1)
Ceramic is attractive as a material as it is extremely predictable in behaviour, stable and very hard wearing. The main body will typically have ASIC chip(s) mounted at the rear, which will optimise performance and accuracy, along with a temperature sensor to compensate for changes in the process temperature and thermal expansion. As a mark of the stability, low nominal range cells as low as 25mBar, with a 20:1 turndown are now available.
Self Monitoring ceramic cells
Some ceramic cells can have self-monitoring right through to the diaphragm surface itself. This is done via a ‘reference’ electrode inside the measuring cell, it is achieved via a comparator with a known relationship versus the actual ‘measurement’. Any misalignment in expected performance is reported, this enables the smallest of defects to be detected if the ceramic diaphragm doesn't behave/flex in its expected way (see fig 2). This means potential issues can be anticipated, rather than just ‘failure’. With oil filled systems, self monitoring, to this degree it isn’t possible.
Overload, pulsation and vacuum resistance
Most dry ceramic cells have high overload resistance, with an integrated overload bearing design. The highest overload is now up to 200 times the nominal measuring range in latest designs, in these the diaphragm, once in ‘overload’ will ‘press or seat’ against the rear main body of the sensor, there it cannot deform or drift and unlike other materials. It doesn't have a yield point like metal, where in this range, irreversible damage occurs.
Ceramic doesn't age, fatigue or stress harden either, so it doesn't react to pressure pulsation or shocks, which can permanently cause drift or damage many other metallic pressure element types. Ceramic has real benefits when it comes to high vacuum conditions, oil filled cells and seals can ‘degas’, this creates an air bubble behind the diaphragm, as the gas is ‘compressible’ it causes drift, and this error can go unnoticed by the user until the next calibration check, with a dry ceramic cell, this situation cannot occur.
If the diaphragm membrane ruptures in an oil filled cell, the process will be contaminated by the fill oil, however compatible with the process, its always preferable to avoid it! Sometimes the pressure measurement may still continue; the leakage and the consequential product contamination may not be detected for some time. A dry ceramic cell with comprehensive diagnostic monitoring means neither a fluid-based process contamination event, or an undetected failure, can occur.
Chemical resistance
Ceramic as a substance is of course resistant to many chemicals, the finer, higher purity ceramics offer 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 e.g. Kalrez®, they can be fully process compatible with some fairly aggressive and corrosive media.
Some combinations can even have all ceramic/polymer based mountings, threads and flanges (see figure 3), providing all non metallic wetted parts, for excellent resistance to aggressive process environments e.g. sea water, which will readily corrode many standard metals.
These options mean ceramic cells can save costs over large flanged oil filled chemical and transmitter with a PVDF threaded capillary seals, using special and often expensive coatings or alloys.
Condensation resistance
The majority of measurements are ‘gauge’ pressure, they have to be referenced and breathe to atmosphere/air. A gauge pressure dry cell will always have the ability to ‘ingress 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 or months and, even if ‘dried out’ – the sensor is never the same. It can occur many months or years after a new plant is commissioned, then suddenly sensors can start to ‘misbehave’. 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 its still not always successful in the long run – humid air finds a way inside. To counter this, a new extra measure has been introduced to protect against this. Using an ‘insulating’ coating on the inside surfaces of the cell, protects the sensitive gold measuring elements 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, this is not limited by the ceramic itself, but the electronic components. Temperature measurement iextremely 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 measuring 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 (e.g. flush diaphragm). This 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 equilibrium with the process temperature to stabilise.
However, an innovative new design seeks to improve this with a temperature circuit mounted directly onto the rear of the diaphragm, (see fig 3) and this also means that temperature measurement can now be transmitted as an additional process measurement, reducing connections and costs.
Sealing: Abrasion resistance
With an oil filled sensor, the metal diaphragm is welded to the connection body material, giving it excellent sealing integrity, but the thin metal diaphragms are also easily susceptible to damage. Ceramic materials are already well known for their abrasion and wear resistance, delivering the same benefit for pressure cells.
A low range ceramic cell can be cleaned with a wire brush and a flush mounted unit resists build up and clogging and comfortably withstands abrasive particles in slurry or pulp, such as the fine metals found in raw paper stock for example. For example, Ceramic cells have been used in mining slurry pumps, lasting many times longer than metal cells in the same application, with no drift, recalibration or replacement required (see Fig 4).
Flush mounting and sealing
Ceramic cells need an elastomer to seal it into the body of the sensor, and it is important to have the right elastomer to match the process. If this isn’t correct, the elastomer will expand, deform and leak.
How this sealing is done varies, the majority of cells use the means of pressing the measuring cell outwards against a circular seal around the outer edge of the diaphragm against a lip slightly smaller in diameter, this compresses and forms the seal (See fig 5).
It means the ceramic face can never be truly flush on the sensor, it will have a slight recess. The designs with ‘truly flush’ ceramic diaphragms perform best, as there are no elastomers exposed to abrasive scouring or contamination by sharp particles, as well as virtually ‘nil’ product retention in the fitting.
A design uses glass welding that keeps both the pressure and chemical integrity of the cell intact, thus enabling sealing on the side to produce a fully flush face(see fig6, 7 & 8). This is very important for industries where absolute flush mounting is needed, with minimal product retention for hygiene or reduced cross process contamination in products such as foods, pharmaceuticals and paper stock.
However in some chemical applications with highly toxic products, double sealing and a second line of defence is needed, the design in figure 5 is perfect for this – note the double sealing arrangement (red and black), as here a fully flush mounting is generally less important. This double sealing concept is used, sometimes along with a ‘second line of defence’ closer to the inside of the housing to avoid any risk of harmful process leaks into the environment.
Small fittings
Generally on lower pressure ranges, especially on flush mounted tank level installations, most oil filled cells and chemical seals on DP transmitters need a minimum 3”/DN80 chemical seal diaphragm to deliver the resolution, linearity and accuracy of measurement needed.
Flush mounting, dry ceramic cells are more sensitive and accurate as the materials are more stable, so they can be as small as ¾”/20mm, even on low ranges, which is ideal on smaller pipes and process vessels. These smaller connection sizes can reduce the cost and weight of process connections for vessel level measurement for example, as well as aiding cleaning via flush mountings (discussed previously) when its important for the user.
Summary
Dry ceramic measuring cells avoid the risk of process and product contamination with fill fluid, but they also bring other advantages.
The material 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 a particular issue for ceramic cells, the latest designs in this technology have an innovative temperature compensation system, which maintains stable measurement accuracy and reliability, whatever temperature swings the process delivers.
The new ‘electronic DP’ systems, some of which are also featuring the same ceramic technology too – 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 is worth consideration over more ‘traditional’ oil filled designs.
