Ceramic measurement benefits
03 August 2015
Pressure sensor manufacturers are now widely employing ceramic materials because it can offer some real advantages for a range of measurements.
In pressure applications where process protection and reliability are paramount there is a wide choice of pressure sensing materials and styles. For flush mounted installations, oil filled pressure cells are frequently the first choice, sometimes integrated into the sensor and often used in conjunction with capillary’s and ‘chemical seals’.
These solutions are adaptable and well engineered. However, with ceramic substrate technology now being widely used by a number of pressure sensor manufacturers, the use of a dry, oil free ceramic cells can now offer real benefits in a range of measurements.
In addition to improved performance, accuracy, linearity and process hygiene, dry ceramic cells are generally 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 delicate by design, to allow them to transmit the pressure, which means that they can be easily damaged or compromised.
Careful consideration also needs to be given to the type of fill-oil used for the application. There are many variants, all designed to minimise contamination in the event of a rupture. However, 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 contrast, in many situations, just one type of ceramic sensor can be an ‘all rounder’ across a site, but still be the optimum choice for each application. They have been shown to operate longer in process conditions, where traditional filled pressure cells require regular recalibration, or even replacement.
Not all the same
Not all ceramics are structurally the same, with the finer and purer ceramic materials producing the highest performance. The best materials are sapphire ceramic based because the very uniform, dense crystal design provides good mechanical strength, corrosion resistance and reliable long-term stability. These materials also have a very smooth surface finish which makes them suitable for use in hygienic applications, including those regulated by the FDA.
The ‘all ceramic’ measuring cell – formed of a main ceramic body ‘block’ and a thinner front process diaphragm – most often use a capacitive technique to measure tiny deflections of the diaphragm as the pressure changes. Some also use the piezo-resistive system. Ceramic is attractive as a material as it is predictable in behaviour and is stable and hard wearing. The main body will, typically, have an ASIC chip 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.
Some ceramic cells can be 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, enabling the smallest of defects to be detected if the ceramic diaphragm doesn't behave/flex in its expected way (see figure 1). This means potential issues can be anticipated, rather than just a ‘failure’ alert. With oil filled systems, self monitoring to such a degree is not possible.
Overload, pulsation and vacuum resistance
Most dry ceramic cells have high overload resistance, with an integrated overload bearing design. The highest overload in the latest designs is now up to 200 times the nominal measuring range. In these the diaphragm, once in ‘overload,’ will ‘press or seat’ against the rear main body of the sensor, where it cannot deform or drift. Ceramic doesn't age, fatigue or stress harden either, so it does not react to pressure pulsation or shocks, which can permanently cause drift or damage in other metallic pressure element types. Ceramic also has benefits when it comes to high vacuum conditions. Oil filled cells and seals can ‘degas’, creating an air bubble behind the diaphragm, as the gas is ‘compressible’ it causes drift, and this error can go unnoticed 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. Sometimes the pressure measurement may still continue so the leakage and the product contamination might 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.
Ceramic is resistant to many chemicals, although care does need 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 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 2), providing all non metallic wetted parts for resistance to aggressive process environments such as sea water, which will corrode many standard metals.
The majority of measurements ‘gauge’ pressure, they have to be referenced and breathe to atmosphere/air. A gauge pressure dry cell has the ability to ingress moisture from the environment around it. The air will inevitably have moisture in it and in humid areas microscopic droplets can form on the electronics, causing micro short circuits that can result in drift or an offset.
Gore-Tex style membranes and filters are used to keep this at bay, but it is still not always successful in the long run. Using an ‘insulating’ coating on the inside surfaces of the cell, however, will protect the gold measuring elements against moisture and the microscopic droplets causing the ‘short circuits’ and drift, delivering long-term reliability, even in very humid environments.
Higher specification ceramic cells can handle direct process temperatures of up to 150°C and this is limited only by the electronic components. Temperature measurement 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 measuring cells there is 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. 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. However, a new design seeks to improve this with a temperature circuit mounted directly onto the rear of the diaphragm and this also means that temperature measurement can be transmitted as an additional process measurement, reducing connections and costs.
Ceramic materials are also 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 can withstand abrasive particles in slurry or pulp.
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 3in/DN80 chemical seal diaphragm to deliver the necessary resolution, linearity and accuracy of measurement. Flush mounting, dry ceramic cells are more sensitive and accurate as the materials are more stable, so they can be as small as 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.
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