CERAMIC BEATS METAL: Measuring cells with long-term stability for use in the chemical industry
For a long time, metallic diaphragm sensors have been the standard for pressure measurement in offering high safety, long-term stability, and reliability while avoiding common problems associated with impulse lines and oil fillings. Ceramic sensors are now seen as an ideal solution for hydrogen applications.
When an abnormal pressure change occurs in a reactor, it raises concerns about system safety and batch quality. Abnormal readings, if not caused by temperature, flow rate, or level changes, might indicate a sensor defect. Pressure is a critical variable in the chemical industry, vital for safety and product quality. Metallic sensors, made of stainless steel or high-quality alloys, withstand harsh conditions but can corrode and age, requiring regular recalibration.
Ceramic sensors, introduced in the 1990s, are resistant to corrosion, making them suitable for aggressive chemical processes. They ensure excellent long-term stability and minimal drift. Robin Müller, International Product Manager at VEGA, notes that ceramic sensors can be used in 60-70% of chemical applications, although they are often perceived as fragile. However, ceramic diaphragms are more resilient than metallic ones, able to return to their original position after deflection without permanent deformation.
Ceramic sensors also avoid the issues associated with oil-filled metallic sensors. They operate without oil, eliminating the risk of leaks. Their design functions like a capacitor, with measuring electrodes and air forming an electric field. Pressure changes alter the capacitance value, which is used to calculate pressure. This design ensures high long-term stability and minimal recalibration.
VEGA’s ceramic measuring cell, CERTEC®, is used in their VEGABAR sensors. These sensors are crafted from ultra-high purity aluminium oxide ceramic, joined with glass solder. VEGA has improved ceramic sensors’ performance against temperature shocks and moisture. A second temperature sensor on the ceramic diaphragm detects minor temperature changes, and an algorithm compensates for temperature shocks.
Ceramic sensors perform better than metallic ones in high vacuum and hydrogen applications. In a vacuum, oil in metallic cells can boil and form bubbles, causing measurement errors. Hydrogen can penetrate metals, react with transmission oil, and cause inaccuracies and material brittleness. Ceramic cells, unaffected by hydrogen, are ideal for low-pressure hydrogen production via electrolysis.
For toxic and aggressive gases, ceramic sensors provide high safety. VEGA uses highly resistant plastics and a “Second Line of Defence” with gas-tight, glass feed-throughs to separate the measuring cell and electronics compartment. This safety feature is available in the VEGABAR 82 and VEGABAR 83 series, with an option for fully welded sensors for relative pressure measurement.
Differential pressure measurement, crucial in flow, level, and vessel pressure measurement, often uses impulse lines prone to freezing and inaccuracies from gas inclusions. VEGA addresses this by using two electronically connected sensors, eliminating the need for complex impulse lines.
In conclusion, ceramic measuring cells are a robust alternative to metallic pressure sensors in the chemical industry. Their high safety and long-term reliability make them suitable for demanding applications.
Web: https://www.vega.com/en-za
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