Ceramic measuring cells – corrosion-resistant, robust, precise

Metallic-based pressure sensors, just like the VEGABAR 83, have been ensuring safety and reliability in chemical plants for decades. They are usually made of high-quality stainless steel such as 316L or other specialist alloys. This makes them ideal for use under harsh conditions. With the correct materials in there design, they can withstand high-pressure applications and extreme temperatures. 

VEGA is one of the few manufacturers in the world that develop and produce their own ceramic measuring cells like CERTEC^®. It is the key element of a ceramic pressure sensor. In an extremely complex manufacturing process, the diaphragm and the base body of aluminium oxide ceramic are printed with gold paste under clean-room conditions and joined together with glass solder at high temperature to form the measuring cell.

The many advantages of ceramic

Besides high corrosion resistance, the ceramic measuring cell also offers the following advantages:

• First-class long-term stability
• High reliability
• Outstanding overload resistance

It enables precise measurement over a long period of time. The ceramic measuring cell is often met with initial skepticism, as the material is assumed to be susceptible to breakage. “This is an unjustified concern,” explains Robin Müller, product manager at VEGA. “Ceramic measuring cells have a significantly higher overload resistance than metallic cells. Metallic diaphragms can deform irreversibly when exposed to very high pressures or shocks, a ceramic diaphragm simply deflects until it touches the base body and then returns to its original position.”

The sensitivity of ceramic sensors to moisture and temperature shocks – once a great disadvantage compared to metallic measuring cells – is no longer a problem. For one thing, the process temperature is measured, and the value is calculated to compensate for the influence of temperature on the measured pressure value. At the same time, a second temperature sensor in the glass seam behind the ceramic diaphragm detects even the smallest temperature changes. An algorithm built into the sensor electronics ensures that temperature shocks are compensated for. “This quite sensitive temperature measurement value can be utilised for process temperature monitoring and also transmitted as a separate signal,” says Robin Müller, describing another useful function.

Applications involving vacuum or hydrogen present special challenges for measurement technology. The boiling point of liquids drops in a vacuum. As a result, the oil in the metallic measuring system may begin to boil at temperatures below the atmospheric boiling point, causing significant errors. Also, hydrogen presents another, but no less significant, hurdle: Hydrogen molecules can penetrate metal – even the thin diaphragm of a metallic pressure measuring cell. “If hydrogen diffuses into and through the diaphragm, it contaminates the transmission oil behind it,” explains Robin Müller. The resulting hydrogen deposits can lead to a permanent change in measurement performance.

Besides diffusion, a metallic diaphragm material can also become brittle with hydrogen. But it is not an issue if diffusion-tight fittings and resistant materials, such as gold/gold-rhodium coatings are used, as VEGA provides with sensors in hydrogen applications. The dry ceramic measuring cells play an important role here: “Even if hydrogen were to get into the measuring cell, it will not cause any damage,” says Robin Müller. “Ceramic pressure sensors are a good solution for use in hydrogen production via electrolysis, which works at low pressures.” 

In chemical applications, differential pressure measurement delivers important data on flow, level and tank pressure. It can be used, for example, to

• measure the level in a pressurised vessel
• determine the flow rate at orifice plates and other flow elements
• monitor a filter or heat exchanger

In this episode of VEGA Talk, Stefan and Tom show what makes the ceramic measuring cell from VEGA so stable: