High performance ceramics
Electromagnetic flowmeters with zirconium oxide-sintered ceramic measuring tubes

Versatile application

Electromagnetic flowmeters with zirconium oxide-sintered ceramic measuring tubes feature an outstanding combination of material properties including surface hardness and quality, mechanical stability and resistance to corrosion and temperature change. Benefits for the operator include ultimate accuracy and long-term stability as well as vaccuum resistance and diffusion resistance. Due to this versatile profile combined with the high accuracy, these flowmeters are used in all sectors of industry: in the paper industry, pharmaceuticals, the food and beverage sector, in the mining and construction materials industry and in the chemical processing industry.

Electromagnetic flowmeters could not be more versatile as OPTIFLUX devices feature ceramic measuring tubes and are available in either a flange or sandwich version. On request, to save on costs, you can opt for a wafer (sandwich) instrument or, to increase process reliability, flanged instruments are available.

The capacitive pick-ups are fully integrated into the ceramic tube using LTCC (Low Temperature Co-fired Ceramic) technology.
Capacitive signal pick-up

Capacitive signal pick-up

Instead of using conventional electrodes that come into direct contact with the process liquid, the ceramic measuring tube on the OPTIFLUX 7300 features non-wetted capacitive signal pick-up. The electrodes are designed as large-area capacitor plates, mounted behind the ceramic liner.

Even if a non-conducting layer of product forms on the metallic electrodes, with capacitive pick-up it is still possible to pick up a clear signal, e. g. latex emulsion or bitumen emulsion.

Because the electrodes do not come into contact with the product, undesired catalytic action with metallic parts is ruled out. Any possible noise is also minimised.

In the case of the OPTIFLUX 7300, noise is reduced not only through the large-scale area of the capacitive electrodes but also through the rigidity. All electronic parts are fully integrated with the ceramic tube using LTCC (Low Temperature Co-fired Ceramic) technology. LTCC tape is used to fuse the capacitive electrodes, shielding and pre-amplifiers to the ceramic tube in three layers.

This results in a very rigid and firm construction, eliminating relative movements of these parts.

The use of non-piezo electric materials prevents piezo-electric noise that can otherwise be induced when using plastic insulation materials. The benefits of this highly rigid design include high immunity to noise and vibration. The considerably larger detection area when compared to metallic conducting electrodes also contributes to the high quality of the signal.

Electron microscope shot of the fused-in-place cermet electrode: left, the light points (platinum) are the only way to recognise that there are two different materials.
Gap-free fused-in-place platinum electrodes

Gap-free fused-in-place platinum electrodes

The OPTIFLUX 5300 electrode is fused-in-place without gap. Any leakage at this point could, depending on the product, pose a safety risk and result in undesired extra costs. For this reason, much effort was invested in developing the following solution:

In the blank ceramic shape, electrodes are either brought in as a massive pin or in the form of a cermet electrode. A cermet electrode is a composite component made up of ceramic and metal, in this case platinum. During the subsequent fusing process, the ceramic portion of the cermet electrode bonds to the surrounding ceramic while the platinum is optimally embedded as regards thermal and electrical properties. The result is a homogenous ceramic fusion zone entirely without gap between the measuring tube and the electrode. The homogeneity of the fusion zone was proven by way of destructive material testing (bursting test). In addition, the two different materials are joined in a fashion so that temperature shocks cannot cause any damage.

Helium leak testing
Wear-resistant and diffusion proof

Wear-resistant and diffusion proof

The fact that ceramic has no open porosity and consists of a relatively fine crystalline structure leads to extremely smooth surfaces when processing, and with the OPTIFLUX to roughness values = Ra 0.8 µm. In addition, the result is a very low leakage rate of typically less than 10 -10 mbar * l/s at 60 bar interior pressure (Helium).

Besides the application with acids and bases in chemical processing, e.g. for use with filling machines in the beverage industry, the flowmeters are almost exclusively fitted with ceramic measuring tubes. The abovementioned remarkably low surface roughness and thus good hygienic suitability plays a significant role in sanitary applications.

Extraordinary hardness (almost adamantine) is one of the properties responsible for the extreme wear resistance of this ceramic. Plastic coatings, on the other hand, are malleable, exhibit vacuum sensitivity and are much less resistant to wear.

Cleaning tanks and pipelines with high-pressure water jets is a proven way to assure productivity, safety, environmental responsibility and low costs. Obviously plastic lined electromagnetic flow meters are quickly damaged by the water jet, whereas a ceramic tubed meter is not harmed.

Often the cause of failures of plastic lined electromagnetic flowmeters is either mechanical wear or chemicals or corrosive components penetrating the liner.

Subsequent research into root causes underlines the great advantage of ceramic measuring tubes: outstanding long-term stability and operating safety whereby production continues without stops or potential environmental contamination. In many cases, several of the properties of this high performance ceramic are required at the same time, e.g. when measuring nitric acid or hydrochloric acid. The PFA or PTFE coatings typical to EMFs are not sufficient in this case as they are not suitably diffusion resistant. For this reason, EMFs with measuring tubes or measuring tube liners made of ceramic material have been available for decades for this type of application. KROHNE Messtechnik was and is the pioneer in this area, introducing the first ceramic device as early as 1982. In close cooperation with Mannheim based FRIATEC AG, ceramic measuring tubes continue to be the state-of-the art for a wide range of applications.

Long-term drift EMF DN 15
Unrivalled long-term measuring accuracy

Unrivalled long-term measuring accuracy

In addition to the properties of the electrical and mechanical components, the form stability of the measuring tube under temperature and compressive stress in particular is crucial to the reproducibility of the electrical signal of an EMF, as this considerably affects the distance on either side of the electrodes and thus the precision of the measurement. The choice of materials for the measuring tube and the electrodes as well as the appropriate technique for joining the electrodes to the measuring tube are therefore key to the lasting reliability of EMFs.

The long-term stability of the ceramic devices was scientifically examined in a joint effort with the German National Metrology Institute (PTB). The temperature stress test showed that the ceramic devices had an average deviation of only 0.05 % compared to the tests in new condition. The PFA sample deviation was eight times higher. The test was designed to include a change in product temperature from 81°C to 18°C every 5 minutes. This simulation was carried out over 600 cycles in succession. Using the same test specimens, 60 cycles were then conducted to simulate a superheated steam sterilisation (SIP), each time switching between water at room temperature and hot steam at 134°C for 30 minutes.

Here, too, the long-term stability (i.e. the accuracy over many measuring cycles) of the measuring tube is achieved thanks to the extraordinary form stability of the ceramic. The high geometric stability is one of the main reasons the OPTIFLUX 5300 is so widely used as a reference device / MasterMeter in flow measurement plants in national institutions, measurement offices, nationally recognised testing bodies and notified bodies. One further application is the highly precise dosing of concentrated media such as flocking agents, acids or catalysts.

The previously mentioned SIP (steaming in place) sterilisation with superheated steam in the food and beverage industry exceeds the temperature changes commonly found in processes in the chemical industry, with temperature differences over 100 K. Thanks to the skilful adaptation of the thermal expansion properties of the measuring tube and cermet electrode, the material bond easily withstands this stress over the long term.

Process reliability, resistance to fire

Process reliability, resistance to fire

Production plant performance in the chemical industry depends not only on the size of the plant but also on factors such as quality, yield, availability and safety. The safety of a process and its technical systems is influenced, among other things, by the process and the selection of the materials.

One basic principle is the creation of inherent reliability through the selection of the best suited design, e.g. through pressure resistance and resistance to corrosion even in the event of a fault. The application of the principles of inherent reliability should take place at the very start of product and process development, even before choosing process equipment and safety instrumeted systems. Devices with reserves in the application prevent stops in production and product spils due to their extended technical limits. OPTIFLUX devices with ceramic measuring tubes are available in both a wafer (sandwich) and a flange version. Thanks to the short screws in the flange version, superior safety against product spills is provided even in case of fire outside the instrument.

Nowadays it is not uncommon to operate plants at part load at the start of their life cycle only to switch them over to full load at a certain point in time based on economic considerations. Temperature levels as well as the composition of the products (e.g. the concentration of acids and bases) can change significantly when this happens. The result is a wide range of requirements - even as regards the measuring technology installed. In cases such as this, OPTIFLUX devices with ceramic measuring tubes are an intelligent solution as they offer large performance reserves. The same is true for some multi-purpose plants whose material requirement profiles must be designed to accommodate the maximum requirements.

Unlike plastic lined devices, occasionally operating under vacuum or below ambient pressure is completely unproblematic. Measuring accuracy is not affected and liner peeling due to blistering is impossible. The chemical industry along with the paper industry as well as mining and construction material industries rely on state-of-the-art technologies, which must be extremely robust and long-lasting. In special test laboratories (for example with NAMUR members) and in practical use in the field, the devices are also tested by users as to their suitability for the application.