Pressure Measurement & Control

Jewels for pressure measurement

July 2001 Pressure Measurement & Control

Pressure measuring instruments for hydrostatic level and process pressure measurement are often operated nowadays under extreme conditions. Only pressure transmitters with all-round chemical resistance can cope with the great variety of process conditions and environments found in manufacturing.

As a result of the high degree of automation in the plants of different industries, measurement and control components play an important role in the smooth operation of a process. Due to the increasing complexity of process parameters, the requirements placed on the sensors must be individually defined for each measurement site. This is a difficult job for the plant operator because, in many cases, the parameters and their exact values may not be known in the planning or even in the test phase. Even in a plant that has already been put into operation, the process conditions often cannot or may not be exactly defined. Also, there are extraordinary cleaning processes (eg CIP / SIP cleaning) to be reckoned with, as well as emergency situations. The chemical resistance of the applied sensors has great significance since the corrosivity of the processed substances is very difficult to determine in advance. It is thus completely understandable that a plant operator will want to use sensors having the widest range of application. That is why, beside high overload resistance and broad operating temperature range, universal chemical resistance has become increasingly important for pressure-measuring instruments.

Ceramics in pressure measurement technology

Pressure measuring cells based on ceramic materials have had a solid footing in the market for over ten years now. Like many new technologies, ceramic technology also had its share of teething troubles. The 'nearly perfect' corrosion resistance of Al2O3 proved to be a disappointment after a short time because corrosion analysis of heavier, solid ceramic components could not be directly applied to the wafer-thin diaphragms required in ceramic capacitive pressure measuring cells. Manufacturers of flowmeters also had some discouraging experiences with this material. It was discovered that, besides the concentration and temperature of the medium, the flow velocity of the medium has a critical influence on corrosion. An ongoing optimisation and development program ensued, being pushed forward by intensified cooperation between measurement technology suppliers, ceramics manufacturers and research institutions. One of the biggest challenges was to convert the results of the research into viable solutions for the manufacture of ceramic diaphragms. In order to understand the corrosion of ceramic materials - which is completely different to that of metal - the microstructure of Al2O3 ceramic has to be viewed more closely:

Of sapphires and rubies

The structure of aluminium oxide ceramic can be best illustrated by means of a micrograph. The material is composed of many Al2O3 single crystals held together at their boundaries by inter-crystalline phases (eg glass phases) and atomic bonding forces. The Al2O3 single crystal is more commonly known as sapphire, ruby or corundum. These three precious stones are chemically identical and differ only in the shade of colour caused by different doping agents. An Al2O3 sintered ceramic can thus be seen as a conglomerate of many microscopic gemstones.

Beside their degree of purity, different types of ceramic are characterised mainly by their microstructural design. Grain size and form, grain size distribution, porosity, and defect incidence are, for example, some of the parameters that have a far-reaching influence on the mechanical and solvent chemical properties of the ceramic.

Al2O3 sintered ceramic with a 99,5% degree of purity is one typically offered as 'standard ceramic' by many manufacturers. Quite noticeable here is the extremely inhomogeneous grain size distribution as well as the very coarse microstructure with grain sizes up to 40 µm. In addition, defects (pores) are also visible on the grain boundaries as well as within the grains.

The newly developed Sapphire ceramic has a 99,9% degree of purity in contrast to 'standard ceramic', the main focus of the development process was placed on an optimised structural design, which is reflected in the extremely fine-grained, homogeneous and nearly defect-free microstructure (average grain size <3 µm). This microstructure fundamentally influences the mechanical properties of a sintered material (eg ultimate bending strength). Furthermore, all detrimental foreign phases (eg SiO2) were eliminated. The individual sapphire grains are so close together that atomic bonding forces between the grains ensure microstructure cohesion. Foreign phases (especially glass phases) are no longer needed to hold the structure together. These foreign components fill in spaces between the grain boundaries and form ideal points of attack for corrosion.

Since the grain boundaries represent a weak spot in the structure, it would seem like a good idea to eliminate them altogether, using monocrystalline sapphire for the sensor diagram instead of sintered ceramic. Indeed, the best possible corrosion resistance could be achieved this way. But a single crystal also has serious disadvantages. For one thing, the price is ten times higher than sintered ceramic, and for another, the risk of mechanical failure is greater. Due to the monocrystalline structure, even the smallest material defects or cracks can propagate freely, as no grain boundaries are there to stop them. Mechanical stress on the pressure sensors (eg overload, pressure shocks, or concentrated mechanical loads caused by solid components in the measured medium) would very probably lead to breakdown of the pressure transmitter.

Sapphire ceramic thus represents an ideal symbiosis of single crystal sapphire (with its excellent corrosion resistance) and fine-grained sintered ceramic (with its optimal mechanical properties). This led to the name of the ceramic type developed primarily for pressure sensor diaphragms.

Real-world corrosion tests

Corrosion tests according to DIN 50905 measure the weight loss or surface erosion of a material specimen. These methods are generally not useful because damage to the ceramic microstructure can occur at the grain boundaries long before weight loss or surface erosion is detected. With pressure sensors, this internal structural damage can lead to drift of the zero point or measuring range. That is why all corrosion tests of Sapphire ceramic were carried out on completely assembled pressure transmitters. In this way, even the smallest drift of the zero point or measuring range could be registered and practical results obtained. The overall result of these tests was a 10 to 100-fold improvement of corrosion resistance in a large number of aggressive mediums. Substances like hydrochloric acid, sulphuric acid, caustic soda, potash lye, nitric acid, acetic acid and phosphoric acid (at 80°C and 10% concentration) were used in tests over a period of four weeks.

Universally employable pressure transmitters

Through the use of Sapphire ceramic, pressure transmitters (based on ceramic-capacitive pressure measuring cells) covering a broad spectrum of applications in level and process pressure measurement are now available. Through the use of different process connections, eg of stainless steel and PVDF, and sealing materials, eg Viton, EPDM, Hiflour and Kalrez, almost any application in any industry can be accommodated. Especially in the food, pharmaceutical, chemical and paper industries, the advantages of this technology have made an impact.

The pressure transmitters have excellent long-term stability (0,1%/year), high overload resistance (eg 1,5 MPa (15 bar) with 10 kPa (0,1 bar) measuring range) and measuring accuracy <0,1%. The standard electronics module has a 4 to 20 mA + HART interface, and a Profibus PA version for field bus applications is also available.

VEGA Instruments SA

(011) 958 1901

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