Pressure Measurement & Control

Pressure measurement - the art of specialisation

January 2002 Pressure Measurement & Control

Pressure measuring ranges of 0 to 500 Pa (5 mbar) using pure pressure sensors with ceramic elements are no longer unusual. From monitoring container and pipe pressures for safe operation to measuring levels in tanks and measuring flow by means of differential pressure for cost savings, pressure measurement has evolved into a complex field of technology on its own.

With the impact of external influences such as very high operating temperatures, often combined with a vacuum, corrosion resistance and condensation, it has become impossible for one instrument to cover all sectors. In fact, high demand has arisen for sector-specific products.

Endress+Hauser has anticipated this market trend and now offer some of the most specialised instrumentation available. One area that requires such precise attention is that of electrical differential pressure measurement at columns.

When operating rectification columns, pressure drop in the column is an important control variable. This is determined by differential pressure measurement between the head and the base, which can be carried out using differential pressure transmitters. In practice, however, this type of measurement is often complicated by certain peripheral conditions. For one, columns are often very high, which means that even laying the pressure piping or capillaries can take a considerable amount of time and effort. In addition, exact measurements, especially when using capillary pressure transmitters are only possible if the capillaries are kept at the same temperature.

The Endress and Hauser FMX 167
The Endress and Hauser FMX 167

When rectifying mixtures that are sensitive to temperature, vacuum rectification is used to reduce the boiling point. However, vacuum is problematic near the absolute zero point especially for oil-filled systems, as the oil filling then tends to form bubbles. In practice, this effect is indicated by increased zero point drift of the transmitter installed. This situation can be resolved if the differential pressure is determined by means of what Endress+Hauser calls electrical differential pressure measurement. In contrast to a differential pressure transmitter, the pressure in the head and in the base is measured separately at both points by means of a pure pressure transmitter, the Endress+Hauser Cerabar S. The difference between the two measured values is then calculated in a separate switching unit or in the plant controls. This situation has a number of advantages.

Firstly, installation is considerably easier, since now only electrical lines must be laid and the column can be practically of any height. Secondly, pressure piping can no longer freeze up or get clogged. Measuring errors caused by capillaries of different temperature (irradiation) can also be ruled out. Absolute vacuum-proof, oil-free ceramic cells can be used. This appears to have the disadvantage that two instruments must now be used instead of one differential pressure transmitter. However, on closer inspection, this is actually an advantage, since three measured variables can now be taken from the two instruments, ie in addition to the differential pressure over the column, head pressure and base pressure are also available for controlling and monitoring purposes in the plant.

Sterile sectors, such as found in food and pharmaceutical processes, present another specialised challenge to which Endress+Hauser has risen. Pressure and hydrostatic level are two important process parameters here. With the Cerabar M pressure sensor, the user has the choice between two expert sensor principles. The heart of the pressure sensor is either a piezo-resistive sensor with metallic measuring membrane, or a capacitive ceramic measuring cell that works without filling oil. However different the technology behind both pressure-measuring methods may be, long-term stability and overload-proof measuring with a high level of accuracy is common to both versions.

The Cerabar M pressure sensor
The Cerabar M pressure sensor

Only materials which the FDA categorises as nonhazardous to health are in contact with the process. These materials include the high-purity Ceraphire ceramic (99,9% A2O3) and the seal (eg EPDM, NBR). All metallic parts of Endress+Hauser's Cerabar M which are accessible from the outside, including the housing, are made from CrNi-steel AISI 316 L, which is standard in pharmaceutical plants. The construction of this housing was tailor-made to fulfil the requirements for hygienic applications. The level of safety required for external cleaning is also provided, thanks to the corner and edge-free design, together with ingress protection to IP68. All connections meet qualifications such as the 3-A certificate for hygienic construction and those regarding unrestricted ability to be cleaned.

Level measurement in water treatment is one of the areas benefiting most from Endress+Hauser innovations, as pressure-measuring instruments have a wide range of applications in the area of water treatment. Hydrostatic pressure measurement especially, is often used for level measurement. Level measurement with hydrostatic pressure sensors has clear advantages over other measurement methods, as the measuring principle is not influenced by foam formation or other objects floating on the surface. However, a prerequisite for long-term stable measurement is that the sensors are not corroded or destroyed by the fluid (eg contaminated chemical-wastewater). The Endress+Hauser Waterpilot level probes are equipped with high-precision ceramic sensors, which are chemically stable and mechanically resistant. These probes can even be used in difficult applications without any problem, such as in the sand collector of a sewage treatment plant. Probes with an external diameter of just 22 mm - thanks to a specially-developed very small ceramic sensor - can be used for measuring purposes in very narrow sounding pipes. An integrated temperature sensor means that in addition to measuring the level, you can also measure the temperature online at the same time. The materials used are suitable for unrestricted use in drinking water. This conformity to drinking water is inspected and certified by various independent bodies (ACS, KTW, NSF).

Waterpilot level probes have a 4 to 20 mA output signal that can be processed directly in an SPS, or in a switching unit that has been specially designed for the measuring task. In addition to a wide range of accessories for mechanical integration, intelligent feed and signal processing units provide tailor-made solutions. In this way, using the Endress+Hauser RMA 422 process transmitter, for example, voltage can be supplied to the level probe and, at the same time, the level and temperature signals can be evaluated and downstream consumers can be controlled.

Water measurement using the Deltapilot S
Water measurement using the Deltapilot S

Exact solutions are at hand when associated intelligent feed and signal-processing units are used. In this way, using the RMA 422, for example, voltage can be supplied to the level probe and, at the same time, both the level and the temperature signals can be processed and downstream consumers can be controlled. To prolong the service life of pumps and to significantly reduce maintenance costs, several pumps can be controlled alternately. In other words, the pump that was out of service the longest is the next to be switched on or the pump in service the longest is the next to be switched off. It is also not a problem to detect level tendencies for optimum early reaction by means of the corresponding signal processing unit and the Waterpilot. Such solution packages clearly show the trend of the future - away from the pure 'simple' sensor and towards the intelligent compact instrument with the corresponding signal processing unit, which offers the user tailor-made solutions.


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