Pressure Measurement & Control


Precision pressure and temperature transmitter

January 2003 Pressure Measurement & Control

Frequently there is a need in the process industry to measure several properties at the same time. This is most often the case with pressure and temperature, as there is often a direct connection between them. A combined sensor, which can measure both properties, would be ideal. It would reduce the number of openings that must be made in a system for installing sensors and so reduce the hygiene risks.

Food production and processing requires above all, cleanliness and hygiene. Filling levels and pressures are important factors, but temperature is the most frequently measured property in the food industry. It is often required at the same points where level and pressure are measured, in containers, reactors, fermenters and ponds.

Up to now, pressure and level were recorded with pressure transmitters, and temperatures with temperature sensors. In the food industry, pressure transmitters must be installed so that they are absolutely flush with the surface of the process vessel. Any gap increases the risk of contamination and the threat of bacterial growth. Every sealed joint must be maintained; every process connection must be designed and manufactured; and every sensor whether for pressure or temperature is a cost factor.

Vega, a specialist in measurement and sensor technology, recognised the need for a dual-purpose sensor and has developed pressure transmitters that can measure both pressure and temperature.

The pressure transmitter supplies the values for pressure and temperature digitally through a simple 2-core cable and works on Profibus PA. Digital technology makes it possible not only to supply power, but also to transmit the pressure and temperature values from seven pressure transmitters over this single 2-core cable.

Before the temperature signal can be used, the question of accuracy must be considered. Generally temperatures are measured using NTCs, PTCs, temperature crystals or Pt100s. In pressure transmitters, NTCs are frequently employed. Unfortunately, they are not very linear and have even less long-term stability. Attempts to compensate for these factors in the microprocessor meet with only limited success.

However, in the oil-free Vega Multisensors, linear and long-term stable silicon double-diodes are employed. An oil-free sensor was developed because the oil in an oil-filled pressure transmitter is a severe hindrance to the flow of heat from the sensing diaphragm to the measuring cell. A silicon double-diode temperature sensor in the measuring cell records the temperature of the cell in the range -30 to +150°C.

Vega's dual-purpose sensor concept has been successfully adapted for aseptic installations for food industry-approved variants. The sensor has been approved by the EHEDG (European hygienic equipment design group) and the American FDA. The diagram shows a section through the flush-mounting Vega D84 pressure transmitter.

For more information contact Alan Wynn, Vega Instruments SA, 011 958 1901, alan.wynn@za.vega.com



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