Just when we get comfortable, they change the rules. This seems to apply to much of life, and pressure calibration is no exception. ISO 9000 is de rigeur, increasing the calibration load, at the same time as staffing levels are being reduced. Suppliers of transmitters, operating in an intensely competitive market, continuously strive for a competitive advantage.
Control systems get more advanced, and plant management want more and better information. The pressure transmitter is one of the more important instruments from which this information is obtained. Mass-flow measurement is in demand, and multivariable transmitters are growing in usage. Their market share is expected to more than double in the next 4 years. Multivariable transmitters incorporate low DP pressures and higher absolute pressures in one package.
40 years ago the typical pneumatic pressure transmitter had an accuracy of 0,5%. 20 years ago the typical electronic pressure transmitter had an accuracy of 0,25%. Today's smart instruments are offering better than 0,05%. Tomorrow? Who knows? (Well, some do, but they are not telling.)
Mass flow computation from velocity-based measurements requires measurement of absolute pressures. Furthermore, most process reactions are governed by absolute pressures, but absolute pressure instruments have been more expensive - and historically, few calibrators have offered absolute mode.
Things are changing for the better - Mensor has released its first 21st century design, the 8100 pressure controller, designed to address these matters. A number of state-of-the-art features are used to achieve the design objective. The front panel display is a flat VGA touch screen. There are no conventional switches anywhere. Apart from the power switch, the only moving parts are the solenoid valves and pressure regulators.
The unit can accept up to two interchangeable pressure transducers. There is no relationship between the transducers, and full auto-ranging is applied if desired. These transducers use the latest quartz resonator digital sensing technique, and are fully mapped for temperature and pressure response. Up to two sub-ranges can be characterised for each transducer. All calibration data is stored in the transducer assemblies, making them truly portable.
The controller may be configured as either absolute mode or gauge mode, but both versions include an on-board precision barometric reference transducer, so that a gauge model offers absolute emulation, and vice versa.
The gauge version is used to allow best accuracy at low gauge ranges. Where a low pressure of 6 or 7 kPa gauge is needed to test for example a DP flow transmitter, a gauge transducer is more accurate, in this example by a factor of 17. Using an accuracy figure of 0,01% of span for simplicity, we can use a 20 kPag transducer with a 7 kPa sub-range, offering an uncertainty of 0,7 Pa. If we used a 120 kPaA transducer, this would provide a roughly -100 + 20 kPa gauge emulation range at the coast, with an uncertainty of 12 Pa.
As it is a pressure controller, using an external source of pressure, the instrument delivers a precisely controlled pressure (or series of pressures) to the instrument under test. The device can be remotely controlled via a GPIB bus - and drivers exist for National Instruments' Labview, and DH Budenberg's Pascal software packages. This makes it possible for fully automatic calibration procedures to be established.
Mensor have therefore succeeded in providing a counter to the problems discussed above. The system offers improved productivity, sufficient accuracy to handle today's state-of-the-art transmitters, the ability to switch between absolute and gauge modes, and the ability to combine high and low pressure ranges in one instrument.
The author
Rod McLeman is part owner, with his family, of Blanes Pressure Solutions. Rod holds a BSc.Mech.Eng degree from the University of Glasgow, and a B.Com degree from UNISA. He has been involved with pressure calibration for the last ~30 years.
For more information contact Rod McLeman, Blanes Pressure Solutions, 011 397 7365, [email protected], www.pressuresolutions.co.za
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