Pressure play

Pressure is a fundamental and prevalent physical parameter throughout today's industrial, scientific and aerospace worlds.

Pressure measurement therefore plays a key role in communicating the status and behaviour of gas or liquid media throughout a wide range of applications. Each market presents its own requirements in terms of performance and packaging. As a result, a vast multitude of pressure instrumentation and manufacturers have evolved.

Over the past 30 years, compact yet rugged, high performance electronic pressure sensors have been available as integrated products, incorporating on-board signal conditioning where required. These have evolved with increasing popularity.

Within the natural environment, pressure sensors can be found measuring open river depths - for example, water table levels and barometric air pressure. Man-made industries use pressure transducers and transmitters for research, development and production purposes on cars, oil rigs, chemical plants, power stations and aircraft.

Currently available pressure transducers and transmitters employ various technologies and a wide range of packaging designs to suit the many applications encountered. High profile technologies such as piezo-resistive 'wet' silicon diaphragm sensors (exemplified by the GE Druck PDCR/PTX series) have demonstrated extreme flexibility of application throughout many markets and installations.

Pressure ranges can be up to 1000 bar and beyond, with a typical accuracy of 0.05% to 0.1% and even better. Other types, such as the 'dry' diaphragm inductive type sensor, provide the optimum solution in certain specialist areas — typically in very low pressure and ultra high vacuum/high temperature measurement. Full scale pressure ranges are as low as 0.1mbar/10Pa.

A further choice available to the user is signal format. In addition to the basic 4-wire/millivolt output and 3-wire/voltage output transducers, a popular selection for harsh industrial duty is the pressure transmitter, which is a 2-wire device where supply voltage and output current, typically 4-20mA, share a common electronic loop; this remains a simple and effective method for many applications.

However, installations with many pressure transmitters can benefit from the simultaneous analogue signalling and digital communication provided by the HART (Highway Addressable Remote Transducer) protocol. Used by over 80 vendors, Hart is used in smart transmitters which enable the 2-wire interface format to be retained but a digital signal superimposed, providing 2-way communications for centralised operator control.

For ultimate precision of measurement, resonant micromachined silicon sensors, such as GE Druck's RPT series, use the principle of a 3D-etched silicon structure whose mechanical resonant frequency varies with applied pressure. To obtain a proportional signal, the output is conditioned by an on-board microprocessor. This system produces outstanding levels of accuracy and stability within an externally compact package, typically 0.01%, for aerospace, meteorological and laboratory applications amongst others. Digital outputs can also be supplied.

For testing pressure sensors and other instrumentation in situ, handheld portable pressure calibrators can be supplied fully self-contained with pressure generation, electrical supply/measurement and PC communications on board.

Generally, today's leading field calibrators embrace a wealth of developments in measuring, interfacing and packaging technologies which directly benefit the user in terms of overall test efficiency, speed and quality.For example, high accuracy measurements can be made readily with precision low-doped and resonant silicon pressure elements.

This advanced measurement technology is then complemented by task-oriented dual parameter graphic displays which can be configured for simplified routine calibration and maintenance or advanced troubleshooting at the touch of a button.

Multifunction hand pumps can now precisely and very rapidly source up to 60 bar pneumatic or 700 bar hydraulic test pressures and 95% vacuum. Compact, rugged and ergonomically designed packaging eases practical use of the calibrator and prolongs its useful life in harsh field environments. Complete test routines can be pre-loaded into the calibrator and recorded for automated field implementation. At the end of a workshift, the results can be uploaded to a PC.

Internal electronic data storage, interfacing to a PC via RS 232 communications protocol or PCMCIA card, reduces the possibility for errors because the recording of data and its transfer to a centralised test and calibration management system is both automated and paperless.

Approved calibration software can also be supplied to enhance this 'low risk' documentation process further and to simplify the necessary compliance with quality standards such as ISO 9000. For users who do not have their own established system, proprietary software packages are available, often from the calibrator manufacturers themselves, which satisfy the full range of test, calibration and documentation management requirements.

This includes, for example, the creation of multiple databases to organise data storage, a powerful search engine for simplified data retrieval and a multi-level security password. Moreover, test procedures and individual specific work orders can be generated for downloading to the portable calibrator.

At the end of the daily workshift, resulting data collected in the field is simply uploaded back at the office PC for creating records and further analysis/processing. Detailed and quality compliant calibration certificates can then be automatically generated, making the whole process far easier and more reliable than the traditional manual paperwork approach.

These and many other developments recognise the importance of the field calibrator's role in improving process safety, efficiency, quality and reliability.

Tony Cuttill is Marketing Communications Manager with GE Infrastructure Sensing, previously known as GE Druck.