TechArchive

This article was originally written in the period 1995-2000

Dave Maundrell, Worldwide Product Manager at Gems Sensors (formerly TransInstruments), takes a look at pressure transducer specifications.

Pressure transducer specifications vary from manufacturer to manufacturer and from instrument to instrument, often with each company claiming that their instrument is more accurate, faster, more reliable and so on. Typically, these claims are supported by detailed technical specifications which, at first glance, may well appear to back up the manufacturers’ claims and are designed to help end users of pressure monitoring or control systems to select the correct device for each application.

Although these specifications vary in complexity, they generally include a section covering the errors inherent within each device. This should indicate how well the transducer or transmitter will perform, either under a range of conditions or at a predetermined status.

The problem, however, is that there are inconsistencies in the way these figures are presented by manufacturers. Some manufacturers show calculated error bands, with the method of calculation and the base figures or coefficients used varying between manufacturers, while others list only individual coefficients, leaving end users to carry out the calculation themselves. As a result, figures quoted by different manufacturers, although looking similar, may in fact conceal significant discrepancies with, for example, errors due to thermal hysteresis or repeatability sometimes being ignored completely in order to produce more favourable data.

Calculating Error Bands

Two of the principal factors to be considered when calculating error bands are the performance of the sensor as pressure rises or falls and the impact of changes in temperature on the output of the sensor.

In essence, the factors to be taken into account for rising and falling pressures are linearity, hysteresis and repeatability. The majority of manufacturers group these factors together under the heading ‘Static Error Band’ as, generally, there is no real advantage in specifying them separately, leaving the user to calculate the likely errors themselves.

On a thin film transducer, the static error remains constant as a percentage of the pressure excursion. For example, if the Static Error Band is stated as being 0.1% maximum span on a 10 bar transducer and the measured pressures are from 0 to 7 bar, the static error band will be 0.1% maximum of 7 bar. Thus the user can readily assess the performance for their particular requirements.

One factor which is particularly important when reading specifications for the stability of measurement, is the Thermal Error Band, which specifies the effect of temperature variations on the transducer. Most manufacturers quote temperature coefficients for zero and span, but rarely quote thermal stability figures, sometimes referred to as thermal repeatability.

Gems Sensors, however, has adopted an error band method which takes into account both the change in the zero pressure output and the change in span with temperature, plus thermal zero and thermal span stability, giving a total Thermal Error Band.

Basically, the latter is thermal hysteresis, and measures the error after a given temperature cycle; that is the difference between the output prior to, and after, the temperature cycle. This gives an important indication of measurement stability; however, many manufacturers do not make it clear whether their thermal error band specifications include thermal stability, so the figures quoted can sometimes give an inaccurate impression of stability.

With all pressure transducer specifications it is important to realise that every sensor used in pressure transducers and transmitters will contain a certain degree of error. This fact applies to silicon, thin film, bonded strain gauge, CVD devices and others, regardless of how well designed and manufactured they are. The degree of error will, of course vary between the different types of device and will reflect the construction techniques used. For example, bonded strain gauge sensors may exhibit poor error characteristics which become worse at temperature rises, while thin film sensors, with their atomically bonded structure, function to particularly tight tolerances over a wide range of operating conditions.

To help users gain a full understanding of specifications and standards, Gems Sensors has issued a comprehensive range of technical fact sheets, which includes an unbiased Definition of Terms used in specifying transducer performance. For copies of the fact sheets contact Cindy Lane at Gems Sensors on 01256 20244.

• Gems Sensors
• Tel: 01256 20244
• Fax: 01256 473680