Checklist: Selecting pressure transducers

Bob Torsiello -- Control Engineering Europe, 1/1/2000

Pressure transducers are found in numerous OEM applications, and they are used widely in process control. The introduction of the microprocessor has increased the functionality and expanded the use of pressure transducers over the past 15 years.

Selecting the proper pressure transducer for any application requires a close look at the following criteria:

• Isolation: Should a transducer be isolated from the medium being measured? For corrosive, high-temperature, or viscous media, isolation is generally required. Frequently, a metal or ceramic diaphragm, with possibly a fill fluid, is incorporated.

• Accuracy: Pressure instrumentation is available in a wide range of accuracies. High accuracy devices usually have improved performance both with temperature changes and over time. Greater stability comes at a premium price.

• Pressure range: Commonly available ranges exist from vacuum to very high pressure, with vacuum, gauge, absolute, or differential pressure references. When selecting a transducer's range, it is desirable for the application's normal operating pressure to be 50-90% of the range chosen.

• Temperature effects: Temperature changes have the greatest effect on a pressure transducer's environmental performance. Most manufacturers provide temperature compensation specifications that define thermal effects over a given range. Performance shown as a coefficient or error band is guaranteed over that temperature range. Outside of that range, larger errors should be anticipated.

• Vibration/shock effects: Vibration and shock are highly application-specific environmental issues. They should reviewed for fit with manufacturer's specifications.

• Electrical effects: "CE"- marked products usually have built-in radio frequency interference (RFI), electromagnetic interference (EMI), and electrostatic discharge (ESD) protection. These are fast becoming a requirement for usage within today's operating environments.

• Hydraulic Applications: When applying transducers in hydraulic systems, it may be necessary to consider use of 'snubbers' to dampen hydraulic spikes. These dampening devices prevent sensor failure due to over-range readings from phenomena such as 'water hammer.'

• Outputs: Transducer outputs are available in industry-standard, millivolt, voltage, or current signals. Digital outputs with communication capability are available as well. The 4-20 mA output is the simplest since it is usually a two-wire configuration.

Bob Torsiello is product manager for transducer products
at NOSHOK Inc., Middleburg Heights, Ohio USA.