Selecting pressure transducers
Bob Torsiello -- Control Engineering, 11/1/1999
Pressure transducers are found in numerous OEM applications including appliances, on- and off-road vehicles, medical equipment, industrial machinery and military/aerospace applications. They are also used widely in process control. Introduction of the microprocessor has spurred both functionality and expansion in the use of pressure transducers over the past 15 years. Selecting the proper one for any application requires a close look at the following criteria:
Need for isolation-One of the first questions to be asked is whether the transducer's sensor needs to be isolated from the medium being measured. If the medium is a clean and a noncorrosive gas or liquid then a nonisolated transducer is acceptable. For corrosive, high-temperature, or viscous media, isolation is generally required. Frequently, a metal or ceramic diaphragm with or without a fill fluid is incorporated. Diaphragm seals can be attached to most pressure transducers.
Accuracy-This key selection criterion is determined by the performance level required for the application. Pressure instrumentation is available in a wide range of accuracies. Keep in mind, high accuracy devices usually have improved performance both with temperature changes and over time. This greater stability comes at a premium price.
Pressure range-Commonly available ranges exist from vacuum to 60,000 psi-in steps-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-Built-in radio frequency interference (RFI), electromagnetic interference (EMI), and electrostatic discharge (ESD) protection are fast becoming a requirement for usage within today's operating environments. "CE"- marked products usually have RFI, EMI and ESD protection built into the transducer's electronics.
Hazardous area applications-Transducers to be used in a hazardous environment must be approved explosion-proof or intrinsically safe models.
Type of process connection-Pressure ports of 1/8-, 1/4-, 1/2-in. NPT and 7/16-in. straight threads are common in industrial applications. Applications in low-pressure ranges may only require hose barbs or simple push-on connections. User preference is typically dependent upon the industry and application.
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. Some of the more common outputs are 0-30 mV, 0-100 mV, 4-20 mA, 0-5 V dc and 0-10 V dc. The 4-20 mA output is the simplest since it is usually a two-wire configuration. Other nonstandard outputs are usually the result of specific requirements of a large-volume OEM.
Electrical connections-Electrical terminations possible include conduit, cable, circular, and DIN style. DIN-style connectors, both full size and miniature, have become popular options across the application spectrum because they offer the convenience of screw terminals and moderate cost.
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Bob Torsiello holds a masters degree in electrical engineering from Stevens Institute of Technology, Hoboken, N.J., and is product manager for transducer products at NOSHOK Inc. He is a 28-year veteran of the instrumentation field, serving the last 10 years specifically in the pressure transducer area. Comments? E-mail djohnson@cahners.com |
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