While selecting a right kind of pressure transducer for an application one must consider following important points:
Corrosion protection: Decide whether the sensor is required to be isolated from the medium being measured. If the medium is a clean and a non corrosive gas or liquid then a non isolated transducer is adequate whereas for corrosive, high-temperature, or viscous media, isolation is usually required. Often, a metal or ceramic diaphragm with or without a fill fluid is integrated.
Accuracy: Another key selection criterion is the performance level i.e. accuracy required for the application. In general, high accuracy devices have improved performance both with temperature changes and over time. However this better stability comes at a premium price.
Pressure range: Available pressure ranges exist from vacuum to 60,000 psi-in steps-with vacuum, gauge, absolute or differential pressure references. While 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: This is another crucial selection criterion. Temperature variations can have considerable effect on a pressure transducer’s environmental performance. Nearly all manufacturers provide temperature compensation specifications which define thermal effects over a specified range. Performance shown as a coefficient or error band is guaranteed over that temperature range. Beyond that range, larger errors should be estimated.
Vibration/shock effects: Since these are highly application-specific environmental issues, they should be reviewed carefully for fit with manufacturer’s specifications.
Electrical effects: In today’s operating environment, radio frequency interference (RFI), electromagnetic interference (EMI), and electrostatic discharge (ESD) protection have become an integrated feature of pressure transducers. “CE”- marked products generally have RFI, EMI and ESD protection built into the transducer’s electronics.
Hazardous area applications: If transducers are to be used in a hazardous environment then they must be approved explosion-proof or intrinsically safe models.
Hydraulic Applications: If transducers are to be employed in hydraulic systems, it may possibly be required to consider use of “Snubbers”. Snubbers are dampening devices used to dampen hydraulic spikes. They can avoid sensor failure because of over range readings from phenomena called “water hammer.”
Outputs: Transducer output is another key decision area. Outputs are available in following forms:
- industry-standard
- millivolt
- Voltage or current signals.
- Digital outputs with communication capability
A few of the regular 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 as it is usually a two-wire configuration. Other nonstandard outputs are by and large the result of specific requirements of a large-volume OEM.
Electrical connections: Another important decision in selecting a pressure transducer is based upon type of electrical connection needed. Following Electrical terminations are possible:
- conduit
- cable
- circular
- DIN style
DIN-style connectors, both full size and miniature, are the popular options across the application spectrum since the advantage of screw terminals and moderate cost is associated with them.
There are two conflicting considerations which appear in the selection procedure of pressure transducers. They are:
- Accuracy
- Overpressure Protection
From an accuracy point of view, the range of a transmitter should be low so that error, generally a percentage of full scale, is reduced. Whereas on the other side, one must always consider the consequences of overpressure damage due to operating errors, faulty design or failure to isolate the instrument during pressure-testing and start-up. Consequently, it is imperative to specify not only the requisite range, but also the amount of overpressure protection needed. Almost all pressure instruments are granted with overpressure protection of 50% to 200% of range.
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