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Rangeable

Rangeable instruments

Rangeable instruments offer valuable flexibility in measurement applications. They allow users to adjust the output signal scaling to match specific process needs, effectively tailoring a single instrument to various measurement ranges. This adaptability simplifies inventory management and reduces instrumentation costs.

A rangeable instrument can be rescaled by adjusting the zero and span setting to a measurement between zero and the 100% full-scale range of the instrument. Re-ranging methods include:

  • Externally accessible variable potentiometers
  • Push-button controls
  • Digital communication

Some rangeable instruments have a limit on how much the scale can be reduced, typically expressed as a ratio of the maximum full scale. For example, a 10 bar range pressure transmitter, reducible by up to 80%, would be described as having a 5 to 1 turndown ratio.

Several critical factors should be considered when selecting a rangeable instrument:

  • Measurement Range and Turndown: Understanding the minimum and maximum expected process values is crucial for determining the appropriate instrument and its achievable span.
  • Re-ranging Method: While potentiometers offer a simple analog approach, digital communication provides greater precision and often allows remote configuration.
  • Accuracy: The impact of re-ranging on accuracy is essential. Analog devices may exhibit reduced accuracy at lower spans, while digital instruments often maintain accuracy within a specified turndown ratio. Engineers should carefully review manufacturer specifications to ensure the instrument meets required performance criteria across the desired range.

Often it is assumed all rangeable instruments offer the same performance across their entire turndown range. As mentioned, accuracy can be affected, and other parameters like stability and linearity might also vary. Often, accuracy performance is compromised when a range is re-scaled to a reduced span, particularly with analog-based devices. Some digital rangeable devices offer a fixed percent of full-scale accuracy to a certain turndown level, beyond which the percent full-scale accuracy typically increases pro-rata.

Another mistake is overlooking the complexity of digital configuration. While offering advanced features, digitally configurable instruments require specialized tools and training. Engineers should assess their team’s capabilities and the availability of necessary equipment before opting for a digitally ranged instrument.

Finally, failing to consider the long-term stability of re-ranged settings can lead to inaccurate measurements and process upsets. Regular calibration and verification are crucial for maintaining measurement system integrity.

  • Rangeable Pressure Transmitters - Rangeable pressure transmitters, also known as adjustable range pressure transmitters, offer the flexibility to modify the 4-20mA output signal to match various pressure ranges.
  • Rangeable DP Sensors - Rangeable differential pressure (DP) sensors provide a high degree of flexibility in measurement applications by allowing users to adjust the output signal.

Help

Turn down accuracy

For a rangeable transmitter of lets say 100 bar g full scale, and a 100:1 turn down with an accuracy of 0.1%FS, the error would be +/-100 mbar over the range of 0 to 100 bar g.  What would be the accuracy if it was re-ranged to 60 bar. Would it be +/-100mbar or +/-60mbar?

It will depend on the make and type, since some will improve the accuracy at lower ranges by using a % of reading method. The worse case error should be within 100 mbar at any point or range. You should be able to find some statement on the product data sheet explaining how the accuracy is affected by turndown.

Related Technical Terms

Glossary of Pressure Range technical terms

Help from Pressure Range resources

Guidance with Pressure Range requirements