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.
DPS300 User Switchable Pressure Range, Volts or Current Output Low DP Sensor - The DPS 300 is a low range HVAC differential pressure sensor. The lowest possible pressure range is 0...100 pascals. 2 or 3 switchable pressure ranges, plus volts or current output are included with most standard configurations.
DPT200 High One Side Overload Differential Pressure Sensor - DP transmitter for measuring pressure difference between two pressure points which can tolerate up a very high overload pressure independently on either the positive or negative side port or both together at the same time.- DPT200 Pressurised Tank Level Differential Pressure Transmitter - Pressurised tank liquid level differential pressure cell transmitter for process plant storage.
36XW Digital Submersible Hydrostatic Level Sensor - The 36 X W is a high accuracy sensor with analogue 4-20mA / 0-10Vdc or digital RS485 output for submersing into a liquid to measure depth.
DS210 Combined Low Pressure Switch, Indicator and Sensor - Low range three in one pressure switch, pressure indicator and pressure sensor for use on pneumatics and HVAC applications.
DMP333i Rangeable High Pressure Precision Transducer - High performance transducer with adjustable scaling for modifying the pressure range for different applications. Ranges from 0-60 bar up to 0-600 bar.
- Hydrostatic level sensor for 7m high tanks with 4-20mA plus HART coms
- Water tank level gauge transmitter with adjustable range
- HART flameproof (Ex d) re-scalable level transmitter for hydrostatic head up to 2 metres
- Adjustable range pressure sensor for scaling -1 to +1 bar g
- 100 mbar abs vacuum transmitter with a 5 bar overpressure
- Zero and span adjustable 10 bar ATEX pressure transmitter
Find out more about our Rangeable to determine which product options and capabilities will best meet your application requirements.
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.
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
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