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Instrumentation

Instrument Range / Span Calculator

An instrument range is defined by its lower range value (LRV) and upper range value (URV); the span is the difference between them. This calculator works out the span and the relationship between a process value and its percent of span for a linear range. Pick a mode: get the span from LRV and URV, find the process value at a given percent of span, or find the percent of span for a given process value. It handles linear range/span only — not transmitter configuration, calibration certificates, or non-linear characterisation.

TypeInteractive engineering calculator

Calculator

Results
Span200
Range-50 to 150
Zero (LRV)-50

Formulas

Span
span = URV − LRV
Percent of span
% span = (PV − LRV) / span × 100
Process value
PV = LRV + span × % span / 100

Diagram

Instrument Range / Span: span = URV − LRV0LRV−50 · 0%URV150 · 100%span = URV − LRV = 200

Worked example

An instrument is ranged LRV = −50, URV = 150. What is the span, and what percent of span is a process value of 50?

  1. 01span = 150 − (−50) = 200
  2. 02% span = (50 − (−50)) / 200 × 100
  3. 03% span = 100 / 200 × 100 = 50%
  4. 04Check: PV = −50 + 200 × 50 / 100 = 50
Result

Span = 200; a process value of 50 is 50.0% of span.

FAQ

What is the difference between range and span?
The range is the pair of endpoints, LRV to URV (for example −50 to 150 °C). The span is the single number that separates them, URV − LRV (here 200 °C). A range can cross zero; the span is always the width of the range.
Why can the lower range value be negative?
Many instruments are ranged across zero — for example −50 to 150 °C, or a compound pressure range from vacuum to positive pressure. This is an elevated zero. The span is still URV − LRV, and the calculator accepts a negative LRV.
How does this relate to the 4–20 mA signal?
Percent of span maps directly onto the signal: 0% of span is 4 mA and 100% of span is 20 mA. Use the mA to Process Value and Process Value to mA calculators to convert between percent of span and the loop current.
Does this configure or calibrate a transmitter?
No. It computes the range/span relationships only. Actual configuration and calibration require the transmitter datasheet, loop drawings, a calibration procedure, and site standards.

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