Instrument Range vs Span
The range is the pair of endpoints LRV and URV; the span is the width URV − LRV. Learn why a range can cross zero, how elevated and suppressed zeros work, and how range and span drive signal scaling.
Definition
An instrument's range is the pair of endpoints it is calibrated between: the lower range value (LRV) and the upper range value (URV), written as LRV to URV. The span is a single number — the width of that range, span = URV − LRV. A 0–100 kPa instrument has a range of 0 to 100 kPa and a span of 100 kPa; a −50 to 150 °C instrument has a range crossing zero and a span of 200 °C.
Why it matters
Range and span are the two numbers that define how an instrument maps the physical world onto its signal, and confusing them is a classic source of error. The span sets the resolution and the percent-of-span scaling; the range endpoints set where zero sits and whether the measurement can go negative. Ranging a transmitter, choosing turndown, sizing for the expected operating window, and reading a calibration sheet all depend on keeping range and span distinct. Many process ranges deliberately cross zero — compound pressure from vacuum to positive, or temperatures spanning freezing — which only makes sense once you separate the endpoints (range) from the width (span).
Formula
Units involved
- •LRV — lower range value, the process value at 0% of span (4 mA)
- •URV — upper range value, the process value at 100% of span (20 mA)
- •range — the endpoint pair, LRV to URV, in engineering units
- •span — URV − LRV, the width of the range, in the same units
- •PV — process value in engineering units
- •% span — position of PV within the range, 0% to 100%
Concept diagram
Worked example
Compare two instruments: a 0–100 kPa pressure transmitter and a −50 to 150 °C temperature transmitter. Find the span of each, and the percent of span at a process value of 50.
- 01Pressure: span = 100 − 0 = 100 kPa; at PV = 50 kPa, % span = (50 − 0) / 100 × 100 = 50%
- 02Temperature: span = 150 − (−50) = 200 °C
- 03At PV = 50 °C, % span = (50 − (−50)) / 200 × 100
- 04% span = 100 / 200 × 100 = 50%
Both read 50% of span at PV = 50, but the spans differ (100 vs 200) and the °C range crosses zero with an elevated zero at LRV = −50.
Common mistakes
- •Using the URV as the span. Span is URV − LRV, not URV. A 10–50 bar instrument has URV = 50 but span = 40.
- •Assuming the LRV is zero. A suppressed zero (LRV > 0, e.g. 50–250 °C) and an elevated zero (LRV < 0, e.g. −50 to 150 °C) are both common. Percent of span is always measured from the LRV.
- •Thinking a range cannot be negative. Many ranges cross zero — compound pressure or sub-zero temperature. The span is still URV − LRV and stays positive as long as URV > LRV.
- •Comparing instruments by range when you mean resolution. Two instruments can share a span but sit at different ranges, or share a range endpoint but have different spans. Be explicit about which you are comparing.
When to use the calculator
Use the Instrument Range / Span calculator to compute the span from LRV and URV, find the process value at a given percent of span, or find the percent of span for a process value — including ranges that cross zero. Pair it with the mA to Process Value and Process Value to mA calculators to see how the same range maps onto the 4–20 mA signal.