Instrument Error Converter — % Span ↔ % Reading ↔ Absolute

Three languages for the same error

Instrument accuracy gets quoted three ways, and they are only interchangeable at one specific reading:

A "% of span" spec describes a fixed absolute band — the instrument is equally wrong (in units) everywhere. A "% of reading" spec describes a proportional band — tighter at the bottom of the range, wider at the top. The same ±0.5% means very different things in each language, and datasheets love the ambiguity.

Worked example — why it matters

A 0–250 kPa transmitter with ±0.5% of span accuracy, read at 50 kPa:

1. Absolute: 0.005 × 250 = ±1.25 kPa
2. As % of reading at 50 kPa: 1.25 ÷ 50 = ±2.5% — five times the headline number

Same instrument, same error, wildly different-sounding specs. This is the strongest argument for ranging transmitters close to the operating point rather than leaving factory-wide ranges in place.

Field notes

• Composite specs exist: quality calibrators quote "±(0.01% reading + 0.005% FS)" — evaluate both terms at your point and add. This tool handles each form; sum the pieces.
• The mA equivalent shown in the readout (error × 16 mA ÷ span) is what you'll actually see on a loop calibrator — handy for writing test procedures.
• Turndown eats accuracy: a transmitter with ±0.5% of span used at 5:1 turndown effectively delivers ±2.5% of reading at the bottom of its used range.

Frequently asked questions

What is the difference between % of span and % of reading?

% of span is a fixed error band across the whole range (0.25% of a 0-100 span is always ±0.25 units). % of reading scales with the measured value (0.25% of reading at 20 units is ±0.05, at 100 units ±0.25). At low readings, % of span specs are much worse in relative terms.

How do I convert % of span error to engineering units?

Multiply: absolute error = (%span ÷ 100) × (URV − LRV). A ±0.5% of span error on a 0-250 kPa transmitter is ±1.25 kPa everywhere in the range.

Which specification is better, % of reading or % of span?

For the same number, % of reading is the stronger spec everywhere below full scale. A 1% of reading device at 10% of scale is 10× more accurate than a 1% of span device at the same point. Always check which one a datasheet means.

Provided for reference and education. Verify independently before use in safety-critical work. See our disclaimer.