What “accuracy” means on a transmitter datasheet
The headline accuracy of a pressure transmitter, often 0.075 % or 0.04 %, is its reference accuracy: the error at constant laboratory temperature, shortly after calibration. In the field two more contributions appear:
- Temperature effect. Changes in ambient temperature shift zero and span. Datasheets quote it per 10 K, per 28 °C (50 °F) or as a band over the whole range.
- Long-term stability (drift). The output changes slowly over months and years. Datasheets quote it per year or over five or ten years.
The calculator combines these into the total probable error (TPE) for your calibrated range.
Span versus URL
The key to reading accuracy specifications is what each percentage refers to:
- Reference accuracy is usually a percentage of the calibrated span: the range between the 4 mA and 20 mA values.
- Temperature effect and stability are usually a percentage of the upper range limit (URL): the largest range the sensor can be set to.
Turning a 100 bar sensor down to 0–40 bar keeps the reference error small in proportion to the span, but the URL-based errors stay the same in bar, so they grow as a percentage of the span. The ratio URL / span is the turndown.
How the errors are combined
The three contributions are independent, so the usual approach is the root-sum-square:
TPE = √(E_ref² + E_temp² + E_stab²)
The calculator also gives the worst case, the simple sum of all three, which is the conservative bound if every error appears at full size and in the same direction.
Worked example
A transmitter with a 100 bar URL is calibrated 0 to 40 bar. Its datasheet gives a reference accuracy of 0.075 % of span, a temperature effect of 0.1 % of URL per 10 K and a stability of 0.1 % of URL per year. The ambient changes by 20 °C, and it was calibrated one year ago.
- Reference accuracy: 0.075 % × 40 bar = ±0.03 bar
- Temperature effect: 0.1 % × 100 bar × 20/10 = ±0.2 bar
- Stability: 0.1 % × 100 bar × 1 = ±0.1 bar
- Total probable error: √(0.03² + 0.2² + 0.1²) = ±0.2256 bar, or ±0.564 % of span
- Worst case: 0.03 + 0.2 + 0.1 = ±0.33 bar (0.825 % of span)
- Turndown: 2.5 : 1
- Equivalent loop current error: ±0.0902 mA
The stacked bar under the result shows each contribution’s share. Here the temperature effect is almost 80 % of the total: improving it, for example with a transmitter that has a better temperature specification or with a more stable installation, helps far more than a better reference accuracy.
Converting datasheet formats
Datasheets phrase the same information in many ways:
- “±(0.025 % URL + 0.125 % span) per 28 °C”: split it. Convert the URL part to per 10 K (× 10/28) and enter it as the temperature effect; add the span part, scaled the same way, to the reference accuracy, or calculate it by hand.
- “0.1 % of URL per 5 years”: enter 0.02 % per year.
- Total performance figures: some manufacturers publish a combined “total performance” figure that already includes reference accuracy and temperature effect for a given ambient change. Don’t add the components again.
Static-pressure effects on differential-pressure transmitters are not included. They can be significant for low differential ranges at high line pressure.
Common mistakes
Comparing transmitters on reference accuracy alone. In the example, reference accuracy is the smallest contribution.
Ignoring turndown. A 0.075 % transmitter with a 400 bar URL set to 0–10 bar is a 40:1 turndown; URL-based errors dominate.
Assuming calibration resets everything. Recalibration removes accumulated drift but not the temperature effect.
Frequently asked questions
Which value should I use for ambient change?
The difference between the calibration temperature and the extreme ambient the transmitter will see in service, for example 20 °C calibration and 40 °C in summer: 20 °C.
Does the unit field matter?
Only for display. Enter the unit of your range, such as bar, psi or kPa; all errors are shown in that unit.
How do I convert the result to 4–20 mA?
The calculator shows the error in milliamps. For scaling between current and pressure, use the 4–20 mA converter.
What range should I choose for a new transmitter?
Choose the smallest URL that comfortably covers the maximum process pressure, so the turndown stays low.