Measurement uncertainty is the range within which the true value is expected to lie, given everything known about the instrument and the way it was used, stated with a confidence level. A humidity reading of 52 percent with an expanded uncertainty of ±3 percent (k=2) means there is about a 95 percent probability the true value is between 49 and 55 percent. Every measurement has uncertainty; the difference between a laboratory and a consumer device is whether it is stated.
Uncertainty is not error
Error is the difference between a reading and the true value, which you can never know exactly. Uncertainty is your quantified doubt about that difference. A calibrated instrument has small uncertainty even though its exact error is unknown; an uncalibrated one may happen to be accurate but you cannot claim it, so its uncertainty is large. This is why “we checked it against a good one” is worth something and “it seems about right” is not.
Where it comes from
- The sensor: nonlinearity, hysteresis, noise, resolution and drift since calibration. See what sensor drift is.
- The calibration: the reference used has its own uncertainty, which is inherited.
- The environment: temperature effects on the sensor, pressure, humidity, electromagnetic interference.
- The method: placement, settling time, sampling interval, operator reading. See why placement changes measurements.
- The quantity itself: a room does not have a single temperature; the spread across the room is part of the uncertainty of “the room temperature”.
These are combined, usually as the square root of the sum of squares, into a standard uncertainty, then multiplied by a coverage factor (k=2 for about 95 percent) to give the expanded uncertainty quoted on certificates.
How it is stated
| Statement | Meaning |
|---|---|
| 21.4 °C ± 0.3 °C (k=2) | 95% confidence the true value is 21.1 to 21.7 °C |
| Accuracy ±0.5 °C | Manufacturer tolerance; usually a maximum permitted error under stated conditions, without a confidence level |
| Resolution 0.1 °C | Display step; says nothing about uncertainty. See range vs resolution |
Using it against a limit
Suppose a storage room must stay below 25 °C and the logger reads 24.7 °C with ±0.5 °C uncertainty. The true value could be 25.2 °C. Whether that counts as compliant depends on the rule (some require the reading plus uncertainty to be under the limit; some accept the reading alone), but knowing the range prevents a false claim either way. The same logic applies to a humidity reading near 60 percent or a CO₂ reading near 1,000 ppm: a reading within the uncertainty of a threshold is neither clearly over nor clearly under. See accuracy vs precision.
Reducing it
- Calibrate against a better reference and keep the certificate.
- Control the conditions: settle time, placement, shielding from sun and drafts.
- Average repeated readings to shrink the random part (not the systematic part).
- Use an instrument whose range and resolution suit the measurement.
Consumer monitors rarely state uncertainty; the manufacturer’s accuracy figure plus a sensible allowance for placement and drift is the practical substitute. See how to read a calibration certificate for instruments that do.
Recommended Tools
Fluke 117 Digital Multimeter, Non-Contact AC Voltage Detection, Measures Resistance/Continuity/Frequency/Capacitance/Min
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Frequently Asked Questions
Is uncertainty the same as accuracy?
Related but not identical. Accuracy is a manufacturer's tolerance. Uncertainty is a stated range, with a confidence level, around a specific measurement, including calibration and conditions. Uncertainty is the more rigorous concept.
What does k=2 mean on a certificate?
The coverage factor. Multiplying the standard uncertainty by 2 gives a range with about 95 percent confidence. k=1 is about 68 percent.
Why does uncertainty matter if I just want to know if a room is too humid?
Because a reading of 59 percent with ±4 percent uncertainty does not tell you whether the room is under or over a 60 percent limit. Knowing the range prevents false confidence either way.
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