Accuracy is how close a reading is to the true value. Precision is how closely repeated readings agree with each other. A CO₂ monitor whose baseline has drifted reads 900 ppm every time in a 1,100 ppm room: precise, inaccurate. A monitor whose readings scatter between 1,050 and 1,150 ppm in the same room is accurate on average but less precise. Datasheets quote accuracy as a tolerance (±50 ppm + 3%) and precision, when they quote it at all, as repeatability or noise.
The target analogy, applied to monitors
- Accurate and precise: readings cluster tightly around the true value. This is what a calibrated NDIR monitor does at steady state.
- Precise, not accurate: readings cluster tightly in the wrong place. A drifted baseline, an uncompensated altitude, a monitor placed by a vent.
- Accurate, not precise: readings scatter widely but average out right. Noisy sensors, very short sampling, a monitor beside a door that keeps opening.
- Neither: a metal-oxide “eCO₂” estimate.
Which specifications describe which
| Specification | Describes | Typical CO₂ figure |
|---|---|---|
| Accuracy / tolerance | Accuracy | ±(30 to 50 ppm + 3 to 5%) |
| Repeatability | Precision | ±10 ppm |
| Noise / RMS noise | Precision | A few ppm |
| Resolution | Display granularity (limits precision) | 1 ppm |
| Long-term drift | Change in accuracy over time | 1 to 2% per year |
| Response time (T63/T90) | Neither; speed | 1 to 3 minutes |
Consumer monitors usually publish accuracy and resolution and skip repeatability. The sensor module datasheet (Senseair, Sensirion, Cubic) has the full set. See CO₂ monitor accuracy explained for reading the tolerance, and measurement range vs resolution.
Which one your decision needs
Trend in one room over time (is the meeting room getting worse through the afternoon?): precision matters, accuracy barely does. A consistent 100 ppm offset does not change the slope.
Comparison against a guideline (are we above 1,000 ppm?): accuracy matters, but the guideline bands are hundreds of ppm apart, so ±100 ppm is fine.
Comparison between monitors or rooms: accuracy matters most, and specifically that every unit has been calibrated in the same reference. See why two CO₂ monitors show different readings.
Research, compliance, commissioning: both, plus documented calibration.
The display can mislead
A reading shown to 1 ppm invites more trust than it earns. Precision to 1 ppm on the display, with ±80 ppm accuracy, means the last two digits are decoration. Humidity monitors that show 0.1 percent RH with ±3 percent accuracy are the same story. Judge a monitor by its stated tolerance and its calibration method, not by how many digits it shows.
How to improve each
- Accuracy: calibrate in a known reference; place the monitor where it reads the room, not a person or a vent.
- Precision: let the monitor warm up; use longer averaging where the app allows; keep it away from doors and drafts.
Recommended Tools
Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.
Frequently Asked Questions
Which matters more, accuracy or precision?
For comparing readings over time in one room, precision. For comparing a reading against a guideline or another monitor, accuracy. Ventilation decisions need modest accuracy and good precision.
Does a stable reading mean the monitor is accurate?
No. Stability shows precision. A monitor with a drifted baseline is perfectly stable and consistently wrong.
What does resolution have to do with it?
Resolution is the smallest step the display can show, such as 1 ppm. It limits precision but says nothing about accuracy; a 1 ppm display can be 200 ppm wrong.
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