Measurement range is the span of values an instrument can read, such as 400 to 5,000 ppm for a CO₂ monitor or 0 to 600 V for a multimeter. Resolution is the smallest change the instrument can display, such as 1 ppm or 0.001 V. Neither tells you how close the reading is to the truth; that is accuracy, a separate specification. A wide range and fine resolution cost little to print on a box, which is why they are advertised more loudly than accuracy.
Range
Below the bottom of the range the instrument reads its floor or nothing useful; above the top it saturates or shows an over-range warning. For CO₂ monitors the interesting question is the top: 2,000 ppm ranges exist on some cheap units and will clip in a stuffy bedroom; 5,000 ppm covers every ordinary indoor situation; 10,000 ppm and beyond are for greenhouses, breweries and safety. The bottom of the range, often 400 ppm, is set at outdoor air and is where automatic calibration anchors. Accuracy is frequently only guaranteed over part of the range, for example 400 to 2,000 ppm, with a looser figure above.
Resolution
Resolution is a property of the display and the analog-to-digital converter. A CO₂ monitor showing 1 ppm steps has 1 ppm resolution; a hygrometer showing 0.1 percent RH has 0.1 percent resolution; a 6000-count multimeter resolves 1 part in 6000 of its range. Resolution caps precision: you cannot see a 0.5 ppm change on a 1 ppm display. It does not create accuracy: the same 1 ppm display can be 80 ppm from the truth.
Accuracy is the one that matters
Accuracy is stated as a tolerance, typically a fixed part plus a percentage of reading, sometimes different across sub-ranges. It is the only specification that tells you how much to trust the number. See CO₂ monitor accuracy explained and accuracy vs precision.
Examples across instruments
| Instrument | Range | Resolution | Typical accuracy |
|---|---|---|---|
| Consumer NDIR CO₂ monitor | 400 to 5,000 ppm | 1 ppm | ±(50 ppm + 3%) |
| Hygrometer | 0 to 100% RH | 0.1% RH | ±3% RH |
| 6000-count multimeter, 6 V range | 0 to 5.999 V | 0.001 V | ±(0.5% + 2 digits) |
| Laser PM2.5 sensor | 0 to 500 µg/m³ | 1 µg/m³ | ±10 µg/m³ or ±10% |
| Thermal camera spot | -20 to 400 °C | 0.1 °C | ±2 to 3 °C |
In every row the resolution is far finer than the accuracy. The last displayed digit or two is noise.
How to use the three together
- Check the range covers the values you expect, with margin at the top.
- Check the accuracy at the values you care about, not just the best-case figure.
- Treat resolution as a display convenience and ignore digits finer than the accuracy.
A monitor advertising 1 ppm resolution and a 10,000 ppm range but no accuracy figure is telling you nothing useful. The best CO₂ monitors guide lists stated accuracy for every pick for this reason.
Recommended Tools
Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.
Frequently Asked Questions
What range do I need in a CO₂ monitor?
Occupied rooms rarely exceed 3,000 ppm, so a 400 to 5,000 ppm range is enough for homes and offices. Wider ranges matter for greenhouses, fermentation and safety applications.
Is higher resolution better?
Only up to the point where it matches the sensor's noise and accuracy. A 1 ppm display on a ±50 ppm sensor adds nothing except false confidence.
What are counts on a multimeter?
A resolution figure: a 6000-count meter can display up to 5999 in each range, so it resolves 0.001 V on a 6 V range. It is unrelated to accuracy.
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