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Guide

How to Compare CO₂ Monitor Accuracy Specifications

Two monitors quote ±30 ppm + 3% and ±50 ppm + 5%. A third says "±5%". A fourth says nothing. Here is how to turn those into comparable numbers at the levels you care about, and what to look for in the small print.

By the Akermin Editorial Team · 3 min read

To compare CO₂ monitor accuracy, convert each specification to an absolute ppm figure at the levels that matter (800, 1,000 and 2,000 ppm) using the fixed part plus the percentage of reading, then check the conditions under which the figure applies: temperature, the ppm sub-range, and whether it is after calibration. Monitors that state only a percentage, or nothing, should be traced to their sensor module’s datasheet or treated as unverified. For home and office use, any NDIR monitor within about ±100 ppm at 1,000 ppm is adequate; the comparison mostly separates honest specifications from missing ones.

Step 1: Write the spec as fixed + percentage

The standard form is ±(A ppm + B% of reading). Some listings give only one part. If only a percentage appears, the fixed part is probably in the module datasheet; if only a fixed ppm appears, it usually applies at one reference level. See CO₂ monitor accuracy explained.

Step 2: Compute at three levels

Specification At 800 ppm At 1,000 ppm At 2,000 ppm
±(30 ppm + 3%) ±54 ±60 ±90
±(40 ppm + 5%) ±80 ±90 ±140
±(50 ppm + 3%) ±74 ±80 ±110
±(50 ppm + 5%) ±90 ±100 ±150
±5% only (fixed part unknown) ±40 plus unknown ±50 plus unknown ±100 plus unknown

Step 3: Check the conditions

  • Sub-range. Many modules specify the figure for 400 to 2,000 ppm and a looser one above. If you care about stuffy bedrooms at 3,000 ppm, find the second figure.
  • Temperature. Typically stated at 25 °C; some add a temperature coefficient. Cold rooms and warm devices both shift readings.
  • Pressure. Modules with a pressure sensor compensate; those without read low at altitude.
  • After calibration. The spec applies to a freshly calibrated sensor. Add drift for a monitor that has not been calibrated in a year; see what sensor drift is.

Step 4: Look for the other figures

  • Repeatability (±10 to 20 ppm typical): how steady the reading is; matters for trends. See what repeatability is.
  • Long-term drift (1 to 2% per year typical): matters for monitors that will not be calibrated.
  • Response time (T63 or T90, 1 to 3 minutes): matters for alerts. See how to check CO₂ monitor response time.

Step 5: Trace the module

Consumer monitors rarely publish more than the accuracy line. The sensor module inside does: Sensirion SCD30 (±(30 ppm + 3%)), SCD40 (±(50 ppm + 5%)), SCD41 (±(40 ppm + 5%)), Senseair S8 (±(40 ppm + 3%)), Cubic CM1106 (±(50 ppm + 5%)) and so on. If the monitor names its module, the datasheet is the authoritative spec. If it names none, the stated accuracy is a claim without a source.

Worked comparison

Monitor Stated accuracy At 1,000 ppm Module named? Verdict
Aranet4 Home ±(30 ppm + 3%) ±60 ppm NDIR, Senseair Tightest consumer spec
GoveeLife H5140 ±(40 ppm + 5%) ±90 ppm Sensirion SCD4x Good, traceable
SwitchBot Meter Pro CO₂ ±(50 ppm + 5%) ±100 ppm Sensirion, per maker Adequate, traceable
Unnamed “10-in-1” device “High precision” Unknown No Unverified; likely eCO₂

How much accuracy you need

The action thresholds (800, 1,000, 1,500 ppm) are far enough apart that ±100 ppm does not change a decision. Spend the comparison effort on placement and calibration instead; see how to place a CO₂ monitor. Monitors with stated, traceable specs are compared in best CO₂ monitors for home and office.

Aranet4 Indoor Air Quality Monitor, Wireless CO2 Detector for Home, Office

Aranet4 Indoor Air Quality Monitor, Wireless CO2 Detector for Home,

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GoveeLife Smart Air Quality Monitor CO2 Detector H5140, Display & Alarm

GoveeLife Smart Air Quality Monitor CO2 Detector H5140, Display &

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SwitchBot CO2 Detector, Air Quality Monitor Indoor, CO2 Monitor for CO2/TEMP/Humidity, Portable Carbon Dioxide Detector for Home/Car/Office/Greenhouse

SwitchBot CO2 Detector, Air Quality Monitor Indoor, CO2 Monitor for

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Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.

Frequently Asked Questions

Which is better, ±(30 ppm + 3%) or ±(50 ppm + 5%)?

The first, at every level. At 1,000 ppm it is ±60 versus ±100 ppm; at 2,000 ppm ±90 versus ±150 ppm.

What if a monitor only states a percentage?

Assume a fixed term is hidden. Look up the sensor module's datasheet, which will state both parts. If the module is unnamed, treat the spec as unverified.

Does a better accuracy spec matter for home use?

Rarely. Decision thresholds are hundreds of ppm apart. Placement and calibration cause larger errors than the difference between ±60 and ±100 ppm.

Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.