Guide · measure
Choosing a CO₂ monitor: NDIR or nothing
A CO₂ monitor is the single most clarifying $60–$250 you can spend on indoor air — if it contains the right sensor. Many cheap "air quality monitors" don't measure CO₂ at all.
The one spec that matters
NDIR (non-dispersive infrared) sensors measure CO₂ physically, by how much infrared light the gas absorbs. The trap: many gadgets list "eCO₂" — a number estimated from a VOC sensor. Open a marker near an eCO₂ device and watch "CO₂" spike: it's guessing from unrelated chemistry. If the listing doesn't say NDIR (or "dual-beam NDIR"), it's not a CO₂ monitor.
Worth having; skippable
- Logging / history. The overnight bedroom curve and the 3 pm office peak are the two most useful charts you'll ever make; you need memory, not a live number, for both.
- Battery + size, if you'll carry it between rooms and meetings — portability is how one unit audits a whole life.
- Calibration you control. Most sensors drift; good ones recalibrate against outdoor air (~420 ppm). Auto-calibration (ABC) assumes the room regularly reaches outdoor levels — fine for living rooms, wrong for a sealed office that never gets there.
- Skippable: apps, clouds, subscriptions, "AI insights." The number is the product.
Editorial examples (unpaid, no links)
To make the classes concrete: the Aranet4 is the well-known premium portable (logging, e-ink, years on a battery); Qingping's monitors are the mid-range with screens and logging; AirGradient sells respected open-source kits; and there are $60-class NDIR units from various brands that do the job with fewer comforts. We name these as examples of their categories, not endorsements; nobody paid, nobody reviewed this text.
The sensor types, in one table
| Listing says | What it is | Verdict |
|---|---|---|
| NDIR (dual-beam) | Infrared absorption, self-correcting reference | Best — buy this |
| NDIR (single-beam) | Same physics, drifts more between calibrations | Fine with occasional recalibration |
| Photoacoustic (PAS) | Newer optical method, compact | OK; more sensitive to pressure/humidity swings |
| "eCO₂" / "CO₂ equivalent" | A guess derived from a VOC sensor | Not a CO₂ measurement — avoid |
Calibrate in three steps
- Put the monitor outside (shaded, sheltered) or on a windowsill with the window wide open, 10–15 minutes.
- It should read roughly 420–450. If it's off by more than ~75, run the manual calibration routine (buttons or app) while it's still in that air.
- Repeat every few months, or when readings drift suspiciously low — an ABC unit that lives in a busy room "learns" the wrong baseline.
Using it well in one week
- Night 1: nightstand, door as usual. Read the morning curve against the scale.
- Night 2: door ajar. Compare. That's your headache experiment, done.
- Workdays: desk, then one meeting room. Numbers over 1,200 by mid-meeting? Now you have evidence, not vibes.
- Weekend: kitchen while cooking — and notice PM is the cooking story, which is the purifier's department, not this device's.
Placement: breathing height, not directly in front of a face, window or vent. And remember what this instrument is: a ventilation meter. For smoke and particles you'd pair it with a PM2.5 monitor — that's the AQI page's world.