Claire Airbox CO2 meter at EUR 59.95: the margin at 1,200 ppm is 110

The moment the heating season starts, a Dutch house enters a contradiction: closing the windows saves gas, but carbon dioxide slowly builds up in the living room. The Claire Airbox turns that into a number. It is EUR 59.95 on bol.com, rated 4.2 from 146 reviews, delivered next day if ordered before 23:59. The unit is 10.2 x 8.5 x 3.5 cm, can stand or hang, measures with an NDIR sensor, carries a 2400 mAh battery, ships with an adapter, shows a red-amber-green indicator and lets you set your own ventilation alarm. It is a Dutch brand with a Dutch manual.

The most useful line on the listing is its tolerance: a CO2 range of 400 to 6000 ppm at plus or minus 50 ppm and 5 percent. Add those two parts together to get the real error at any given reading. Take 1,200 ppm: 50 plus 5 percent of 1,200, which is 60, gives plus or minus 110 ppm, so the moment the screen says 1,150 the true value may already be 1,260. Set your alarm 110 below the number you actually care about rather than on it, and that single step is the difference between owning a meter and owning a number.

• CO2 range 400 to 6000 ppm at plus or minus 50 ppm and 5 percent; temperature -10 to 40 C at plus or minus 1 C; humidity 20 to 80 percent RH at plus or minus 5 percent RH. (source 1)
• A self-calibrating NDIR sensor, a red-amber-green indicator, a ventilation alarm you set yourself, and adjustable backlight brightness. (source 1)
• A built-in 2400 mAh battery for places with no socket, an adapter in the box, and a 10.2 x 8.5 x 3.5 cm body that stands or hangs. (source 1)
• EUR 59.95, rated 4.2 from 146 reviews: 91 five-star, 25 four-star, 10 three-star, 7 two-star and 13 one-star. (source 1)

The second check you can run yourself comes out of the words self-calibrating. A sensor like this calibrates by treating the lowest value it has seen over a period as outdoor air, and the listing states that its range bottoms out at 400 ppm. So take a baseline before it settles indoors: leave it on a balcony or an open windowsill for a while, and the reading should land a little above 400. Whatever the gap is, that is the systematic offset sitting on every indoor reading afterwards. A meter that never sees outdoor air drifts, and the screen will not say so.

The reviews deserve an honest line: 13 of the 146 are one star, roughly one in eleven, against 91 five-star ratings. That is a split distribution, not a uniform chorus. It also answers a different question from a hygrometer. A hygrometer tells you whether to dehumidify; a CO2 meter tells you whether to open a window, and in the heating season opening a window has a price. Read the two together and you know how long that window stays open and when to shut it.

Price

EUR 59.95 including VAT (bol.com, checked 10 September 2026)

Rating

4.2 from 146 reviews (bol.com)

Specs

Self-calibrating NDIR; CO2 400 to 6000 ppm at plus or minus 50 ppm and 5 percent; temperature -10 to 40 C at plus or minus 1 C; humidity 20 to 80 percent RH at plus or minus 5 percent RH; 2400 mAh battery; 10.2 x 8.5 x 3.5 cm

Platform

bol.com (affiliate link)

Checked

10 September 2026, on the bol.com listing and the bol catalogue interface (EAN 8720256916239)

Hokimi field note: measure the bedroom first. Shut the door at night and look at the number the moment you wake up. The biggest gap is usually the bedroom rather than the living room, and the bedroom is usually the one room a gap under the door can fix.