Dimlux co2 sensor

[THE DARK KNIGHT]

So I “auto calibrated” it, taking it outside for 10 minutes and then calibrating it to 400. Now it’s been in my room and is showing that co2 levels are around 1300, haven’t seen it below 1200 for a week.

I haven’t turned the co2 on yet because of these readings. Im in a sealed room with no extraction and these readings have been during lights on I keep forgetting to check during lights off. I’m assuming this is wrong as I’m meant to need to supplement co2 the room. I was of the understanding a sealed room would have less co2 than outside of a regular room due to the plants producing oxygen in the day.

have I done something wrong or what? And I’ve also taken the readings as soon as I’ve gone in the room and literally held my breath near the sensor taking the reading so it’s not high because I’ve been in there.