A CO2 meter only gives a reliable picture of the air quality if you place it in the right spot. If you place it incorrectly, you will mostly measure random fluctuations: a draft from outside, the breath of someone walking past, or the airflow from a fan. A few simple rules can prevent this.
Measure at breathing height, not on the floor or high up on the wall
CO2 exhaled by people mixes with the ambient air, but the concentration is most representative at a height of about 1 to 1.5 metres, roughly the height at which you sit or stand. So place the meter on a cabinet, bookshelf or desk at that height, not on the floor (where colder, sometimes less mixed air lingers) and not right against the ceiling.
Keep it away from windows, doors and ventilation grilles
A meter placed right next to a gap or ventilation grille will mostly measure the incoming fresh air, giving an overly optimistic picture of the rest of the room. Place the meter about one or two metres away from windows, doors and grilles, somewhere in the middle of the room or where you spend most of your time.
Do not place it directly in the airflow from a fan or heater
If a fan or heater blows directly over the meter, the air will be mixed locally, giving you a snapshot that is not representative of the entire room. Place the meter outside the direct airflow from devices such as a fan or heater.
The bedroom deserves a permanent spot
In the bedroom, the CO2 level is most important at night, because the room is usually closed up then and very little fresh air enters. Place the meter on the bedside table or a shelf at breathing height, not directly next to the head of the bed (otherwise you will mostly measure the air you have just exhaled) and not right next to a window that is slightly ajar.
One meter per living space is enough to start with
You do not need to place a meter in every room right away. Start with the rooms where you spend the most hours and sleep: the living room, bedroom and possibly a home office or children's room. If you consistently see high readings there, you will know that ventilating those rooms is a priority.
What a CO2 meter can and cannot do for you
A CO2 meter only measures; it does not ventilate. A high reading is a signal to open a window or ventilation grille, not something that a device will solve for you. Dynter does not sell CO2 meters itself, so this is honest and independent advice: it is mainly about choosing a good spot and then actually taking action based on the reading you see.
If you want to address other aspects of your indoor air besides CO2, such as fine dust, pollen or household dust, an air purifier with a HEPA filter can help (note: this is a different problem than CO2; an air purifier does not refresh the air). For that purpose, for example, check out the AirPure Plus PA244, or visit dynter.eu to see the full range.