Product review (5 stars): Very good CO2 monitor
This is a really terrific device. It worked for me right out the box and within 15 - 20 seconds it was giving CO2 readings. It seems quite sensitive: if I  exhale on to it, the reading change abruptly whereas if I place it in the living or bedroom, the values stay fairly constant.  I have used it in the bedroom at night to see how stuffy the room is if the windows aren't open enough. Very handy device. The only negative is that it has to be plugged into a wall socket so one can't go walking around on battery power to check on readings throughout the house. Am considering getting a second one.