Where you permanently mount a carbon dioxide transmitter depends on why you are monitoring indoor air quality. There are 3 places it can be mounted:
Indoor Air Quality: In most homes or offices, wall-mounted CO2 level transmitters are used to monitor carbon dioxide levels as they would be experienced by occupants. They can be located at the same height (48 inches or 122cm is standard) and on the same wall as you would mount a thermostat. Like a thermostat, they should not be near outside doors or windows that can make the space appear to have more fresh air than it actually does.
Commercial HVAC Control: As an alternative to wall mounting, commercial HVAC contractors will use duct-mounted CO2 transmitters. By sampling the air from the return air ducts, they are assured of getting the most consistent average air quality for each zone in the building.
Safety: Wherever compressed CO2 is stored or created, CO2 safety alarms should be mounted 18 inches (45cm) off the floor. Because carbon dioxide is heavier than air, it will first pool at floor level, then fill the enclosed space. The rate at which the space is filled is dependent on the size of the leak, but in a small room CO2 levels can become life-threatening very quickly.