Only recently has there been any popular demand for Carbon Dioxide detectors. They have been primarily the concern of those who monitor industrial environments and public building safety. Home carbon dioxide detectors (not to be confused with carbon monoxide detectors) are a relatively new phenomenon.
Carbon monoxide (CO) is a gas produced when fuel is burned in the absence of oxygen. Carbon dioxide (CO2) is exhaled by animals and absorbed by plants. Plants in turn give off oxygen to complete the cycle. But additional CO2 is emitted when fossil fuels such as natural gas and oil are burned.
As the environment becomes more and more overloaded with people who burn fossil fuels and cut down forests, the natural balance of the carbon cycle is changed. At present the average concentration of CO2 in the earth's atmosphere is about 375 ppm (parts per million). This is not a dangerous level, nor an unprecedented level in the history of the earth. However, since CO2 is a greenhouse gas, increasing its concentration also increases the warmth of the atmosphere. Hence, the growing concern about CO2 levels. Indoor areas, if not well ventilated can have much higher concentrations of CO2. At 1000 ppm some people begin to feel discomfort, but at 2000 ppm most people will be uncomfortable. "Sick building syndrome" has a high correlation with increased levels of CO2. The cure usually includes better ventilation.
In closed environments, especially industrial environments where fuels are used, CO2 levels can become dangerously elevated. Dry ice is frozen CO2, so industries that use a lot of dry ice might also need CO2 monitors. In coal mines the miners have traditionally used canaries to monitor the level of CO2. When the canary dies the miners get ready to leave. But there are more sophisticated instruments for CO2 detection. The "Testo Model 535 Carbon Dioxide Monitor" is a hand-held monitor which detects levels from 0 to 9999 ppm and has a dual probe to measure airflow through both an incoming and out-going duct. It's cost is about $730 U.S. Dollars.
Green houses use CO2 monitors in order to assure that plants are receiving enough CO2 to carry on photosynthesis. Often these monitors are built to trigger a CO2 enrichment device when levels drop below that which is optimal for plant growth. However Green Air Products does produce a "Portable Carbon Dioxide Monitor Model CDM-7001" which is hand held and would be well suited for a variety of applications. It also gives a temperature reading and is able to register a CO2 level within 30 seconds. It has a data port that can output data to a windows based application. This monitor operates 70 hours on 4 AA batteries. The cost is about $925 US dollars. GE Telaire makes a device that looks identical in every aspect and sells for about $425.
On the low end of the scale GE Telaire makes a wall mounted unit marketed as "Ventostat 8000 Series" which measures up to just 2000 ppm and claims an accuracy of plus or minus 100 ppm. It sells for less than $200 US dollars but it's specifications do not measure up to any of the other monitors mentioned here. If it can be trusted in could at least give some warning that you should get out of the building!
For about $1800 US dollars you can get a hand held sensor that monitors a dozen different air quality indicators including CO2. The "BW GasAlertMicro 5 IR" is a hand held monitor with an audible, visual or vibrator alarm. Other features include a full function self test capability, automatic calibration and pass-code protection.
For twice that amount you can purchase an instrument that monitors for combustibles, carbon monoxide, oxygen, and carbon dioxide. The "K103Q-BM25 Multiguard sensor" has a 103 decibel alarm and a bright red visual alarm beacon which can be seen from all directions. Weighing about 15 pounds, it operates up to 170 hours on a battery pack, or it can be connected to a 110 or 22O volt power supply. It includes two outputs for connection to remote alarms which can communicate danger to more distant locations. This sensor has a manual alarm activation feature so it can be used to signal distress to a remote location. It looks like a great product in spite of its formidable price tag!
It would appear that these are the best of the CO2 monitors that are be appropriate for diverse applications, and that most are made for industrial use. As our awareness of increase CO2 concentration grows, it is likely that more affordable units with good features will become available.