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Early carbon monoxide detectors had sensors with a white or beige color element that would fade to a brownish or blackish color if carbon monoxide was detected. Today, CO detectors use alarms.
CO alarms differ from smoke detector alarms in that they incorporate a concentration-time function. At lower concentrations (100 parts per million), a detector will not sound an alarm for up to an hour. At 400 ppm, the alarm will sound within a few minutes. This concentration-time function is intended to mimic the uptake of carbon monoxide in the body while also preventing false alarms due to common sources of carbon monoxide such as cigarette smoke.
The types of sensors available vary in cost, accuracy, and speed of response. Any of the three are basically adequate, given the level of your concern, because CO levels normally increase very slowly, and there is a large range between safe and unsafe levels.
1. Biomimetic. A biomimetic sensor (chem-optical or gel cell) works with a form of synthetic hemoglobin that darkens in the presence of CO and lightens without it. You can either see the colors directly or have them connected to a light sensor and alarm.
2. Electrochemical. This type of fuel cell is designed to produce a current that is precisely related to the amount of carbon monoxide in the atmosphere. Its advantages over other technologies are that it is highly accurate in its readings of carbon monoxide, requires minimal power, and has a five-year life.
3. Semiconductor. This sensor has thin wires of the semiconductor tin dioxide on an insulating ceramic base, which is monitored by an integrated circuit. The carbon monoxide reduces resistance and by doing so allows a current high enough to trigger the alarm.
