Flame Detectors Carbon Monoxide Detectors Carbon monoxide is a colorless, odorless gas that is a  byproduct of incomplete combustion. Its presence in the  breathing air of human beings can be deadly. To ensure crew  There are several types of carbon monoxide detectors.  Electronic detectors are common. Some are panel mounted  and others are portable. Chemical color-change types are  also common. These are mostly portable. Some are simple  buttons, cards, or badges that have a chemical applied to  the surface. Normally, the color of the chemical is tan. In  the presence of carbon monoxide, the chemical darkens to  grey or even black. The transition time required to change  color is inversely related to the concentration of CO present.  At 50 parts per million, the indication is apparent within  15 to 30 minutes. A concentration of 100 parts per million  changes the color of the chemical in as little as 2–5 minutes.  As concentration increases or duration of exposure is  prolonged, the color evolves from grey to dark grey to black.  If contaminated, installing a new indicating element allows a  carbon monoxide portable test unit to be returned to service. When radiation emitted by the fire crosses the airspace  between the fire and the detector, it impinges on the detector  front face and window. The window allows a broad spectrum  of radiation to pass into the detector where it strikes the  sensing device filter. The filter allows only radiation in a  tight waveband centered on 4.3 micrometers in the IR band  to pass on to the radiation-sensitive surface of the sensing  device. The radiation striking the sensing device minutely  raises its temperature causing small thermoelectric voltages  to be generated. These voltages are fed to an amplifier whose  output is connected to various analytical electronic processing  circuits. The processing electronics are tailored exactly to the  time signature of all known hydrocarbon flame sources and  ignores false alarm sources, such as incandescent lights and  sunlight. Alarm sensitivity level is accurately controlled by  a digital circuit.  [Figure 17-9] Optical sensors, often referred to as flame detectors,  are designed to alarm when they detect the presence of  prominent, specific radiation emissions from hydrocarbon  flames. The two types of optical sensors available are infrared  (IR) and ultraviolet (UV), based on the specific emission  wavelengths that they are designed to detect. IR-based optical  flame detectors are used primarily on light turboprop aircraft  and helicopter engines. These sensors have proven to be very  dependable and economical for these applications. and passenger safety, carbon monoxide detectors are used  in aircraft cabins and cockpits. They are most often found  on reciprocating engine aircraft with exhaust shroud heaters  and on aircraft equipped with a combustion heater. Turbine  bleed air, when used for heating the cabin, is tapped off of  the engine upstream of the combustion chamber. Therefore,  no threat of carbon monoxide presence is posed. Carbon monoxide gas is found in varying degrees in all smoke  and fumes of burning carbonaceous substances. Exceedingly  small amounts of the gas are dangerous if inhaled. A  concentration of as little as 2 parts in 10,000 may produce  headache, mental dullness, and physical lethargy within a  few hours. Prolonged exposure or higher concentrations  may cause death. 17-9 Figure 17-9.  Infrared (IR) based optical flame detector. − + − + − + − + Duflation Dischminator Flicker rate Dischminator Fire Counter Clock 4.3 Micrometer sensor F +12V +12V +12V GND GND Signal Threshold Voltage F 2.55