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In most commercial airliners, the air that is used to pressurize the cabin and provide safe, breathable air to the passengers comes through the bleed-air system which draws compressed air from the engines of the aircraft. However, contamination of this air supply from gases like carbon monoxide (CO) is a potential problem which might compromise the safety of the passengers and crew; therefore, the Federal Aviation Administration has set a maximum allowable concentration of CO gas in the cabin. There are ongoing efforts to find reliable sensors that are capable of accurately measuring the amounts of CO onboard aircraft. In this study, a total of ten commercially available, electrochemical CO sensors and sensor packages have been selected for testing in an environmentally sealed chamber where the total pressure and CO gas concentration could be controlled. The sensors were evaluated at total pressures of 101.3 kPa (1 atm), 87.5 kPa (4000 feet altitude equivalent), and 75.3 kPa (8000 feet altitude equivalent) which are a selection of the pressures typically encountered during the various stages of commercial airline flight. In addition, the effect of molecular oxygen on the sensors' performance was evaluated. The concentrations of CO used in this study ranged from 18 to 250 ppm CO and were supplied from pre-mixed tanks with the balance of the gas being nitrogen. The concentration of the CO inside the sensor chamber was monitored by a Perkin-Elmer Spectrum GX Fourier Transform Infrared spectrometer (FTIR). A review of the performance of the commercial CO sensors in this environment is presented.Nomenclature C CO (t) = concentration of carbon monoxide inside the sensor evaluation chamber at time, t (ppm) C FTIR (t) = concentration of carbon monoxide inside the FTIR gas analysis chamber at time, t (ppm) CO = carbon monoxide C s (mg/m 3 ) = Source carbon monoxide concentration (mg CO/m 3 ) C s (ppm) = Source carbon monoxide concentration (ppm CO) FAA = Federal Aviation Administration F i = flow rate of carbon monoxide test gas entering the sensor evaluation system (cm 3 /min) F leak = leak rate of room air into the sensor evaluation system (cm 3 /min) F o = flow rate of the gas mixture leaving the sensor evaluation system (cm 3 /min) FTIR = Fourier Transform Infrared spectrometer/spectroscopy M = molar mass of carbon monoxide (28.0101 g/mol) P = atmospheric pressure (101.3 kPa) PPM = parts per million P sys = pressure inside the sensor evaluation system (kPa) R = universal gas constant (8.3145 m 3 ·Pa/K·mol) SCCM = standard cubic centimeter per minute 2 TCP = tricresyl phosphate T = room temperature (293 K) t = duration of carbon monoxide test gas flow into the sensor evaluation system (min) Vol chamber = Volume of the sensor evaluation chamber (cm 3 ) Vol FTIR = Volume of the FTIR gas analysis chamber (cm 3 )