Exhaust gas analyzer
An exhaust gas analyser or exhaust CO analyser is an instrument for the measurement of carbon monoxide among other gases in the exhaust, caused by an incorrect combustion, the Lambda coefficient measurement is the most common.
The principles used for CO sensors (and other types of gas) are infrared gas sensors (NDIR) and chemical gas sensors. carbon monoxide sensors are used to assess the CO amount during an MOT test.[1] In order to be used for such test it must be approved as suitable for use in the scheme. In UK a list of acceptable exhaust gas analysers for use within the MOT test scheme is listed on the Department of Transport website.[2]
Lambda coefficient measurement
The presence of oxygen in the exhaust gases indicates that the combustion of the mixture was not perfect resulting in contaminant gases. Thus measuring the proportion of oxygen in the exhaust gases of these engines can monitor and measure these emissions. This measurement is performed in the MOT test through Lambda coefficient measurement.
The Lambda coefficient (λ) is obtained from the relationship between air and involved in gasoline combustion of the mixture. Is actually a measure of the efficiency of the gasoline engine by measuring the percentage of oxygen in the exhaust.
When gasoline engines operate with a stoichiometric mixture of 14.7: 1 the value of LAMBDA (λ) is "1".
Mixing ratio = weight of the fuel / Weight of air mass
- - Expressed as mass ratio: 14.7 kg of air per 1 kg. of fuel.
- - Expressed as volume ratio: 10,000 litters of air per 1 litter of fuel.
With this relationship theoretically a complete combustion of gasoline is achieved and greenhouse gas emissions would be minimal. The coefficient is defined as Lambda
If Lambda > 1 = poor mixing, excess of air. If Lambda < 1 = rich mixture, excess of gasoline.
- A poor mixture will generate a high content of oxygen in the exhaust and therefore a high content of Nitrogen Oxide.
- A rich mixture will generate a small content of oxygen in the exhaust gases so abound emissions of carbon monoxide and hydrocarbide.
- Carbon dioxide emitted is directly proportional to the fuel consumed.
Chemical CO sensors
- Chemical CO gas sensors with sensitive layers based on polymer- or heteropolysiloxane have the principal advantage of a very low energy consumption, and can be reduced in size to fit into microelectronic-based systems. On the downside, short- and long term drift effects as well as a rather low overall lifetime are major obstacles when compared with the NDIR measurement principle.[3]
- Another method (Henry's Law) can be also be used to measure the amount of dissolved CO in a liquid, if the amount of foreign gases is insignificant.
Nondispersive Infrared (NDIR) CO Sensors
NDIR sensors are spectroscopic sensors to defect CO in a gaseous environment by its characteristic absorption. The key components are an infrared source, a light tube, an interference (wavelength) filter, and an infrared detector. The gas is pumped or diffuses into the light tube, and the electronics measures the absorption of the characteristic wavelength of light. NDIR sensors are most often used for measuring carbon monoxide.[4] The best of these have sensitivities of 20–50 PPM.[4]
Most CO sensors are fully calibrated prior to shipping from the factory. Over time, the zero point of the sensor needs to be calibrated to maintain the long term stability of the sensor.[5] New developments include using microelectromechanical systems to bring down the costs of this sensor and to create smaller devices.Typical NDIR sensors cost in the (US) $100 to $1000 range.
See also
References
- ↑ http://www.cryptontechnology.com/files/290_295%20gas%20analysers%20manual.pdf
- ↑ "List of Acceptable Exhaust Testers". VOSA. Retrieved 9 February 2015.
- ↑ Reliable CO Sensors Based with Silicon-based Polymers on Quartz Microbalance Transducers, R. Zhou, S. Vaihinger, K.E. Geckeler and W. Göpel, Conf.Proc.Eurosensors VII, Budapest (H) (1993); Sensors and Actuators B, 18–19, 1994, 415–420.
- 1 2 Carbonate Based CO Sensors with High Performance, Th. Lang, H.-D. Wiemhöfer and W. Göpel, Conf.Proc.Eurosensors IX, Stockholm (S) (1995); Sensors and Actuators B, 34, 1996, 383–387.
- ↑ http://sstsensing.com/sites/default/files/AN0117_4_CO2SensorAutoCalibrationNote.pdf Co Auto-Calibration Guide]