Carbon dioxide sensors principle

Up to now, sensors using this parameter have not been taken into consideration, as they are generally not selective. Water for instance is traceable in air because its velocity of sound is significantly higher. The VOS of carbon dioxide is just around 1/3 the VOS of air. In a mixture of air, water and C02 none of the compounds can be quantified. As the VOS of carbon monoxide is similar to that of air, CO cannot be quantifiedby this method either. Hence, sensors based on acoustic principles cannot be taken into consideration neither for the measurement of single species in a flue gas flow nor for the identification of fuel gases. 

The original design was developed specifically for sensing of carbon dioxide (Severinghaus, 1965), but the principle on which these gas sensors operate is general. The key element is a small compartment into which the gas can penetrate through a semipermeable membrane (Fig. 6.28). 

The first gas-sensing electrode based on a similar principle was the carbon dioxide electrode, developed to determine CO2 in blood. Later, sensors for other gases (e.g., SO2, NOx, and HCN, etc.) appeared on the market. 

Carbon dioxide can be assayed via infrared absorption, or electrochemically by measuring the changes in the pH of a buffer solution as a result of varying CO2 partial pressure above the solution. The latter principle can also be applied to optical sensors. 

Carbon dioxide is chemically inactive so that some kind of sensors such as the semiconductor sensor is not adequate to detect CO, because the semiconductor sensor operates on the principle based on chemical reaction of gases at the surface. Most of simple and handy sensors are based on the effect of adsorption, desorption and electrochemical reaction. In Table 7, current status of CO, sensors are summarized together with the requirement of environmental monitoring and conventional analytical equipments[6]. Recently, electrochemical sensors, especially solid electrolyte CO, sensors have been actively studied and developed[56-63]. 

Although it is generally agreed that fiber sensors will greatly extent the range of instrumentation useful for air pollution studies, there is a certain lack of methods that work with real samples. Most papers describe principles rather than applications. Thus, the sensors developed for sulfur dioxide, hydrogen sulHde, carbon monoxide, and other species (as described in Section 17-2) await their application to practical environmental studies. 

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