1. Field of Invention
This invention relates generally to gas sensors and methods of detecting gases.
2. Related Art
Gas detection methods based on the thermal conductivity transport property have been widely used because such methods do not show significant selectivity toward a specific gas. Thermal conductivity is defined as the rate at which heat flows through an area of a body or material. Since thermal conductivity is a bulk property of gases, methods based on this property are considered to be near universal or non-specific gas detection methods.
Typically, the thermal conductivity of a gas is measured by heating a resistance element such as a hot wire filament or a thermistor, and contacting the heated element with a gas sample. The temperature of the resistance element is determined by the thermal conductivity of the gas sample, with a change in temperature reflected as a change in resistance of the resistance element. In this method, the resistance element behaves as a thermal conductivity sensing element.
Certain gases, such as helium and hydrogen, have thermal conductivities that are much greater than the thermal conductivity of air, while other gases, such as nitrogen, argon, carbon dioxide, carbon monoxide, ammonia and nitrogen have thermal conductivities that are less than or similar to that of air. A detector such as a gas chromatograph, which measures thermal conductivity of a gas typically uses a carrier gas of high thermal conductivity to detect a specimen gas of low thermal conductivity, or a carrier gas of low thermal conductivity to detect a specimen gas of high thermal conductivity. For example, helium is used as a carrier gas for nitrogen detection, and nitrogen or argon is used as a carrier gas for hydrogen detection. In other thermal conductivity detector applications, specific gas analysis based on thermal conductivity has been limited to either binary gas mixtures of known gas species, or hydrogen gas in a mix of gases having similar thermal conductivities significantly different from hydrogen. Because these methods measure only a single bulk parameter of a gas sample, different gas mixtures having similar thermal conductivities cannot be distinguished.