Gas analyzer with background gas compensation

Abstract

A sensor responds to a magnetic wind produced by the magnetic susceptibility of a target component gas, such as oxygen, which is present together with a background gas in a sample located in a measurement field produced in a measurement device. A background sampler also determines thermal and physical characteristics in a conditioned flow path, and a processor performs a computational correction on the detected magnetic wind signal to more accurately determine the concentration of the target gas. The background sampler includes a mass flow cell in series with a laminar flow device to measure heat capacity and viscous properties of the sample. It derives the gas viscosity from a differential pressure measurement; and preferably determines the heat capacity and thermal conductivity of background gases, the composition of which may be unknown, to compensate for their effects on the magnetic wind sensor elements. A further correction for diamagnetism of the background is made using one or more correlations between the measured parameters in certain common gases or classes of gases. The determination of gas bulk properties allows the concentration of the target gas to be made with enhanced accuracy in the presence of unknown, unquantified or changing background components, and the necessary hardware may be used in conjunction with existing magnetic wind sensor and processor. In that case, the processor program is modified to apply the additional sensor inputs to correct the magnetic wind reading. A thermal conductivity correction is also made, and this may be directly derived from the bridge control signal when operating the magnetic wind sensors in mode to maintain constant temperature. The system may be used in petroleum tanks and hydrocarbon vent systems to monitor for explosive levels of oxygen.