US Patent 7752887 Detection of impurities in gas streams

A method of measuring the level of an impurity in a gas comprises providing a gas detection tube having a internal volume containing a reagent which can react with said impurity to produce a compound capable of producing a color change in said tube such that the extent of the color change is proportional to the concentration of the impurity in said gas. A volume of the gas at least 1×103 times the test volume of the gas detection tube is passed through the tube and the extent of the color change of the detector element is measured.