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Abstract

A method of measuring external leakage, which comprises the steps of introducing a first gas different from main component gasses of an external atmosphere into a pipe having a leak, adding H.sub.2 gas (or H.sub.2 -containing gas) to the first has at a down-stream point, and measuring gas components entering from the external atmosphere of the leak by an atmospheric ionization mass spectrometer (atmospheric pressure ionization mass spectrometer) at a further downstream point (for example, N.sub.2 H.sup.+ is detected when the external atmosphere is the N.sub.2 gas). A method of measuring the external leakage of a gas supply piping line having a plurality of branch piping lines comprises the steps of introducing H.sub.2 gas (or a H.sub.2 -containing gas) into at least one of the branch piping lines, introducing a first gas different from main component gases of an external atmosphere into the rest of the branch piping lines, and measuring gas components entering from the external atmosphere by the atmospheric pressure ionization mass spectrometer at a downstream point of the branch portion. The present invention can measure the external leakage very simply and with high levels of sensitivity and accuracy. Particularly, the present invention can be suitable applied to the cases where an inspection portion is elongated or branched such as feed pipe of a semiconductor plant.

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First Claim