A packaging machine for producing closed packages includes a chamber that can be sealed in an airtight manner around one or a plurality of packages, a gas flushing unit for flushing with a gas the at least one package contained in the chamber, at least one gas line leading into the chamber or out of the chamber as well as a measuring device for determining a concentration of the gas. The measuring device comprises a measuring head for reading, through the use of electromagnetic radiation, an indicator for the concentration of the gas. The indicator is arranged in an interior of the chamber, in an interior of a package and/or in an interior the at least one gas line.