A metering unit is provided for metering hydrogen peroxide into an evacuated vessel, for controlling the concentration of hydrogen peroxide in the vessel. The metering unit includes a body defining a metering passage having a fixed volume and upstream and downstream ends, an upstream connection for connecting the upstream end to a hydrogen peroxide supply, a downstream connection for connecting the downstream end to a hydrogen peroxide vaporizer, an upstream valve for selectively closing the upstream end and a downstream valve for selectively closing the downstream end, and a controller for operating the valves in a non-overlapping and opposite manner for selectively preventing opening of both valves at the same time. With the metering unit of this disclosure, expensive peroxide concentration measurement systems are replaced with an economical monitoring of the number of injection cycles using a passage of fixed volume.
