A reader for a sterilization indicator having sterilizing agent sensitive indicia is described. The method includes a step of providing a threshold signal, which is dependent on a quotient of a first signal and a second signal. The waveguide is aligned with the sensing region such that the waveguide is operable to transmit the radiation towards the aligned sensing region. The present invention achieves increased thermal efficiency through utilization of a regenerative heat exchanger to recapture a portion of the heat energy transferred to the gas during the purification process.