Abstract: A portable device for testing the performance of an oxygen-depletion-sensing pilot light jet comprising a gas mixture applicator means, a pressurizable vessel forming a reservoir for containing a gas mixture which has an oxygen content just below the level at which the oxygen-depletion-sensing pilot light should safely operate a gas mixture flow control means or valve means controlling flow of said gas mixture to said applicator means; said gas mixture applicator means being connected to the reservoir by ducting for receiving said gas mixture from the reservoir and to enable supply of said gas mixture as a localized ambient environment to the pilot jet and light.