(57) A portable device for testing the performance of an oxygen-depletion-sensing pilot light jet comprising 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 in communication or communicable with gas mixture applicator means; said applicator means in a first arrangement permitting said gas mixture to flow from said flow control means to atmosphere at ambient pressure and said applicator means including a supply duct locatable adjacent and/or over the air inlet aperture of an oxygen-depletion-sensing pilot light jet to supply said gas mixture thereto at ambient atmospheric pressure, orinasecondarrangement said applicator means comprising a housing locatable to surround an oxygen-depletion-sensing pilot light jet and the air inlet aperture therefor and preferably also the ignition means for the pilot light, to enable supply said gas mixture to the air inlet aperture and to provide said gas mixture as a localised ambient environment to the pilot light.