An apparatus is described for calibrating at least one sensor within a
gas detector (22), which
detector has a gas inlet in fluid communication with the or each sensor, the apparatus comprising a housing (10) that contains: a) a surface (24) for abutting against the
detector (22); b) a holder (14) for holding the
gas detector (22) with respect to the housing (10) in such a manner that a region of the
detector containing the gas inlet abuts against the surface (24) of the housing to form a sealed gas interface between the surface (24) and the detector (22); c) a connector for connecting a source of pressurised
calibration gas (12) to the apparatus, d) a conduit (32) for delivering a
calibration gas from the connector (34) to the interface between the detector and the apparatus housing, e) electrical connections (40) within the holder for forming electrical connections between the apparatus and a detector held within the holder, and e) a flow controller (34, 36) within the conduit for providing
calibration gas at a predetermined level to the interface, the flow controller including an electrically-operated valve (36) being controllable for initiating and terminating the flow of calibration gas through the conduit (32) by means of signals received from the detector via the electrical connections (40). The whole calibration is controlled by he detector and not the calibration apparatus.