NOVELTY - A chemical sensor apparatus comprises a sensor material (2) having an intrinsic material property sensitive to a target species (5); a filter structure that allows transfer of the species to sensor material; a measurement device (3) for monitoring the material property to provide a primary sensor response; an energy source (9) for inputting energy to the sensor material; a feedback controller including a comparator (6) for controlling the energy source depending on the primary sensor response; and a control unit (8) for providing a measure of the energy input to the sensor material.    USE - As a chemical sensor apparatus useful in a fire detector and a fire detector system for sensing a chemical (claimed); useful for sensing gases (e.g. hydrogen, oxygen, ozone, water, halogens, acids, nitrogenous gases (e.g. ammonia), hydrocarbon or carbonaceous gases (e.g. carbon monoxide (CO), carbon dioxide (CO2) and methane), organic solvents (e.g. benzene) halocarbons (e.g. carbon tetrafluoride), boronic gases, silicon, germanium, and arsenic gases, sulfurous gases, and metal halides; for sensing toxic gases (e.g. hydrogen chloride, hydrogen fluoride, fluorine, hydrogen cyanide, hydrogen sulfide and CO), combustible gases (e.g. hydrogen, hydrocarbon/carbonaceous gases, gases emitted from petroleum and petroleum derived fuels, vapors of organic solvents (e.g. benzene and xylenes), ammonia, silanes and boranes); for sensing gases emitted by combustion process, products of combusted fuels, products of nitrogen (e.g. nitrogen oxides), combusted products of contaminants in the fuel (e.g. sulfur oxides from coal or diesel containing sulfur); gases associated with a chemical process (e.g. chemical synthesis and semiconductor manufacturing), products of refining petroleum, coal, biomass and solvents in coating processes; gases emitted from bacterial process (such as ethanol, carbon dioxide, hydrogen sulfide, methane, ammonia and acetone), a gas emitter from food processing/cooking/storage, metabolic gases (e.g. CO2) from e.g. humans, mice, dogs and cats; for sensing gas (e.g. oxygen and nitrous oxide) administered during medical treatment; for detecting gases to monitor air quality in vehicle cabins of automobiles, planes and trains, and buildings (e.g. homes, hospitals and laboratories); for detecting chemical warfare agents (e.g. tabun, sarin, soman, mustard-Lewisite, nitrogen mustards, sulfur mustards, hydrogen cyanide, chlorine, chloropicrin and phosgene), its chemical precursor and its decomposition products with oxygen, water, sunlight and biological tissue; and for sensing explosives (e.g. trinitrotoluene) and compounds (e.g. nitrogen oxides) emitted by the explosives.    ADVANTAGE - The feedback control of energy delivery to the gas sensor material allows operation at low power at target gas concentrations below a selectable threshold and elevation of energy inputs above the threshold allowing faster response to target gas exposure and recovery after cessation of exposure. The sensor and mode of operation provides low energy requirements for sensor applications with rare or intermittent exposure to target gases such as in fire detection and security applications.    DETAILED DESCRIPTION - A chemical sensor apparatus comprises a sensor material (2) having an intrinsic material property sensitive to a target species (5); a filter structure that allows transfer of the target species to the sensor material; a measurement device (3) for monitoring the intrinsic material property to provide a primary sensor response; an energy source (9) for inputting energy to the sensor material; a feedback controller including a comparator (6) for controlling the energy source in dependence upon the primary sensor response; and a control/driver unit (8) for providing a measure of the energy input to the sensor material. The measure of the energy input constitutes a secondary sensor response. The energy source comprises heat source or photon source. The apparatus further comprises an indicator for providing an indication that the target species has been detected; a second sensor material and a barrier positioned between the second sensor material isolating it from the target species. INDEPENDENT CLAIMS are included for the following:    (1) a fire detector comprising the chemical sensor apparatus, and optionally at least one other type of fire-detecting sensor (e.g. a temperature sensor and a smoke sensor) and a local signal assessment and control unit; and    (2) a fire detector system comprising several fire detectors and optionally a system assessment and control unit.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a gas sensor system comprising a feedback arrangement.    Sensor structure (1)    Sensitive material (2)    Measurement device (3)    Gas (5)    Comparator (6)    Control unit (8)    Energy source (9)