Abstract: A gas detector apparatus for determining the concentration of a selected one of a plurality of predetermined gases includes a gas sensor which generates a gas output signal related to the concentration of the selected predetermined gas in the gas sample. A K factor storage associated with the gas sensor stores at least one K factor, unique to and generated by using the gas sensor, for each of the plurality of predetermined gases. A processor, in response to a selected gas control signal, selects the K factor or factors from the K factor storage for the selected gas and, in accordance with a predetermined process stored therein, calculates a concentration signal representing the concentration of the selected gas in the gas sample.