1. In a gas analysis instrument ofthe type having at least one gas analyzer for measuring the concentration of a component of interest in human breath, and a gas turbine for producing a number of output pulses that varies in accordance with the volume and rate of flow of human breath therethrough, the improvement comprising: (a) first means for storing a piecewise linear approximation of the response of the gas turbine, (b) second means responsive to the number of output pulses produced by the turbine for determining the operating point of the turbine on the stored approximation, and (c) third means responsive to the stored approximation and the output of the second means for providing volume data for a breath.