Abstract: A vehicular ignition interlock system having a flow rate integrator and a servograph. The integrator is provided to insure that only "deep lung air" is analyzed for alcohol. A pressure transducer is employed in the integrator such that the output of the transducer is proportional to the air flow rate. As the rate of flow of a breath sample varies, a corresponding change in the required duration of the breath sample is made to insure that a deep lung air sample is being analyzed. The servograph comprises a plurality of colored lights and a tone generator which are controlled by a computer to direct the operator to adjust the rate of flow of his or her breath sample in accordance with the visual indication provided by the lights. Thus the servograph and the flow rate integrator insure that only a human subject will be capable of providing the air sample to be analyzed. A gas sensor and flow sensor are mounted in a replaceable module to facilitate field repair, replacement and calibration.