INTRODUCTION AND SUMMARY Olson Laboratories@ Inc.@ under contract with the Coordinating Research Council (CRC) (Project No. CAPE-22-72) investigated the feasibility of two flow measuring techniques in the measurement of mass mobile source exhaust emissions. These techniques utilized a tracer gas injected into the exhaust stream and an ultrasonic vortex flow meter (UVF). Both techniques were used with conventional exhaust gas analyzers. With the aid of an on-line@ computer-controlled system@ the mass emission results could be computed by integration of the instantaneous values and modal emissions analysis performed. A series of experiments were conducted in studying the characteristics of each method@