A constant volume sampler (CVS) with a constant-area critical flow venturi (CFV) has been used widely for measuring auto emissions. The system is suitable for estimation of mass emissions with prevention of the condensation of water in the sampled exhaust gas. By using a CVS system with a variable-area CFV, the flow rate of diluted exhaust gas can be to adjusted so as to accommodate each driving mode. A new-type variable-area CFV has been developed so that the flow rate can be varied continuously within the range of 2-15 m3/min according to the rotation of the stepping motor for positioning the venturi. This new system will be able to minimize the dilution ratio of the sample gas.
