The procurement of representative samples of particulates suspended in gas streams demands that the velocity at the entrance to the sampling probe be precisely equal to the stream velocity at that point. This is accomplished by regulating the rate of sample withdrawal so that the static pressure within the probe is equal to the static pressure in the fluid stream at the point of sampling. The calibrated orifice and inclined manometer indicate the instantaneous sampling rate. The specific points of stack for sampling are selected to ensure that the samples collected are representative of the material to be discharged or controlled. The stack-sampling components are selected on the basis of the source to be sampled, the substances involved, and the data needed. The procedural outlines serve as a starting point in assembling a stack-sampling system, after consideration has been given to the complications that might arise because of the presence of interfering substances in the gas samples.