This chapter describes four subsystems. The four subsystems are: a pitot tube probe or pitobe assembly; a two-module sampling unit; an operating or control unit; and an integrated, modular umbilical cord that connects the sample unit and pitobe to the control unit. The environmental engineer measures the velocity head at various traverse points using the pitot tube assembly. The engineer collects the gas samples at a rate proportional to the stack gas velocity and analyzes them for carbon monoxide (CO), carbon dioxide (CO2), and oxygen (O2). Isokinetic sampling requires precisely adjusting the sampling rate with the aid of the pitot tube manometer readings and nomographs such as APTD-0576 (Rom). The two-module sampling unit has a separate heated compartment and ice-bath compartment. The umbilical cord connects the two-module sampling unit to the control unit, as well as connecting the pitot tube stack velocity signals to the manometers or differential pressure gauges.