Technical Procedures - USEPA Method 1A

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Sample and Velocity Traverses for Stationary Sources With Small Stacks or Ducts – USEPA Method 1A

USEPA Method 1 is used to provide guidance for the selection of sampling ports and traverse points at which sampling for air pollutants are performed. A measurement site where the effluent stream is flowing in a known direction is selected, and the cross-section of the stack is divided into a number of equal areas. Traverse points are then located within each of these equal areas.

This method is applicable to flowing gas streams in ducts, stacks, and flues of less than about O.30 meter (12 in.) in diameter, or 0.071 m2 (113 in.2) in cross-sectional area, but equal to or greater than about O.10 meter (4 in.) in diameter, or 0.0081 m2 (12.57 in.2) in cross-sectional area. This method cannot be used when the flow is cyclonic or swirling.

In these small diameter stacks or ducts, the conventional USEPA Method 5 stack assembly (consisting of a Type S pitot tube attached to a sampling probe, equipped with a nozzle and thermocouple) blocks a significant portion of the cross-section of the duct and causes inaccurate measurements. Therefore, for particulate-type pollutant sampling in small stacks or ducts, the gas velocity is measured using a standard pitot tube downstream of the actual emission sampling site. The straight run of duct between the pollutant sampling and velocity measurement sites allows the flow profile, temporarily disturbed by the presence of the sampling probe, to redevelop and stabilize.

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Last Update: October 25, 2006