When performing  Method 201A (M201A) in smaller stacks where blockage effects will be between 3% and 6%, Section 8.7.2.3 of the Method states that all velocity traverses must be performed according to Method 1A (M1A) criteria.  This involves performing velocity traverses at a separate location in accordance with Section 11.1.1 of M1A (although a standard Pitot tube is not required because the stack will be greater than 12" in diameter), or alternatively for sources with steady flow, performing velocity traverses before and after the test run at the M201A sampling location using a S-type Pitot tube in accordance with Section 11.1.2 of M1A.  When using the latter procedure, the flows calculated from the pre and post velocity traverses must agree within 10% to be considered valid.  Either procedure is performed with a Pitot tube that is not attached to the M201A sampling head.  In either procedure, the Pitot Coefficient (Cp) used for all calculations will be the coefficient determined without the M201A sampling head.  The velocities measured will be corrected for blockage using Equation #27 of M201A.  If performing the Method 1A traverse at a separate location during the test run, the corrected velocities will be used to calculate dwell times (Equation 24), and all post test run calculations, according to the method.  If doing the pre and post velocity readings, the Equation 27 corrected pre readings will be used to calculate dwell times and the method acceptance criteria.  The stack flow rate will be calculated using the uncorrected pre and post readings, with the average of the pre-test flow and post-test flow used for subsequent emission rate calculations.

If the M201A sampling train causes blockage between 3% and 6%, simultaneous testing of another sampling train is not allowed, since this will result in additional blockage.  If the blockage effects of a M201A sampling train are below 3% but are borderline, simultaneous testing of another train is not allowed, unless calculations submitted as part of the protocol demonstrate that the addition of a second sampling train will not cause the blockage to exceed 3%.

To determine whether your stack suffers from blockage effects, refer to Table 1 in the M201A FAQ page (http://www.epa.gov/ttn/emc/promgate/QAs201.pdf ).  M201A cannot be conducted in stacks where the blockage exceeds 6%.