Section 8.1.1.1 of Method 4 specifies traversing the stack in the determination of moisture. Minimum traverse points are listed based on the stack dimensions, but the Method does allow for fewer traverse points with approval. EMS historically has permitted single-point sampling for Method 4 based on this allowance, since gaseous methods previously were all single-point sampling and moisture was considered analogous. However, with the change in instrumental methods to now require a stratification check prior to allowing single-point sampling, we have reassessed our past acceptance of single-point sampling for moisture. From this point forward, we will require the following:

     When the stack temperatures is less than or equal to 212 F, or if moisture droplets are expected, the traverse points specified by Method 4 must be used. You must also compare the measured moisture to the saturation moisture at the stack temperature, and use the lower of the two values.

     When the stack temperature is greater than 212 F, the traverse points will be as determined by a contaminant/diluent stratification check performed in accordance with the procedures found in Method 7E. In most cases, this will result in single-point sampling, as is the current practice. Please note that the Method 4 single point will be consistent with Method 7E requirements (the single point from the stratification traverse that most closely matches the mean concentration of the contaminant/diluent used for the stratification check).

As a reminder, Method 4 requires the sample probe be heated sufficiently to prevent condensation. This is especially important if tubing is used to connect the probe to the impingers, in which case the tubing is considered part of the probe and must also be heated (or shown that it is being kept sufficiently hot to prevent condensation). If tubing is used, the length should be as short as possible and should avoid low spots below the elevation of the first impinger inlet.