Determination of Moisture Content in Stack Gases – USEPA Method 4

USEPA Method 4 is used for the determination of the moisture content of stack gas.  A gas sample is extracted at a constant rate from the source; moisture is removed from the sample stream and determined either volumetrically or gravimetrically.
 
The method contains two possible procedures:

• reference method
• approximation method

The reference method is used for accurate determinations of moisture content.  The reference method is often conducted simultaneously with a pollutant emission measurement run.  When it is, calculation of percent isokinetic, pollutant emission rate, etc., for the run is based upon the results of the reference method or its equivalent. 
 
The approximation method provided is used to estimate percent moisture to aid in setting isokinetic sampling rates prior to a pollutant emission measurement run.  Alternative means for approximating the moisture content (e.g., drying tubes, wet bulb-dry bulb techniques, condensation techniques, stoichiometric calculations, previous experience, etc.) are also acceptable as approximation methods.