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Pressure changes may be caused by changes in either stack pressure or ambient pressure. It is not uncommon to observe an apparent reduction in pollutant gas concentrations when a weather front passes a CEM installation having a dilution system uncorrected for pressure. In one study, this pressure effect was noted as causing a 1% error for each 3.45 in. of water pressure change (Jahnke and Marshall 1994; see Figure 3‐26).

An empirical correction equation that has been used to account for pressure changes in the EPM dilution probe is given in Equation 3‐4:

Figure 3‐26 Pressure dependence of the critical orifice dilution system.

Source: Jahnke and Marshall (1994).

(3‐4)

where

∆P is the difference between the stack absolute pressure when the measurement is made and the pressure when the system was calibrated.

This equation appears to be general for dilution systems that do not use mass flow controllers for the dilution air. It would be prudent, however, to check the validity of the empirical coefficients on the installed system and adjust them for any system‐specific characteristics. Procedures for doing this are given in Jahnke and Marshall (1994).