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Temperature effects contribute approximately 1% error for each 50 °F change in temperature from the initial dilution probe calibration setting (Jahnke and Marshall 1994). The temperature effect, however, is nonlinear. Temperature fluctuations of this magnitude do not usually occur in most operating units. However, for cycling units and other units that operate infrequently, a common practice has been to perform calibration adjustments to the dilution system when the unit is cold (not operating) and assume that the calibration holds during start‐up and when the unit is hot. This is not a valid assumption and can lead to measurement error greater than 10% due to the nonlinearity of the temperature dependence.

Probe temperature dependence can be minimized by heating the probe, although this has not always been successful during start‐up due to cooling of the heater by initially cool flue gas. A better approach, particularly for cycling units, is to use an external dilution system outside of the stack, where the temperature can be better controlled.

Alternatively, but not as satisfactory as maintaining probe temperature, empirical temperature correction equations can be developed. One equation developed for the EPM dilution probe is given in Equation 3‐5.

(3‐5)

This correction equation appears to be system specific (Jahnke and Marshall 1994) and should only be used as a first approximation for small changes in temperature (e.g. ± 50 °F). It is recommended that the user check the expression for his or her own installation and adjust it as appropriate. It does not appear to work well in cycling units where large swings in temperature are frequent.