A method has been found to reduce the high measurement uncertainty of NOx emissions of gas turbines.

Calculation of the measurement uncertainty of NOx emission measurements shows that the high uncertainty is not only due to the uncertainty of the NOx emission measurement itself but is also due to the correction to 15 % O2 in the flue gases.

A second, independent method to determine the percentage oxygen in the flue gases is introduced in addition to the direct sampling method. This second method provides additional information resulting in a significant reduction of the measurement uncertainty.

The solution found has the advantage that the additionally required measurement equipment is kept to a minimum.

Experience with the new method in field tests are excellent.

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