Reference method for measuring emissions of nitrogen oxides from stationary sources: chapter 11
11 Precision and accuracy
11.1 An interlaboratory study of the precision and accuracy of determining oxides of nitrogen in gaseous combustion productsFootnote1 using the phenol-disulphonic acid method yields the following estimates of the variance of the method.
For determinations of nitrogen oxide in the range of 20 to 2000 ppm NO2, the relationship of the between-laboratory component of variance, SB (an estimate of reproducibility), and the mean NO2 concentration, CNO2, is:
(4) SB = 2.21 − 1.18
Description of Equation (4)
SB equals 2.21 times the square root of the concentration of NO2, in ppm on a dry basis at 25°C and 760 mm Hg, minus 1.18
For the same range, the relationship of the within-laboratory component of variance, Sw (an estimate of the repeatability), and the mean NO2 concentration, CNO2, is:
(5) SW = 1.52 − 4.21
Where:
SB, SW, and CNO2 are expressed in ppm NO2.
Description of Equation (5)
SW equals 1.52 times the square root of the concentration of NO2, in ppm on a dry basis at 25°C and 760 mm Hg, minus 4.21
Separate field tests done as part of the same study resulted in a between-laboratory standard error of 14.5 ppm for NO2 concentrations of 90 to 260 ppm. Moreover, the study revealed that there was no significant bias in the determination at the 99-percent confidence level.
11.2 The following equation defines the 95-percent confidence interval (C.I.) for the average of several concentrations determinations by any laboratory.
(6)
Where n - the number of determinations used in calculating the average.
Description of Equation (6)
Confidence Interval equals the concentration of NO2, in ppm on a dry basis at 25°C and 760 mm Hg, plus or minus 1.96 times the square root of SB squared plus the square of SW squared then divided by n, the number of determinations used in calculating the average.
11.3 The accuracy of the method was examined over two concentration ranges with known nitric oxide spikes. The average determinations differed from the true values by -4.6 and -1.8 percent in the respective concentration ranges of ≤700 and ≥700 ppm CNO2. Student's t-test indicates that these biases are not statistically significant it the 99-percent confidence level.
Figure 3. NOx Calculation Sheet
NO2,ppm = 
Description of Figure 3 Equation
Nitrogen dioxide, in ppm, is equal to 24.45 x 103 divided by 46.01 all multiplied by W, the weight of NO2 found in gas sample, in micrograms divided by Vc, the corrected volume of sample gas, ml.
NO2, mg/m3 = NO2,ppm x 1.88
NO2, lb/ft3 = NO2, ppm x 1.17 x 10-7
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Sample Number
|
Absorbance @ 405 nm
|
Weight of NO2 in Gas Sample W µg |
Corrected Volume of Gas Sample VC ml |
Concentration of NO2 ppm |
Concentration of NO2 µg/m3 |
Concentration of NO2 lb/ft3 (standard cubic foot) |
Stack Gas Flow Rate QS m3/h |
NOx Emission Rate ERNO2 kg/h |
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