Uncertainty budget and interlaboratory field tests in SO2 and NOx emission measurements

This paper compares two techniques to assess uncertainty in emission measurements. The first approach, described in ISO 14956, defines a procedure to establish uncertainty budgets from the systematic assessment of all relevant factors influencing the result. The second approach is based on inter- and intra-laboratory field experiments set out according to ISO 5725-2. These two concepts were applied during field validation of reference methods for sulphur dioxide (SO2) and nitrogen oxides (NOx), which are described in the European draft standards prEN 14791 and prEN 14792. For the SO2 manual reference method, uncertainty budgets are in excellent agreement with confidence intervals determined in field tests, due to the fact that the major uncertainty contribution is the chemical analysis of sulphate, which had previously been determined in an inter- laboratory study. Results obtained for the automatic NOx method show pronounced discrepancies between both approaches, illustrating that reliable uncertainty budgets are only obtained if a number of demanding conditions are met.