Accurate measurement of emissions (i.e. volume flow rate multiplied by concentration) is vital to the control and reduction of air pollution. We quantify the uncertainty of flow measurements by S-type pitot tubes in narrow exhaust stacks (diameter < 0.5 m) in support of recent European Union (EU) regulations for emissions from medium-size combustion plants. The contribution of blockage and wall effects to the uncertainty budget is evaluated. The aim of this thesis is to characterize flow uncertainty and to aid the development of more accurate measurement methods for volume flow rates in narrow exhaust stacks. This thesis will provide the scientific basis for implementation of EU regulations concerning emissions from medium-size combustion plants. Characterization of flow fields in narrow exhaust stacks is key to accurate volume flow rate measurements. We use numerical simulations to study the mean velocity profile and turbulence statistics of fully developed turbulent flow. To investigate the impact of wall effects on measurement uncertainty, we simulate the flow eld around an S-type pitot tube in close proximity to the stack wall. In addition, we conduct S-type pitot tube measurements at various wall-distances and bulk velocities. Throughout this thesis, we use several uncertainty quantification techniques to evaluate the uncertainty of numerical simulations and experiments. Studying spatial variation in measurement uncertainty gives insight in the most suitable locations for S-type pitot tube measurements in narrow exhaust stacks. The results of this thesis suggest that wall effects dominate over other sources of measurement uncertainty in the near wall region. Throughout the measurement plane, the combined uncertainty of measurement error sources exceed the impact of blockage. We recommend to determine volume flow rates in narrow exhaust stacks by S-type pitot tube measurements in the stack center. A correction factor can be determined to compute the volume flow rate in a narrow exhaust stack from the maximum flow rate in the stack center.