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The particle size distribution (PSD) of submicron exhaust engine-out soot, is typically determined using a method based on the electrical mobility is used. This measurement procedure is subjected to uncertainty mainly due to inaccurate dilution of the sampled aerosol, unknown flow conditions at the probe inlet and the limited measurement accuracy of the device itself. In order to determine the measurement uncertainty, two different aerosol spectrometers, a TSI EEPS 3090 and a Cambustion DMS500 were installed and operated simultaneously at a single-cylinder heavy-duty diesel engine at the Institute of Internal Combustion Engines of the Karlsruhe Institute of Technology (KIT).The engine was operated at various operating points to evaluate the ability of the spectrometers to correctly determine the PSD and the total particle number concentration (TPNC) at different boundary conditions. Variations of the position and geometry of the probe systems were performed, to evaluate the influence of the flow conditions at the probe inlet. The dilution systems of both measuring devices were verified by varying the dilution factor in three steps. Both particle sizers show good agreement in the TPNC. It could be confirmed that the probe geometry and its positioning had a negligible effect on the properties of the exhaust gas sample. A slight shift in PSD between the EEPS and the DMS was observed at the engine test bed. For a detailed evaluation, a device comparison with monodisperse polystyrene (PSL) particles with a SMPS as reference was performed at the Institute of Mechanical Process Engineering and Mechanics (KIT).