In recent years the need for reliable measurements for the detection and control of low water vapour concentrations in many scientific and industrial applications has promoted the development and use of some low concentration humidity generators as measurement standards for the calibration of diverse trace humidity analysers. Just as other national metrology institutes, the PTB has made efforts to extend the capability of calibration and testing of hygrometers in the low concentration range to nl/I volume fraction of water vapour. Based on an existing PTB trace humidity generator a new coulometric trace humidity generator has been redesigned, constructed and validated. This humidity generator could serve as primary standard for trace humidity measurements. A mathematical model including the quantities and parameters describing the behaviour of the improved coulometric trace humidity generator was used to calculate the uncertainty and can also be used as fundamental equation for the evaluation of the performance of this type of generator. The inherent uncertainties of the main subsystems are discussed, especially with respect to the applied current, volume flow and blank of the zero gas. The expanded uncertainty of the water vapour concentration is presented.weiterlesen