This paper presents an analysis of the correlation effect in the uncertainty estimation of a system for humidity generation based on the two pressures principle. In order to compare the validity of the calculations, two methods for evaluating uncertainty, including the correlation between the input variables, were used. The first one was the classical approximation based on the applicability of the central limit theorem, while the second consisted of the propagation of the input distributions using Monte Carlo simulations. The obtained results show that the usual assumption of negligible correlations between the input variables is valid for uncertainty levels in calibrations of hygrometers.