Light negative air ions demonstrably exhibit positive influence on the human or- ganism. The concentration of these ions can be effectively used as the integral criterion for the evaluation of air purity, and there also exists a multitude of other applications. The presented aspiration method enables us to establish the concentration of air ions in the atmosphere. In a Gerdien tube fitted with a one-piece inner electrode, measurement of the saturation character- istics must be performed to facilitate the determination of the air ion mobility spectrum. This method is very sensitivity to noise and leakage current. In this paper is determination uncertainty of type B in Gerdien tube and electrometric amplifier. This method is very sensitive to noise and leakage current. In this paper, the determination of uncertainty B in a Gerdien tube and an electrometric amplifier is presented. The uncertainty depends on the input bias current and input noise current of the applied operational amplifier. Another significant source of B uncertainty is the temperature and time instability of the Teflon insulator.