For extreme high vacuum pressure measurements a modified gauge of the type described by Lafferty was fabricated [J. M. Lafferty, J. Appl. Phys. 32, 424 (1961)]. To determine the limit of low pressure measurement, its residual current was experimentally estimated. Residual current consists of x-ray-produced photoelectron current and electron stimulated desorption (ESD) ion current. Residual current was studied using the Bayard–Alpert method [R. T. Bayard and D. Alpert, Rev. Sci. Instrum. 21, 571 (1950)]. The linear relationship between emission current and residual current was obtained. The ratio of x-ray-produced photoelectron current to emission current was on the order of 10−7; the ratio of ESD ion current to emission current was on the order of 10−8; both ratios were obtained using a cleaned stainless steel anode. In order to investigate the upper limit of ESD ion current, the anode was exposed to oxygen gas at a fixed pressure and ESD ion currents were measured as a function of exposure. The upper limit of the ratio of ESD ion current to emission current was on the order of 10−5; residual current was on the order of 10−16 A with an emission current of 10−9 A. Sensitivity obtained was 4×10−3 A/Pa with an emission current of 10−9 A. Therefore, the predicted low pressure limit for the modified Lafferty gauge was on the order of 10−13 Pa.