The Korea Research Institute of Standards and Science has three major vacuum systems: an ultrasonic interferometer manometer (UIM) (Sec. II, Figs. 1 and 2) for low vacuum, a static expansion system (SES) (Sec. III, Figs. 3 and 4) for medium vacuum, and an orifice-type dynamic expansion system (DES) (Sec. IV, Figs. 5 and 6) for high and ultrahigh vacuum. For each system explicit measurement model equations with multiple variables are, respectively, given. According to ISO standards, all these system variable errors were used to calculate the expanded uncertainty (U). For each system the expanded uncertainties (k=1, confidence level=95%) and relative expanded uncertainty (expanded uncertainty/generated pressure) are summarized in Table IV and are estimated to be as follows. For UIM, at 2.5-300 Pa generated pressure, the expanded uncertainty is < 4.17 x 10(-2) Pa and the relative expanded uncertainty is < 1.18 x 10(-2); at 1-100 kPa generated pressure. the expanded uncertainty is < 7.87 Pa and the relative expanded uncertainty is < 7.84 x 10(-5). For SES, at 3-100 Pa generated pressure, the expanded uncertainty is < 1.77 x 10(-1) Pa and the relative expanded uncertainty is < 1.81 x 10(-3). For DES, at 4.6 x 10(-3)-1.3 x 10(-2) Pa generated pressure. the expanded uncertainty is < 1.04 x 10(-4) Pa and the relative expanded uncertainty is < 8.37 x 10(-3); at 3.0 x 10(-6)-9.0 x 10(-4) Pa generated pressure, the expanded uncertainty is < 8.21 x 10(-6) Pa and the relative expanded uncertainty is < 1.37 x 10(-2). Within uncertainty limits our bilateral and key comparisons [CCM.P-K4 (10 Pa-1 kPa)] are extensive and in good agreement with those of other nations (Fig. 8 and Table V). (c) 2006 American Vacuum Society