An aerosol sampling project for the Air Force (Whitefield et al 1993), generated the need to make particle concentration measurements in the size range ≤10 nm, and a Met One CNC (Model 1100-5) was used for these measurements. The Met One small size performance and efficiency vs. particle concentration were calibrated against an ALGR/OPC facility (ALternating GRadient continuous flow thermal diffusion cloud chamber (CFD) and Optical Particle Counter), using a laboratory generated NaCl aerosol for which the critical supersaturation spectrum is both well known and low. The advantage of the ALGR is that it and the Model 1100-5 can operate under similar aerosol concentration conditions. These studies show that the Met One CNC begins to drop below 100% counting efficiency at aerosol diameters ≤10 nm, and reaches 75% efficiency at 7 nm. Complementary electrometer measurements were made for comparison.