 In this study, two condensation particle counters, CPCs, were tested to determine their accuracy when measuring concentrations of particles between 10 nm and 100 nm in diameter. The results showed that the CPCs were accurate for particles in the 25 nm to 50 nm size range, but were underestimated and overestimated for smaller and larger particles, respectively. Therefore, it is important to calibrate the CPCs according to the concentration and particle size being measured. This article was authored by Baratsh Jichaskil, Anastasios Melaes, and Athanasius Mamakos.