Based on the viscous flow characteristics of gas through capillary tube, a simple and cheap gas mixing system was developed for controlling gas compositions in CA chambers. The gas flow rate through capillary tube had a linear relationship with pressure, (length)$^{-1}$ and (radius)$^4$ of capillary tube, which agreed well with Hagen-Poiseuille’s law. The relationship between flow rate and combined parameters was described as Q=0.000209724($pi$ r$^4$P/$mu$L) and the coefficient of determination was 0.9984. The developed system could control gas concentrations in CA chambers within $pm$0.3% deviation compared to the preset concentrations. It was possible to predict the required time and required gas flow rate for exchanging the gs in CA chamber to a certain concentration of gas by using the mathematical model developed in this study.