This procedure utilizes Fourier transform infrared spectroscopy to measure the CO2 concentration within carbonated beverage containers.
CO2 absorbs IR radiation in a very specific range of wavelengths. As the concentration of carbon dioxide decreases the measured CO2 absorption correspondingly decreases. The IR absorption value is divided by the length of the path through the container to account for the change in the container shape (there is usually a slightly shorter path length by end of test) over the time of the test. This calculation yields a concentration value for a particular sample. The concentration is then plotted against time to determine the rate of CO2 loss. During the first 10 days after carbonation, the bottle is equilibrating. During this time, the CO2 is dissolving into both the bottle walls and the closure. In addition the bottle is “creeping” by growing from the pressure of carbonation. After 10 days, these processes have theoretically all reached equilibrium so that there is no additional significant creep. The rate of CO2 loss through the bottle and closure is now constant as is the leak rate between the bottle finish and the closure liner.