The measuring chamber is completely filled with sample and sealed. The volume of the measuring chamber is expanded. Equilibrium is generated and pressure and temperature are measured. The measuring chamber volume is further expanded, equilibrium is generated and pressure and temperature are measured again. The two pressures and temperatures are used for CO2 determination and dissolved air compensation.
CO2 in a beverage container is distributed between the liquid phase and head space. Right after filling, the entire CO2 is in the liquid, and little to no CO2 is in the head space. As time progresses, some CO2 migrates into the head space. The warmer the sample, the more CO2 will be found in the head space. Still, the overall amount of CO2 in the container remains unchanged, it is just the distribution that changes.
Here is what the CO2 distribution in a bottle looks like, assuming a head space of around 10%.
Equilibrium means that the migration of CO2 from the liquid to the head space is equal to the migration of CO2 from the head space into the liquid. As CO2 is colorless, we do not see whether equilibrium is reached or not. Therefore, we have to equilibrate the sample to ensure a defined state of distribution. Equilibrium can be reached by gently shaking the container about 15 times back and forth. The container is best held horizontally to increase the liquid and head space interface.
Well, we cannot see how far equilibrium has already progressed in a sample. Therefore, failure to ensure equilibrium is likely to result in very poor repeatabilities, meaning that you can expect results anywhere between 5 g/L and 4.5 g/L when looking at the above example.
In a beverage container, CO2 is distributed between the head space and liquid phase. The solubility of CO2 decreases with increasing temperature. Therefore, the amount of CO2 in the liquid phase will decrease when the temperature increases.