The beer will eventually reach equilibrium with the serving pressure, so if you want to maintain your carbonation level, you need to use the serving pressure that matches your carb level based on the chart. If you have a serving pressure higher than the corresponding carb level, carbonation will immediately begin to increase and keep going up until it reaches the carb level corresponding with the higher pressure. If the serving pressure is lower, the carb level won't drop until you pour a beer and make room in the headpsace for the CO2 to come out of solution, but every time you pour a beer it will drop, until it reaches equilibrium with the lower pressure.
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