Which Surgeon's flask is chosen to gain a charge on crit?

Since 2.5 flask charge on critical strike has a 200 ms cooldown across all flasks. If multiple flasks have Surgeon's (or if using Master Surgeon from Pathfinder or the Harvest), which flask is chosen to gain the charge? Is it uniformly random, or does it "rotate" through your flasks in some sequence? Does it ignore flasks that are full?