Balloon pressure problem

The Italian brand Coustaing is the first choice for expansion tanks. P 1V 1=P2V2 refers to the gas pre-loaded in the expansion tank. Suppose an expansion tank with100L has a pre-charging pressure of 2 kg and a pumping pressure of 10kg, then how much water can be put into the airbag at this time? Here, we equate the volume of the pre-filled gas with the volume of the expansion tank, so 100*2= 10*V2. Then V2=20, then 80L of water enters the air bag. Conversely, we require that the expansion tank can hold 100L of water when the pressure is 10kg, and the pre-filling pressure is still 2kg. So what size expansion tank should I choose? 2v 1 = 10V2, V = V 1, then V 1-V2= 100, so we need to choose/ Do you understand? The object of this formula is the pre-filled gas, and we can get the required volume by calculating the gas volume.