You had posted a similar question earlier, and it was apparently deleted by the moderators. In that one I gave a detailed explanation on how we use our tanks and think about the air usage, and since I still have that answer available I'll repeat it below to save time - I expect you did not see this answer before the deletion. You originally had different volumes in your fill count example, so they won't match what you have in this post, but it should help you understand our use a bit better. Thanks for taking the time to ask and learn about our usage.
You asked if you did the math right, and I'll say it is not really wrong, but not really correct either - dealing with air under pressure is complicated by the pressure level, and the pressures we both fill to and from in our airguns can vary greatly.
First, when we say how much air a tank holds, we have to specify the pressure of the air as that is an important function of the total volume. For example, if the air is at atmospheric pressure, the volume of air will be the same as the water volume. At one bar of pressure (gauge pressure, not absolute pressure), it will hold twice the water volume of air, and so on.
So the key thing is to specify the pressure, along with the volume that one is reporting.
The second thing has to do with the complexities of filling smaller tanks from bigger tanks (or at least higher pressure tanks). Air flows from the higher pressure tank to the lower pressure tank, and it will do so until the pressure either equalizes between the two tanks, or a valve is closed to stop the air pressure at an lower pressure in the tank being refilled before the pressure equalizes.
In your example of filling the 280cc reservoir from the 9L tank, one must know how much pressure is desired in the 280cc tank for it to be considered "full." If the goal is to have the 280 tank at the same max pressure that the 9L tank has at the start, then technically it won't even get one "full" fill because the equalized pressure of the two tanks will be below the starting point of the 9L tank. Now nobody here would be that specific about the situation, and we would all pretty much accept that the first fill will be a "full" one, but it falls off from that point on, getting to a lower pressure on each refill.
In practice, most 280cc reservoirs will be filled to less than the 9L tank's starting full pressure, so that won't be an issue - but it does illustrate the importance of defining the fill pressures involved. Furthermore, we rarely have to fill our smaller tanks (like the 280cc one) from empty - they are often starting at almost half full before we refill them, because we do not shoot them down to empty before they need to have air added - this is very different from activities that use the tanks for breathing air, as they are often very nearly empty after use . An under water diver will plan to spend as much time under water safely as he can from a given amount of air - we airgunners tend to go for a number of the most consistent shots we can take with a full air gun. Thus most air gunners will "need to refill" their air guns when they still have anywhere from 100 to 175 bar still in the reservoir, depending on the gun, in order for the shots to stay consistent in terms of speed of the pellet and accuracy at the target.
So to further your example of the 280cc tank being refilled from a 9L tank starting at 300 bar:
- If refilling the 280cc tank when it drops to 150 bar, and filling it to 250bar, one would get about 19 full fills from the 9L tank, then get several partial fills
- If refilling the 280cc tank when it drops to 100 bar, and filling it to 200bar, one would get about 35 full fills from the 9L tank, then get several partial fills
Both of those examples would have the number of fills reduced by a few if the initial fill of the 280cc tank was from empty. There are numerous possible ways to use the air (in terms of starting and ending pressure) and that is why the online calculators are so helpful.
As I said, knowing the pressure level involved in the equations is crucial to getting the correct answers with air refills . . .
And to your main question, I like to have both a large less-portable tank to use a main refill one, and smaller more portable one to take with to wherever I will be shooting (if I don't need the capacity of the big one), so I have both a 9L and a ~2L tank. My 9L is an Acecare and I really like it. My ~2L is another brand and really like it too - I bought it many years ago. I did consider the 12L when I bought the 9L, but decided the 9L was big enough for my needs given the price difference.
