Smitty... here is how "cascade" filling works: (re-written, for clarity, from another internet source)
To get the most rifle bottle fills possible out of their air cylinders, air gunners often use two air cylinders, in tandem, cascading them, so that they are able to fully top off the airgun bottle many more times than if they had a single air cylinder to refill from.
Let’s say your rifle’s air bottle has a 250 BAR capacity. When your Omega (or other brand) storage air cylinder reaches around, let’s say 200 BAR, you really need to think about getting it refilled it, as your cylinder can only refill your rifle’s bottle up to the pressure remaining in the cylinder: 200 BAR.
If you want to refill your rifle up to its 250 BAR capacity, the air that remains in your air cylinder (now at 200 BAR pressure) is air that you can't really “use" (beyond 200 BAR) because it doesn't have the pressure to fill your rifle beyond the 200 BAR pressure that it contains.
If you have two cylinders, however, you can actually use them to MUCH more advantage than having just one, by a technique called "cascading".
What is done to get the “cascade advantage” is use only cylinder #1, as long as it can completely fill your airgun’s bottle to the pressure you desire, let’s say that’s 250 BAR. Once the air cylinder’s pressure drops below 250 BAR, it can no longer completely fill your rifle to its capacity of 250 BAR. If your air cylinder drops to 200 BAR, it can only fill your rifle’s bottle up to 200 BAR - not the 250 BAR that it is capable of holding.
Now, you use cylinder #1 to “bulk fill” the rifle’s bottle from whatever level it is at (let’s say 150 BAR) up to the pressure that cylinder #1 has available, perhaps that’s 200 BAR at this point.
Then, you use cylinder #2 to just “top off” the airgun -taking it from, say, 200 BAR up to the 250 BAR that you desire. In this way, minimal air is used from cylinder #2, which has for the time being become your “higher pressure” tank.
So, when cylinder #1 is below 250 BAR (or whatever is your max fill pressure), just use it to “bulk fill” your rifle bottle from 150 BAR (or whatever pressure you start to refill your airgun) all the way up to whatever pressure remains in cylinder #1.
Then, only use cylinder #2 to "top off" your rifle’s bottle up to your desired fill pressure, perhaps that’s 250 BAR.
For each fill now, just use cylinder #1 to get as much “bulk air” into the rifle’s bottle, before you switch over to using cylinder #2, just to “top it off” to maximum pressure.
Eventually, you'll get to the point where cylinder #1 is so low that you'll only use cylinder #2 to fill your rifle. But by that time, you've drained cylinder #1 MUCH lower than you could possibly have drained it if it were your only available air cylinder.
In this way, you get the MOST ECONOMICAL use out of the air that you have put into your air cylinders.
