And how many time can you do that before there's not enough pressure left to get the required volume?
I have three O2 cylinders. One by one, as the pressure drops, they are relegated to continuing blending. But eventually the pressure in the other two drop and you just can't get the needed volume. At some point you've to go top off with EAN40. I could just keep ordering new O2 cylidners, but at some point I'm gonna have 10 cylinders strapped to the wall with 500 PSI, all waiting to be drained as I blend EAN into my doubles.
Sometimes you can plan your gas around your dive, but other times you've got to tweak the dive plan around the available gas.
Get smaller cylinders so it doesn't take so much recreational nitrox to use one up and exchange it for a full one. 125cf works great for me at my tech to recreational dive ratio