Its a very simple concept. you put in x% O2 and you top off. No bunch of fancy regulators and flow meters, continuous sampling that you see in shops with lots of cash to invest in a quality CB system. Its not set it and forget it. Its not so portable either. Its great for banking nitrox. Its great for those that have constant supply presure of O2, but for a tank by tank fill PP is the fastest and least expensive way to do it. Yes you can pump the O2 tank dry and not waste cash on refilling partial filled O2 tanks. Even the small shop that used to function in my area attempted CB filling. NO its not just a stick and a hose coming form an O2 tank. The output had a regulator on it to provide air to a flow meter to provide gas to the O2 sampler. Seldom was the fo2 measured on the rig the same as in the tank an hour later after it cooled. Of course yo can get the automatic regulators that adjust the O2 feed. And that is pretty well set and forget it, but only after lots of money is invested. If you are filling for your self. Its really just too expensive to do it with out constant monitoring of the process on the cheap. If one had 2k cuft of storage then it would be worth the CB filling method. And then o have to have an O2 shut off to insure you dont dump your O2 after the compressor stops sucking. No really expensive to put in but another one of many expences required to set up an automatic CB fill system. but then again in a small operation you would sit there all day waiting for the cascade to fill. Can it be done , YES, can it be done easily NO. Can it be done cheaply? NO Is it easy once you get it working YES. Is it worth it to fill your own 2-4 tanks every other weekend? NO. What is fail safe is that you know if you put in 25% O2 and top off you get 40% nitrox. I dont have to adjust the system to make it, cause im not making anything but air adn using it to top off with.
You can make a chart of O2 for a mix per hundred # of gas or as i do i do it in my head. 2500 # of nitrox 32 thats 800 O2 adn 1700 N2. .2/10 of 1700 is the O2 that is with the N2 from theh air. Then 800 - 340 is the Psi of O2 tp put in the tank and then top off. Or you just know that you need 18-19% O2 to mis nitrox 32. You can cut is a bit easier and say 20% of 2400 psi (2500-100) and find out the O2 amount to fill with O2 prior to top off. No matter how you do it,,,,,, it is easier than CB for a small operation wehn it comes to cost of building the set up that does not require constant monitoring. This is not to say that there is not definate benefits to CB blending, There is. Think about the amount of waste of O2 you have when PP blending compared to the cost of bullding the system.
I guess we'll have to agree to disagree. I have a home built stick that I then had my local compressor guy tap and fit the same oxygen line he puts in his sticks he builds professionally. My setup includes the oxygen reg, a high pressure pop off in case oxygen pressure builds too high(which never happens), and a flow meter into the stick, and an oxygen shut off solenoid for when compressor stops. It wasn't that expensive (around $300). It's easy to use. Set the % and it doesn't vary much through fills. Maybe 0.5% max so I just watch it. You say your experience if final % doesn't match input %? I have no idea why. I see that all the time with pp blending, never with cb.
I just don't really see what you're saying as reality, but I don't own a shop. I do fill a ton of tanks via cb with nothing but ease and accuracy. I think the majority here that have pp and cb filled would say cb makes the most sense in any size situation.
It sounds like you haven't experienced a properly setup cb system. How is paying attention to oxygen flow rates into a tank and monitoring pressures in fill bottles easier?