My friend experimented with quite a few empty refillable carts and there is such a large difference between them. A lot of them don't have enough flow or big enough inlets to let a massive amount of dmt through. o2 vari-flow is hands down the best of the best that I tried. There may be others that are just as good or better but the o2 carts are so good I stopped looking anywhere else.
They have an Achilles heal though. They're bullet proof for 3 fill-ups, eventually the collar that the bottom screws into comes off. The adhesive holding it all together comes apart. my friend has been saving them and will eventually experiment with epoxy or something to see if he can't extend each carts life. But that will require a bunch of research he has yet to do because there is concern about doing it right.
Presently my friend uses a label maker to label the amount and ratio of how much dmt is in the cart along with a message to breath deep or music to the soul, things like that, and then wraps it around the base to take pressure off the weak point. This might not make sense without a picture.