I have one of these, too. Holds 3mls. I've changed the heads without issue. Took the head apart, did a dry burn, put it back together, no issue. The only issue I have had with this one is that when it gets about halfway through, it does leak. I like my protank (and mini) better, but it's not too bad..... if I keep it above half full. Someone did suggest I add an addition flavor wick for that, just to see. Haven't done that yet, though.