ginti
Contributor
I have tried twice now to buy a bp/w locally. The first shop said "what a great idea i will find one for you" and then never managed to get one priced let alone landed. The second shop i went to didn't acknowledge they existed and tried to sell me a rear inflation bcd. I ended up buying a bottom of the line standard bcd out of frustration, and instantly regretted it.
I also really want to do fundies but haven't had the coin for it yet.
Since you want to do fundies, try to contact right now a GUE instructor, just to ask for some info; there are two of them in New Zeland, they are going to tell you where to buy a bp/w
Regarding your question about narcosis, it changes a lot depending on several conditions; even external conditions, like water temperature and visibility, can affect narcosis. My guess? Worse conditions -> higher stress -> worse breathing pattern -> higher CO2 -> higher narcosis. If my guess is right, basically anything that affects your breathing pattern (including stress, which changes day by day) have an influence on narcosis.
In my case, I tend to feel narced below 30/35m, depending on conditions, which is why I don't like diving air/nitrox in this depth range - trimix is my thing