Imported post
Normoxic means just that - normal or the same as air in its percentage of oxygen. I think a normoxic qualification allows you to use mixes down to around 18% depending on the agency. As Dom said, the percantage of oxygen is above that critical 10-12% giving you a ppO2 of above 0.1 on the surface and is therefore still life supporting.
The addition of helium then of course gives you an equivilent narcotic depth (END) shallower than your diving depth depending on the amount of helium in your mix. Helium does also have its own set of rules for deco etc
The depth it is used at is limited (as with nitrox) by the ppO2. Normixic trimix, with say 21% O2 will have a ppO2 of 1.4 at about 54m (of there abouts, off the top of my head). Depending on your agnecy, that is about the limit.
As to equipment, it is by nature a technical dive, I wouldn't expect anyone to jump in with a single trimix cylinder with a pony. I would used twins and probably side slings with deco gas.
In answer to your question on blending. You add the helium, add the oxygen (if required) and then top up with air. As far as I know, people don't blend using nitrogen only (I am prepared to be corrected though)
HTH
Paul