But isnt the issue with a J valve that they can be accidentially operated thus giving the feeling of a reserve, and then finding there is no reserve as it was already accidentially set and you have sucked it empty. In your case, you are running to 500 psig, regardless and so it shouldnt be an issue, but if you are tempted to run it to empty with the thought of having the J valve reserve, you could get accidentally caught out.
In australia, J valves are virtually never used. Only those who have had them from many years ago may still have them. I think most people tend to shoot them with silver arrows just to make sure they are dead.
I suppose its horses for courses. I prefer a simple K/DIN valve and know I have to monitor my gas constantly. If I am wanting to be adventurous, I take a 2.7 litre sling tank as my "emergency" supply should something bad happen. If I am going deep, then its all the required gas + reserve+ an emergency 7 litre supply+ a spare emergency deco cylinder under the boat at say 5-10m.
Plan the dive and dive the plan. Emergency air should never have to be used ever. I always run the rule "If it can it will so prepare for the inevitable"
