Question

When should you bring a spare air bottle / pony?

Answer 1

Good sense is to have redundancy when diving. A pony bottle with its own regulator can be a lifesaver in every out of air or other emergency situation. It will give a diver time to make a good ascent, a safety or deco stop,  or a line entanglement  or any similar situation.

Or it save an out of air buddy's life.

Answer 2

I dont, just never have. I always dive with a buddy and she uses almost no gas. we always dive together and start our ascent when the first of us reaches 1000 psi, that way we are back on the boat with atleast 500.

Answer 3

Never had one, did consider it once but I just plan the dive ;stick to the plan and always keep aware of my air.