Yes. When we used the J-Valves, we did not have SPG's and the restriction let us know it was time to surface. They were NOT intended to be a built-in pony, just a reminder that your air was low.... Remember the old J-valve? It had an internal pressure valve which would constrict air flow at (I think) 800psi, and then you could flip the lever down and get the extra 800 "reserve". It was supposed to work like a "built in" pony bottle. Problem was, people were likely to rely on that reserve, and incorporated it into their dive plan. This could have disastrous effects when some realized that the'd forgotten to flip the lever up before the dive, and when the air flow restricted, they were actually out of air, not at their reserve stage. I once had a J-valve tank, but I never used it as such.
Frankly, I don't get all of the rancor over the use of a pony. I could care less if a diver decides to use it or not. One thing is certain however, should I find myself suddenly OOG and my buddy hands me the reg attached to his pony, I'm not going to refuse it; and neither would anyone else.
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