I posted this information on a DecoStop thread several months ago, but I am unable to locate the thread. I will repeat what we found when we tested an Apeks regulator.
We took an ATX-200, with an Apeks ATX100 safe second installed, and put it on our test bench. The second stage was "normally" tuned and the regulator was performing as would be expected for normal diving. The diver-adjustable cracking effort knob was turned one full turn from the easiest opening position, and the opening effort was measured at 1.2 iniches of water. We installed one of the industry standard inflators (ours came from a Dive Rite wing)
to determine the level of safety (or lack of safety) of having high intermediate pressure on a balanced second stage. The results were something like the following.
We increased the intermediate pressure in increments of 25 pounds. We moved the pressure up to 275 psi before the inflator began to "auto fill". At that point, we removed the inflator to determine when the Apeks second stage would open. At 400 PSI, I decided to terminate the test due to other unknown possible safety concerns with the test. At the time of termination, the Apeks second stage had still not opened and provided relief for the pressure. I repeated the same test with an inflator from a ScubaPro jacket. That inflator opened at a pressure of about 250 psi.
Our conclusion was that a diver, using an Apeks regulator set-up, should certainly have some sort of provision for high-pressure seat failure. Our conclusion was that a spring-tension, non-adjustable OPV would be the best solution. These types of OPVs present very little actual practical potential of failure, are extremely low profile, and are not terribly expensive. Of course, you could also use an un-balanced second stage, of some other make and model, as a back-up regulator to accomplish the same thing.
The risk of an inflator "runaway", all of the training and reaction to problems of this sort aside, is much too large, in my opinion, to dive these units without an OPV. There will clearly be some dispute from many with regard to our conclusions, and that is OK. We also represent a fairly intelligent group of divers here at our store, and my IMMEDIATE reaction was to put OPVs on ALL of our personal Apeks first stages. The link below will take you to the type of OPV we used and the type we most often sell.
OPV Valve - 200 PSI Pre-Set Pressure Relief
Go to the bottom of this page for the OPV.
Thanks,
Phil Ellis