I have a Scubapro Mark V, A.I.R. I regulator combination that I bought new in 1979. That combination is probably the finest-breathing regulator ever put together (along with the Mark V/Pilot combination). I plan to dive it this weekend, and it is still working just fine, thank you.
Concerning the Mark V first stage, Scubapro was a bit put out when the U.S. Navy tested it in the late 1970 or early 1980s, and found that the Mark I first stage out-performed it. If you look at the Mark I construction, you'll see that it has only one LP port, but that it comes out on the top of the first stage cap. This is in line with the flow-through piston's air flow. The fancier Mark V, at that time, had two opposing LP ports, but they came off to the side, at a 90 degree angle to the flow-through piston's air flow. So the lower-end first stage, the Mark I, out-performed the higher-end regulator, the Mark V.
When these results were made public, Scubapro had to make a change, and what they did was to re-design the LP system for the Mark V. They put four LP ports at a 90 degree angle to the air flow through the piston, that is on the side, for convenience. But they also put one additional port right in the top of the cap. This is the main port, and all the rest should be (for best air flow results) used for auxillary equipment (safe second stage, BC inflator, dry suit inflation system, etc.).
By the way, the A.I.R. I first stage was designed to be used in extreme diving with two first stages, with two LP hoses coming to each side of the second stage. If two Mark V first stages were to be used, the flow available at the mouthpiece was tremendous. I have used it in this configuration, and in very high demand environments (swimming against high currents in a river) there is a slight difference. This difference would be more noticeable at great depth than at the river depths I typically see.
Someone asked about the interstage pressure, and I have two references in my Technical Manual for Scubapro Regulators that show this to be 140 psi. The intermediate pressure can be changed by adding stainless steel washers at 5 psi per washer additional pressure if the HP spring is not giving 140 psi.
When the Mark V (or other like-engineered piston first stages) are not being used, there is no wear on the seat, as the spring lifts the piston off the seat. Similarly, with the Pilot and A.I.R. I regulators, when the key is inserted into the slot in the diaphragm, it lifts the main valve off the aspirator seat, and again prevents permanent changes in the seat. These regulator parts last a very long time if maintained in this manner.
SeaRat
PS--I used the Scubapro Mark V/A.I.R. I combination in 1986 when I was researching a new BC design in Clear Lake, Oregon (headwaters of the McKenzie River):
