In the U.K. we never had double valves on a single. We went from a single second stage to having an octopus. For a while there were even manifolds which had an a-clamp fitting and could made a single out of two cylinders.
Recreational Scuba Deco Diving
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OK, I see. As always, I am learning a lot about scuba history here on SB.
Here in Italy, instead, when the first 15-liters steel tanks were made available (around 1980, if I remember correctly) they were all equipped with a double valve with reserve. If not, people could not use their two independent regs, which were the standard setup on previous twin tanks (10+10 liters steel or 9+9 liters alu, both at 200 bars).
Single-valve was instead standard on smaller tanks (5, 7, 8 or 10 liters), which had never been considered suitable as the "main tank" here, as most rec dives were below 40m and with deco, so a 15-liters was the minimum.
So it appears that even on twin tanks in UK/US there was just a single valve? I have never seen such a twin tank, indeed...
If that tank had just one valve, it had became entirely unusable. Having two valves, it is still functional, using the other one. This could make all the difference between a serious problem and a minor inconvenience.
Regarding the chances that two failures (broken knob and free flowing reg) happens at the same time on the same valve are truly minimal.
If the tank had a single valve, instead, the chance that one of the two failures occur is much higher, and in that case just one of them is a big problem...
So for me a tank with two valves is far safer than a tank with a single valve (which I did never use in all my life, except in two occasions, when double-valves tanks were not available and I had to adapt - forcing me to much shallower depth and avoiding deco, as I did not feel "safe enough" with just a single reg plus octopus).
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Given how few (zero) double valves for single tanks I've seen in my time, I'd hazard a guess that those have been rather exclusively limited to continental Europe.
Which is almost certainly true.
And, back on topic, this possibly also relates to the fact that here everyone was trained to these RSDD, which of course require a large tank and some redundancy.
Instead in the Americas deco diving was not considered feasible for a basic rec diver, hence also the equipment employed was downsized to smaller tanks and no redundancy.
Also in training this translates in the CESA excercise.
Here it was not included in training, we had instead to free dive down to 15m for proofing our capabilities.
Doing a CESA was not considered an option, due to deco obligation, and we were trained to solve any problem underwater, without ascending to surface.
When later some sort of CESA excercise was introduced, around 1985, the student had to stop the ascent at 5m, for the mandatory deco stop, instead of going directly to the surface.
It was more an exercise about controlling the BCD.
tmassey
Contributor
I think this is a profound comment. As equipment and training have evolved, they have reinforced each other. If deco is not a thing, equipment evolves to take advantage of the lower requirements, which makes deco even less feasible, which supports even more aggressive cuts in gear and training.
The reverse is true: when deco becomes *more* likely, the gear and training evolves to simply make it part of the basic procedure for all dives. And the idea of “basic” becomes much more involved.
(ETA: Shearwater computers illustrate this. I got my Perdix shortly after my deco-level training. Before then, I never considered deco on a recreational dive. After the Perdix and it’s default deco-friendly settings, deco became a consideration on nearly every dive. The dive didn’t change, but the perspective, driven by my choice of computer and its settings did.)
Honestly, that split probably benefits *both* groups. People in the middle of the road tend to get hit from both directions!
This is not different in other areas, either: passenger cars don’t have racing slicks and roll cages for the same reason. Why should daily drivers suffer the cost and performance penalties? But when race car drivers start complaining about not being able to take 1.5g corners and how much rolling the car hurts, the fault is not the car, it’s the expectation. And when they still try to do it, why would anyone expect anything other than serious danger and destruction?
The reverse is true: when deco becomes *more* likely, the gear and training evolves to simply make it part of the basic procedure for all dives. And the idea of “basic” becomes much more involved.
(ETA: Shearwater computers illustrate this. I got my Perdix shortly after my deco-level training. Before then, I never considered deco on a recreational dive. After the Perdix and it’s default deco-friendly settings, deco became a consideration on nearly every dive. The dive didn’t change, but the perspective, driven by my choice of computer and its settings did.)
Honestly, that split probably benefits *both* groups. People in the middle of the road tend to get hit from both directions!
This is not different in other areas, either: passenger cars don’t have racing slicks and roll cages for the same reason. Why should daily drivers suffer the cost and performance penalties? But when race car drivers start complaining about not being able to take 1.5g corners and how much rolling the car hurts, the fault is not the car, it’s the expectation. And when they still try to do it, why would anyone expect anything other than serious danger and destruction?
What follows is a theory of mine and could be off base. The move away from deco as part of normal OW training and to what we do today is the result of a series of consequences born of frustration with standard dive practices decades ago.
Back then, for much of the world and all of the USA, the standard for diving was the US Navy tables. These tables guided dive practices, and they included decompression. For the average recreational diver, who was not doing planned decompression for the most part, these tables had a couple of problems. The first was that their surface interval schedule was based on the 120 minute compartment, resulting in extremely long surface intervals between dives. The second was they treat multi-level dives as if you had dived to the deepest depth the entire time. This led to shorter than necessary dive times and much longer than necessary surface intervals between dives.
A frustrated diving scientist, Dr. Raymond Rogers, worked with PADI's DSAT branch to see if those limits were necessary for the kind of diving people like him were doing. Using new Doppler imaging technology, he discovered that for typical NDL dives, the 120 minute compartment was much longer than necessary. With the goal of shortening the surface interval and making the scheduling of 2-tank dives as we know them today, PADI created a set of tables for that kind of diving, the Recreational Dive Planner. This planning tool was ideal for 2-tank dives. It used the 60 minute compartment, reduced first dive NDLs somewhat, and added pressure groups to reduce rounding, and this all dramatically decreased required surface intervals. With the goal of adding multi-level capacity, they created the wheel, a planning device that never caught on because computers that did the same thing were introduced at about the same time.
The Recreational Dive Planner was of necessity limited to NDL dives, so it did not include decompression. The thinking was that if divers were going to plan decompression, they would have to use different resources to plan and execute those dives. As a result, the world of diving was divided into recreational and technical diving, with the latter term introduced well after that by Michael Menduno.
Back then, for much of the world and all of the USA, the standard for diving was the US Navy tables. These tables guided dive practices, and they included decompression. For the average recreational diver, who was not doing planned decompression for the most part, these tables had a couple of problems. The first was that their surface interval schedule was based on the 120 minute compartment, resulting in extremely long surface intervals between dives. The second was they treat multi-level dives as if you had dived to the deepest depth the entire time. This led to shorter than necessary dive times and much longer than necessary surface intervals between dives.
A frustrated diving scientist, Dr. Raymond Rogers, worked with PADI's DSAT branch to see if those limits were necessary for the kind of diving people like him were doing. Using new Doppler imaging technology, he discovered that for typical NDL dives, the 120 minute compartment was much longer than necessary. With the goal of shortening the surface interval and making the scheduling of 2-tank dives as we know them today, PADI created a set of tables for that kind of diving, the Recreational Dive Planner. This planning tool was ideal for 2-tank dives. It used the 60 minute compartment, reduced first dive NDLs somewhat, and added pressure groups to reduce rounding, and this all dramatically decreased required surface intervals. With the goal of adding multi-level capacity, they created the wheel, a planning device that never caught on because computers that did the same thing were introduced at about the same time.
The Recreational Dive Planner was of necessity limited to NDL dives, so it did not include decompression. The thinking was that if divers were going to plan decompression, they would have to use different resources to plan and execute those dives. As a result, the world of diving was divided into recreational and technical diving, with the latter term introduced well after that by Michael Menduno.
US Navy Divers Handbook was the air diving Bible. I still have my July 1979 version. You would have to exceed 40 minutes bottom time at 190 feet for the dive to be considered an exceptional exposure dive and would require a 24 hour surface interval.
Although I hadn't thought about it quite this way, I concur with this historical summary. PADI drove the term Recreational diving with the introduction of the RDP, which only allow for non-NDL dives under the rubric of "emergency decompression." If you look closely at those emergency decompression rules, they are actually pretty close to what the Navy tables require, except for the added penalty of staying out of the water for 6 or 24h....a kind of penalty box for violating NDL.
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