One or Two first stages?

[Angelo Farina]

I am servicing the whole number of regs of my family (4 divers), and I would like to setup at least two complete setups in an "advanced" configuration more or less adhering to GUE/DIR recommendations.
I was always in favour of the concept of "donate the primary". When I and my wife were working as instructors and divemasters, in the eighties, we did always use this approach with our customers, keeping the yellow reg in our mouth (ready for them) and the not-coloured one as backup.
Here you see my wife at Ikoga (Maldives) in 1986:

It was before modern "jacket-style" BCD appeared...

But let's come to current setup for our regs. Firsts stages are all Scubapro MK5 - MK10, and 2nd stages are all old 109-156 (transformed to BA with S-wing poppets).
They are differentiated by the colour of the hose (yellow for primary-donate and black for secondary-personal backup) and of the exhaust rubber baffle (orange for primary-donate and black for secondary-backup).
I had some troubles finding the two long yellow hoses, but this is not my main concern, finally I got them.
My question regards the first stages. I could use the four MK5 for those two setups: two fully independent regs, connected with the two valves present in our 15-liters cylinders.
But I see that GUE/DIR recommend to use just ONE first stage when employing a single cylinder, and I do not really understand the reason of this choice, which appears significantly less safe than having two fully independent first stages, controlled by two fully independent valves.
Can someone explain me why using just one first stage is better - more safe - than using two?
Is for the "minimal" Hogarthian diving philosophy?

 

[helodriver87]

The assumption is that if you're diving single tank, you're doing diving that doesn't really require redundancy (OW no deco type stuff). There's nothing stopping you from just surfacing.

If you need redundancy, an H valve doesn't truly provide it, as a burst disc or neck o ring (for example) can leave you in the same situation as a busted first stage. Better to just use doubles and have the ability to isolate.

Doubles may be slight overkill for the light technical/deeper recreational application, but a set of 85s is pretty easy to handle and offers true redundancy.

 

[shurite7]

But I see that GUE/DIR recommend to use just ONE first stage when employing a single cylinder, and I do not really understand the reason of this choice, which appears significantly less safe than having two fully independent first stages, controlled by two fully independent valves.
Can someone explain me why using just one first stage is better - more safe - than using two?

You are missing one important aspect - your buddy, who happens to be using a separate first stage. The point is, you and your buddy should be practicing your skills, especially for air donation, and planning your dives for two people; meaning enough gas supply for one diver to bring up two divers (you and your wife for example). UTD has a nice video on how to plan a dive using RBG - Rock Bottom Gas. I would also add, keep your gear properly maintained and serviced to minimize equipment failures, and know how to use it.

Some will advocate using a separate system - pony bottle, for example - when using independent setups. Others, such as Irvine, will state this is a solution to a problem that doesn't exist because you and your buddy should be practicing your skills and planning your dives appropriately.

In short, skills and planning with your buddy takes care of the issue of having independent first stages, at least for recreational diving with a buddy.

 

[Bigbella]

I was taught to ensure redundancy in just about everything; and that also included multiple first stages and additional tanks. An earlier comment about a potential valve failure, was the instructor's -- who had experienced such a thing in the Navy -- rationale . . .

Others, such as Irvine, will state this is a solution to a problem that doesn't exist because you and your buddy should be practicing your skills and planning your dives appropriately.

Sadly, we're that truly the case, I would not be required to carry along an octopus, while on vacation boats . . .

 

[Angelo Farina]

I understand that a single first stage, with DIN connection and properly maintained, can be thought to be safe enough for rec diving within NDL.
But my question is why GUE/DIR forbid to use a dual first stage setup...
I accept that it is not mandatory, but why not allowing it, for additional redundance?

 

[runsongas]

h or y valves fill a niche for regulations in some countries where two first stages are required for cold water diving. you don't see them too often outside of that. in that particular environment, having the extra regulator is more useful than the small possibility of a valve failure. and the h or y valve fulfills the requirement at a lower cost than a pony tank or doubles.

 

[barth]

But my question is why GUE/DIR forbid to use a dual first stage setup...

Why do you think GUE forbid to use single tand and a dual fist stage setup?

"Students may use a single tank/cylinder with a single- or dual-outlet valve."

https://www.gue.com/files/Standards_and_Procedures/GUE-Standards-v9.pdf
Appendix A

 

[barth]

DIR-diver.com - Singles, H-valves or doubles?

This is also my opinion.

 

[Angelo Farina]

DIR-diver.com - Singles, H-valves or doubles?

This is also my opinion.

Ok, this explains it quite well.
Fine, it simplifies the service work I am doing...

 

[Angelo Farina]

Why do you think GUE forbid to use single tand and a dual fist stage setup?

I was reading here:
https://www.gue.com/files/Standards_and_Procedures/GUE-Standards-v9.pdf
Appendix A, point 9.1.
It appears that a single cylinder can have two posts and two valves, but just a single first stage.
From what I am reading, it seems that the assumption is that singles have less capacity than doubles.
In the case of me and my wife, this is the opposite.
Our old Aralu twins (9+9l, 170 bar) contain just 3000 liters, are light and streamlined, and we were using them for small depth.
Instead our singles (15l, 250 bars) contain 4000 liters, and we prefer them for deeper dives with potential deco.
I was not understanding why use a single first stage for more technical dives, and two when diving shallowever...
So all this originates by the assumption that a single has smaller capacity than twins, which in our case in untrue.
Said that, as all I want is to have, as much as possible, a standard setup, I will configure our systems for using just a single first stage. Less work to be done for servicing just two instead of four!