Diving with redundancy.

[JBFG]

Oh, really, eh?

It’s the civilized way. We are not all barbarians eh.

 

[jadairiii]

And to be clear, with recreational diving you should not need a bunch of convoluted "redundancy". Service your gear, use a DIN first stage and monitor you gas consumption. You will be good to go.

 

[Marie13]

It’s the civilized way. We are not all barbarians eh.

Are you saying only the Québécois are barbarians?

 

[johndiver999]

You know, my wife has told me for 38 years that if I did not have a job and take out the trash, I would be completely redundant. Or superfluous. So, every time I dive, I am diving with redundancy.

For some reason, and specifically with respect to the wife, I think I might prefer to be viewed as superfluous rather than redundant.

 

[Angelo Farina]

Here in Italy the standard tank you can rent everywhere is 15 liters, 232 bars, H or, better, Y valve (2 independent valves).
Usually the posts accept both DIN and Yoke.
In the past, when Yoke was the standard, it was highly advisable to employ two independent first stages, as the chances to have an O-ring failure were significant.
As this event was relatively frequent, training here was including the manouvre of pulling the tank above your head in front of you, for closing quickly the valve of the failed post before all the air is lost.
But then DIN became the standard. With DIN, the risk of an O-ring failure are quite remote, and in the rare case the resulting air loss is modest.
Hence it became standard to use a single first stage and two second stages. I do not suggest to use an octopus (a crap yellow second stage), better to have two identical high end second stages, so you can safely employ a "primary donate" approach, and alternate the usage of the two regs for ensuring that they are always both fully functional.
Eliminating one of the two first stages diminishes slightly the redundancy, but makes the system lighter, simpler and cheaper, allowing to get a single, better first stage instead of two not-as-good.
Still I never rent cylinders with just one valve and post. Having two posts opens possibilities of different mountings of the first stage, and gives you some redundancy in case one valve refuses to open or starts leaking from the O-ring of the knob...

 

[PfcAJ]

It’s the civilized way. We are not all barbarians eh.

Why dive with them on your back when you can put them on your stomach! Brilliant!

 

[Open Ocean Diver]

Here in Italy the standard tank you can rent everywhere is 15 liters, 232 bars, H or, better, Y valve (2 independent valves).
Usually the posts accept both DIN and Yoke.
In the past, when Yoke was the standard, it was highly advisable to employ two independent first stages, as the chances to have an O-ring failure were significant.
As this event was relatively frequent, training here was including the manouvre of pulling the tank above your head in front of you, for closing quickly the valve of the failed post before all the air is lost.
But then DIN became the standard. With DIN, the risk of an O-ring failure are quite remote, and in the rare case the resulting air loss is modest.
Hence it became standard to use a single first stage and two second stages. I do not suggest to use an octopus (a crap yellow second stage), better to have two identical high end second stages, so you can safely employ a "primary donate" approach, and alternate the usage of the two regs for ensuring that they are always both fully functional.
Eliminating one of the two first stages diminishes slightly the redundancy, but makes the system lighter, simpler and cheaper, allowing to get a single, better first stage instead of two not-as-good.
Still I never rent cylinders with just one valve and post. Having two posts opens possibilities of different mountings of the first stage, and gives you some redundancy in case one valve refuses to open or starts leaking from the O-ring of the knob...

Initially I thought the H valve would give me redundancy and to some degree it does when it comes to first stage failure, however remote. Since I rent tanks this really isn’t an option. The thought of relying on my buddy (with all the training we do) that maybe 40 feet away looking at something amazing and all of a sudden I have a first stage failure (out of air situation) at 100 ft, yikes. A pony bottle seems to be a good option. I believe under these circumstances however remote, if we have an out air situation panic will happen and bolting to the surface is dangerous.

 

[WinfieldNC]

Doubles give you more gas but what other redundancy does it give you that an H-valve doesn't? You blow an o-ring you shut off that post with either H-valve or doubles. You have a first stage problem you shut off the problem post. You have a second stage problem (free flow) you shut off the problem post. You can take AN/DP through TDI with an H-valve and it meets redundancy requirements. Doubles would offer redundancy if you somehow punctured a tank but that almost impossible. You can run an SPG off both posts in either set up. If you're using doubles with an open manifold you basically have one big tank, if you're not watching you SPG and run out of gas then you're in the same situation as having a single cylinder. What am I missing? I really mean that... is there something I'm not thinking of?

 

[MichaelMc]

If you're using doubles with an open manifold you basically have one big tank, if you're not watching you SPG and run out of gas then you're in the same situation as having a single cylinder. What am I missing?

If you run doubles closed, you have independent tanks and a 'do nothing but switch to the other reg reserve'. But you have to switch regs during the dive to preserve the reserve (basically like sidemount). Yet, if small, they fill like a single tank.

If you run them closed, breath off the right, and occasionally equalize by opening briefly, you get the 'do nothing but switch to backup reg reserve' without having to switch regs during normal dives. And you get frequent re-training/OOA drills if you neglect checking your right SPG.

Tiny doubles (AL40/LP50) are no more weight than comparable single tanks. If you want a hard reserve either of the above is a compact way to go. Look at the weight and buoyancy of 2xAL40 and 1xAL80 and consider if the few pounds of brass/steel for the extra first stage, manifold and bands would just replace some lead. Isolation manifold 2xAL40s are my 'tank' for 80 cu.ft. of gas backmount.

They'd be hard to rent. Depending on you, rental tank sizes, and beach or boat, sidemount also gives a hard reserve. As do ponies..., Sidemounting Stage/Pony Bottle

 

[Angelo Farina]

If you run doubles closed, you have independent tanks and a 'do nothing but switch to the other reg reserve'. But you have to switch regs during the dive to preserve the reserve (basically like sidemount). Yet, if small, they fill like a single tank.

I love doubles.
That is what was in common use when I started diving, 45 year ago. I and my wife stil own two set of alu doubles, Technisub Aralu, 9+9 liters with double post and pull-safe reserve.
However we are not using them anymore, also because these old alu cylinders were partially unsafe. We own 15-liters steel, with double post and reserve, which are not so streamlined, not so redundant, but still perfectly fine for rec diving (and due to age we do not anymore go down to 50-60 meters as when young).
However the point for me is another: when I RENT a cylinder, do I ask for one with double posts? the answer is ALWAYS.
The cost for the rental is the same. The added redundancy is a benefit, even if using a single first stage (of course with DIN mounting - with Yoke mounting it would be unsafe using only one first stage). The additional weight of the valve saves one lead in the belt. I really do no see the advantage of renting a cylinder with just one post...
Why the hell people rents (or buy) them?