Redundancy on twin sets.

[stuartv]

Hogarthian AFAIK describes more than just a harness and wing. It also includes things like the long hose, bungeed necklace etc.

I believe the intention was to describe the reg and hose setup.

I gotcha. I still think it's worth noting that "Hogarthian" does describe a lot more than just reg and hose setup. Most of which is irrelevant to whether doubles are safe for diving solo. And the 7' hose part is also moot for solo diving. So, saying that a Hogarthian rig is needed to be safe when diving solo is like saying that you need a 2017 Volvo XC70 to drive safely when all you really mean is that you need any modern car with a seat belt and driver's side airbag. You don't even need Hogarthian hoses much less a whole Hogarthian rig.

 

[stuartv]

It is not possible to lose all your gas because one manifold failure.

If you think it is possible, please describe in details which o-ring or whatever should fail to lose all your gas.

You're swimming through a cave (or other overhead), you accidentally ram your isolator knob into the ceiling and the whole isolator valve shears off.

I say this because my dad started diving in north Florida in the 60s. We are from Tallahassee. My dad dived a lot of the caves around there back in the day. And lived to talk about it. He has told me stories of shearing off valves on cave ceilings.

 

[Littlerayray]

Ok so maybe not the entire hog setup you can negate the 7ft hose but everything else is a pretty good idea

 

[stuartv]

I don't think the Hogarthian requirements for 1 continuous piece of webbing, exactly 3 D-rings, and no padding make any difference, either, when it comes to solo diving with doubles. In other words, almost none of the characteristics that define a rig as Hogarthian are really needed or necessarily recommended for diving solo with doubles. A lot of those things are recommended, in general, but the reasons have nothing to do with diving solo with doubles. At least, that's how it seems to me. In other words, bringing the term Hogarthian into this, just muddies the waters. It carries a lot of baggage, so to speak, that has nothing to do with the OP's question.

Yes, doubles are safer for diving solo. You NEED a redundant air source for solo diving - whether it's back mount doubles, side mount doubles, or a back mount tank and a pony bottle. You need a 1st and 2nd stage on each cylinder. Beyond that, Hogarthian or not is completely irrelevant.

 

[KevinNM]

Is it not possible for the manifold valve knob to fail? Like I said, a small chance but I don’t use the word impossible when it comes to failure modes.

I am on the phone now so searching the Board is challenging but I seem to recall threads where people have broken manifolds on wrecks caves etc.

I'd argue there is far more danger of you drowning due to going ooa with independent tanks than of a problem with the manifold. Manifolds are ridiculously strong. AJ once mentioned losing a set of doubles out of a truck bed at highway speeds. Destroyed the regs but the doubles set was fine.

There is a failure mode that can result in a slow continuous leak, but I've never heard of it actually happening in the field.

 

[CptTightPants21]

You're swimming through a cave (or other overhead), you accidentally ram your isolator knob into the ceiling and the whole isolator valve shears off.

I say this because my dad started diving in north Florida in the 60s. We are from Tallahassee. My dad dived a lot of the caves around there back in the day. And lived to talk about it. He has told me stories of shearing off valves on cave ceilings.

Isolators are very resilient and, as others have stated, people have hit them while scootering and not had any issues. Despite all the mentions and discussions online, I have never heard of a single sheared off isolator failure happening in real life.

 

[tbone1004]

It's not a matter of safe but it's safer also it's only safer if your manifold has an isolation valve and your rig is setup hogarthian style

of note, while he does have some that have them on his newer tanks, many of Hogarths actual sets of doubles do not have isolation manifolds on them.

@stuartv if you own your own gear, and can handle it, doubles is a lot more convenient than singles plus a pony bottle. Less work to get in and out of the water, one less overall tank, no moving of the rig from bottle to bottle. Downside is not a lot of boats can handle doubles, but if you're going to bother with a pony bottle and have your own gear, may as well just use a set of doubles.

@RainPilot it is possible, but exceptionally rare. With proper orientation of the knob, the risk of shearing off one of the main valve knobs is significantly higher, has happened many times in a cave, and is the main justification for having an isolation manifold. To do that, you pretty much have to scooter into something quite solid, i.e. the keyhole in Ginnie, door of a wreck, etc. I have never heard of the isolation knob itself failing outside of the normal failure mode of all valves which is when you open them they don't seal because the o-ring failed, but I've never had one not seal when it was closed *the seat holds everything back until opened, then a single o-rings holds the gas back from the valve stem

 

[RainPilot]

Can you elaborate on that failure mode? I’m hearing that the o ring failure means it can’t be closed again? Or am I missing something?

 

[Gareth J]

Just to add to the debate.

1. A twinset, doesn't need a backplate and wing. As long as the Stab' jacket can handle the weight.

2. A twinset may, or may not, have a manifold. There are plus and minus on both sides manifold/no manifold.

3. It isn't just caves that can sheer an isolator valve, I had a buddy sheer one in the sea in adverse conditions when the boat pitched on to him. On the upside, it was the manifold that broke -- not his head.

4. Personally, I don't like crossed hoses, left cylinder - all the hoses to the left, right cylinder all the hoses to the right.


Back to the original question
A Twinset gives you GAS redundancy, It doesn't make you a necessarily a solo diver. Solo diving is a totally different thing. It is about more than just redundancy, it is also about being defensive in your diving. I will do far more aggressive things when diving with a buddy or team than I will do if I am solo.

These days a do very little solo diving, mainly because I mostly dive CCR. There are particular risks with CCR diving that are not present on OC. That said, these days I would potentially dive solo on my unit on some of the low risk dives I do. I tend to carry a camera on most of my dives, my attention to my buddy is sometimes poor, so to some degree I am diving solo. My late partner was a much better photographer than I am, we often separated during dives, due to focusing on our subjects rather than each other. More often than not it was me, not her that noticed the separation, and me that would start searching for her. She dived a twinset, I dived a CCR with bailout.

Having gas redundancy will not help if you are caught in line, trapped, injured, black out, or struggling to exit the water. With all of these a buddy may well make the difference.

Gareth

 

[tbone1004]

Can you elaborate on that failure mode? I’m hearing that the o ring failure means it can’t be closed again? Or am I missing something?

when the valve is closed, the seat *#2,3* sits against a crown orifice and stops the gas from moving around. When you open the valve, o-ring #17 seats around the stem #8 and against the bonnet #4 and is what seals the valve knob but allows gas to flow to the outlet. When o-ring 17 fails, the valve will be perfectly fine when closed, but as soon as you open it, it leaks out and is seen from either end of the valve knob. You can fix that without draining the tank thankfully, but it is still annoying.