Question about diving doubles with standard reg setup.

[mahjong]

These are all interesting possibilities for diving indies doubles. However, the OP has a non isolation manifold. He would have to break that down by removing the crossbar to do independent doubles. FWIW, I would say it makes more sense to replace the plain crossbar with an isolation crossbar. Nothing lost from your indie doubles, and some options gained.


QUOTE=DevonDiver;6208298]Whilst not "wrong", the advice isn't optimal either.

Seek out a cheap 2st stage and an extra SPG... and you've got a fully functional set of indie-doubles. This'll bring lots of advantages; primarily that you'll have redundancy and surplus gas. I dive indie doubles all the time, including to teach many recreational dive courses. It's very convenient and provides a lot of options and safeguards.

Depending on your air consumption, you could still do two dives from the two tanks:

Dive 1. Breathe from the LHS tank, until you reach your reserve 50bar/500psi etc. RHS tank is for redundancy on that dive.
Dive 2. Breathe from the RHS tank, until you reach your reserve 50bar/500psi etc. LHS is for redundancy - you'll have the 'extra' reserve left from dive 1.

**That's kind what you were asking, but without the reg on the second tank, you'd lose the redundancy. This will also result in the tanks becoming 'imbalanced' as the drained tank gets more buoyant...and the untouched tank remains more negative. This can screw with your trim (and, hence, comfort) in the water.

Alternatively, you could 'balance' the tanks over 2 dives:

Dive 1. Alternate the tanks, breathing a set amount (i.e. 35bar/500psi) from each, before changing... and repeat.
Dive 2. Continue that process.

Lastly, you can utilise both tanks for single, longer/deeper dives:

Breath 1/3rd of the RHS tank. Switch to the LHS tank and breathe 1/3rd of that. Return to the RHS tank and breath that down to a minimum reserve (can be less than 50bar/500psi). You retain the remaining 2/3rds of the LHS as a contingency reserve only. This meets the requirements of 'rule of thirds'.

To configure the regs for doubles, you need:

RHS cylinder: 1x 1st stage. 1x 2nd stage. 1x SPG. (+ 1x LPI if you have a double bladder or want LPI hose redundancy).
LHS cylinder 1x 1st stage. 1x 2nd stage. 1x SPG. 1x LPI.

It works very nicely if you configure it for long (5'-7') /short (22-24") hoses for the regs... and get shorter (24") SPG hoses also.

Here's how mine looks:

Attachment 114862[/QUOTE]

[DevonDiver]

If your dive count is accurate at less than 24, this is really a premature disscussion. If you want to ditch your singles and move to doubles right now, that is a fine plan with guidance. In the continental US people dive indies for specific reasons or missions. Side mount for example. Diving back mounted doubles with out an isolation manifold, or as indies is a slipery slope for you at 24 dives.

If you want to dive doubles find a mentor that is not trying to get in your wallet to guide you and then go diving. Increaseing your dive count in doubles that are set up in a more traditional way will serve you well prior to getting more formal training.

The guy in the video (below) had 14 dives. He spent 4 days/8 dives with me getting used to the doubles (and doing wreck skills). It takes a while to get truly comfortable in the new kit, which you can see the diver concerned was doing - as waterpirate says, a good mentor/instructor can help fast-track that familiarity and competence.

I don't think that double cylinders are the preserve of technical, or otherwise 'advanced' divers... they're a tool for a job. If you're diving wrecks, bad viz, deep water... then you need redundancy. Double cylinders is the optimum redundancy. Isolated manifold is the best, but indie-doubles gets overlooked way too much nowadays.

[Aqua-Andy]

I don't think that double cylinders are the preserve of technical, or otherwise 'advanced' divers... they're a tool for a job. If you're diving wrecks, bad viz, deep water... then you need redundancy. Double cylinders is the optimum redundancy. Isolated manifold is the best, but indie-doubles gets overlooked way too much nowadays.

I'm just wondering why you feel double with isolated is best? This is what I dive when diving doubles but I sometimes wonder. With indie-doubles you do need one more SPG and have to do air swaps but you do have true redundancy, no one failure could render you with out gas. I have no problem isolating but I often wonder how much gas I would loose if a blown tank neck o-ring, manifold o=ring or burst disk failure. Yes the chances of these failures are very slim and that is why your buddy is there and and we plan for this occurring in our gas plan. But is is something I often question.

I watched your video, boy are you spoiled over there. When I did my adv-wreck/cavern it was in 40 degree water and heavy dry gloves. I would love to have the dexterity of those thin gloves the diver in the vid is wearing.

[DevonDiver]

A reg failure with an isolator preserves all your gas, and leaves all that gas accessible. To me, that's more than a fair trade-off to the risk of a catastrophic failure that necessitated an isolator shut-down. The likelihood of a reg based issue, rather than a cylinder/isolator based issue pushes heavily towards the benefits of an isolator system. It isn't as critical for recreational divers, but when operating in overheads and deco, that extra gas can be the difference between life and death.

Reg swapping adds task loading. That should never be under-estimated, especially at a relatively inexperienced level of diving. If it's all you've got to think about on a dive, then it's fine... but if you've got other stuff to achieve/deal with, then it can add significant pressure and/or lead to human errors.

As for loss of gas... I've never experienced an isolator/burst-disk or cylinder o-ring failure. I did have a frozen first stage free-flow once - it drained a 15L tank in under 2 minutes. On a single cylinder, it wasn't enough time to ascend from the depth I was at (allowing for the usual "wtf!?!?" initial reaction etc). A pony cylinder got me up safely for the remaining ascent (from about 12m).

[waterpirate]

[QUOTE=DevonDiver;6208487]

Reg swapping adds task loading. That should never be under-estimated, especially at a relatively inexperienced level of diving. If it's all you've got to think about on a dive, then it's fine... but if you've got other stuff to achieve/deal with, then it can add significant pressure and/or lead to human errors.

This is the issue in a nut shell. Task loading. Diving indies or two tanks with a cheater bar is fine if your sole focus is on the dive. If you add other activities such as spearing, photography, or an oh $hit moment in a new diver it all goes south. My other issue with the cheater bar is that it is worse than indies in regard to handling a failure.

I am a huge proponant of identifying divers early on who are headed to bigger things, not just the holiday diver. These divers need to meet a bp/w and isolator doubles sooner than later.
Eric

[Aqua-Andy]

A reg failure with an isolator preserves all your gas, and leaves all that gas accessible. To me, that's more than a fair trade-off to the risk of a catastrophic failure that necessitated an isolator shut-down. The likelihood of a reg based issue, rather than a cylinder/isolator based issue pushes heavily towards the benefits of an isolator system. It isn't as critical for recreational divers, but when operating in overheads and deco, that extra gas can be the difference between life and death.

I would not say it preserves "all your gas" it only preserves the amount you have after shutdown. If you're in a tight restriction or for some reason can't get to your valves and can't get it shutdown very quickly you may end up worse off than if you had indie-doubles. As for that extra gas being the difference between life and death, either you did not plan your gas usage correctly and need to reconsider the type of diving your doing or things have gone terribly wrong.

Reg swapping adds task loading. That should never be under-estimated, especially at a relatively inexperienced level of diving. If it's all you've got to think about on a dive, then it's fine... but if you've got other stuff to achieve/deal with, then it can add significant pressure and/or lead to human errors.

Yes it does add task loading, but most dives should be able to be done with one reg swap. breath off tank 1 for 1/3 of the gas supply in that tank, swap to tank 2 and breath 2/3 of that tank.

As for loss of gas... I've never experienced an isolator/burst-disk or cylinder o-ring failure. I did have a frozen first stage free-flow once - it drained a 15L tank in under 2 minutes. On a single cylinder, it wasn't enough time to ascend from the depth I was at (allowing for the usual "wtf!?!?" initial reaction etc). A pony cylinder got me up safely for the remaining ascent (from about 12m).

I guess that puts the real failure point of doubles w/manifold is the diver being able to get the isolator shut down before loosing too much gas. As you said you can loose it very fast. Like I said before with independent doubles no one malfunction can leave you with out enough gas to make it out from an overhead environment (actual or theoretical). I guess this is the fun of tech diving, weighing the costs and benefits of the tools we have at our disposal.

[DevonDiver]

I would not say it preserves "all your gas" it only preserves the amount you have after shutdown.

Ok... to pick straws... it preserves all your gas "after shutdown". In contrast, any failure on indie-doubles means you lose the entire contents of that tank.

Shut-downs are a fundamental part of tech training (or generic preparation for diving doubles). Most agencies set standards (speed) for achieving that. Yes, that speed could be retarded by physical restrictions (which goes way beyond the scope of this thread).. but a suitably experienced diver should be cognizant of that situation at the time.

Yes it does add task loading, but most dives should be able to be done with one reg swap. breath off tank 1 for 1/3 of the gas supply in that tank, swap to tank 2 and breath 2/3 of that tank.

Ideally, a minimum of 2 swaps. Breathe 1/3rd of tank 1. Swap. Breathe 1/3rd of tank 2. Swap. Breathe remainder of tank 1.

Many divers choose to swap more often to assist with trim etc. Same principles with sidemount diving.

...and that's for a single dive on doubles (a tech dive realistically). Balancing a set of indie doubles over 2 dives (as the OP is likely to do) would require more swaps.

I guess that puts the real failure point of doubles w/manifold is the diver being able to get the isolator shut down before loosing too much gas.

Absolutely, but it's a pretty minimal risk. Even then, isolator shut-down should take ~10 to 20 seconds max.

I guess this is the fun of tech diving, weighing the costs and benefits of the tools we have at our disposal.

Absolutely!

[DaleC]

I dive independant St72's and can get away with one reg swap. Left tank to 1000, switch and right tank to 1000. This is for dives to 100' (which is what I use this rig for mostly) and allows for minimum gas in both cylinders at max depth/duration.
I also find breathing left first allows me to have the long hose in my mouth (for donation) during the second half of the dive when an OOA most likely will occur. During the first half it is clipped off on my right chest D ring but the boltsnap is held on the reg with small, tear away tubing, in case it needs to be deployed. Just a little extra reg management to be considered.

One can look at reg swaps as task loading or as building comfort surrounding taking the reg out of your mouth underwater. It also ensures that both your regs are in good working condition because you breath both on every dive. Next time you are in a group setting take a sly look at how much attention most divers pay to the reg they are planning to donate to you in case of an emergency

[MX727]

Ideally, a minimum of 2 swaps. Breathe 1/3rd of tank 1. Swap. Breathe 1/3rd of tank 2. Swap. Breathe remainder of tank 1.

Mathematically, it's the same thing. Breathe 1/3rd of Tank 1, swap and breathe Tank 2, or do it your way. In any case, you have a tank with 2/3rds reserve.

[DevonDiver]

Mathematically the same result, but functionally, it's more optimum should a failure happen during the dive, especially if you're configured with long hose and may need to share air in a restricted space.

If a failure occurs, or you need to share air, at X, Y or Z stage in the dive...

I care more about gas management on the dive, than my cylinder state when I get back on the boat

It's also the method I've been taught on several courses - so it's the method I use within team planning etc, to ensure standardisation.