Twin tanks: manifold or redundant

[Nemrod]

Is it okay to dive twins with two redundant rigs or is it better with a manifold? Part of the reason why I like the idea of twin tanks is that you can potentially set them up independently. However, now that I think about it, it seems that I might run into trim problems when there is significantly more air in one tank versus the other. Also, I don't recall seeing anyone dive twins w/o a manifold.

Independent twins have been used for many years and though currently out of favor now that we have isolation manifolds the concept remains valid.

No, you will not have a "trim" problem. Besides, the process is to breath down both tanks, usually switching over at 750 or 1,000 psi intervals always maintaining the final 1/3 (Rule of Thirds) in both tanks for redundancy (750 or 1,000 psi depending if LP or HP).

N

 

[DCBC]

Is it okay to dive twins with two redundant rigs or is it better with a manifold?

An isolation manifold is preferred for back-gas. There are no trim issues and they can be shut-off independently. All gas is available at all times; the way to go.

 

[clafrieda]

Wow, very informative responses. Thanks! Here's another question. What happens when you have a full tank and a 1/2 full tank on a manifold? Does it suck more from the fuller tank initially?

 

[shoredivr]

Wow, very informative responses. Thanks! Here's another question. What happens when you have a full tank and a 1/2 full tank on a manifold? Does it suck more from the fuller tank initially?

With the isolator valve open, you have two tanks joined by the isolator, so anything you breathe from either regulator comes out of the combined gas in both tanks. Gas pressure throughout the two joined tanks remains the same.

The only time you'd shut down the isolator, which isolates one tank from the other, is if you had a problem and were losing gas from one or the other regulator(s) or tanks valves etc, or if you were doing a valve drill. In the case of having a gas leak, the dive's over, and something needs to be fixed.

If somebody filling the twins didn't know any better and closed the isolator on you while filling , resulting in one tank filled, the other not, then yes, you hear the hiss of pressure equalizing between the tanks. Hopefully this happens in the shop and not on the dive boat where you now have a short fill, and could be the dive's over before it started.

 

[Nemrod]

Wow, very informative responses. Thanks! Here's another question. What happens when you have a full tank and a 1/2 full tank on a manifold? Does it suck more from the fuller tank initially?

That just really is not going to happen unless you made it happen.

Once upon a time twin tank manifolds did not have an isolation valve or in some cases they were only partially isolating, you could shut off a regulator but not isolate the tanks. Modern isolation manifolds allow both tank and regulator isolation. Thus it is rare now for divers to encounter independent twins, though, there are people who still dive them, especially for solo.

N

 

[DaleC]

I dive backmounted independant twins, Al 80's and St 72's using a mounting system that allows me to swap out a cylinder easily in the SI. As noted there are pros and cons to each system. Here's how I see it:

Manifolded twins:
Requires valve shutdown skill in a stressed situation.
Redundancy is dependant on isolator shutdown speed and volumes when shutdown occurs.
Cannot switch out cylinders.
Can utilize more available gas.

Independant twins:
Requires reg switching skill in a non stressed situation.
Fully redundant.
Allows switching cylinders for repetitive dives.
Cannot fully utilize all available gas as reserves levels are left in each cylinder (more about this later).

For the following I am comparing Al 80's (independant or manifolded) and am rounding the volume off to 80cuft at 3000psi to make the math simple.

I do non penetration rec solo dives and generally use a rock bottom reserve value of 1000psi. If I am doing one long dive with independant twins I will leave the water with 2000psi (1000X2) minus the ascent gas used. In a sense I am denying myself access to 53 cuft of gas. For me that is not an issue but someone with a manifold would certainly be able to access more gas than I (which may be important in some circumstances).

If I am planning two dives I leave 1500psi in the left tank and ascend on the right tank at 1000psi. During the SI I then swap out the right tank and begin dive two with 1500+3000psi (or approx. 120cuft.). If I apply the same rock bottom values I can use 2500psi (66cuft.) before beginning my ascent.

Some one on manifolded doubles will have the advantage of more available gas on a single dive with a comfortable buffer for a valve shutdown but not so on a second repetitive dive. If they use half the doubles on the first dive they begin dive two with 1500psi X2 or 80cuft (vs my 120). If they use a rock bottom value of 1000psi (500psi on their spg) and have a failure near the end of their dive they could potentially isolate with less than 500psi gas available to them (you have to follow the logic but it's there). You also have to be pretty fast on the isolator or you could lose all backgas in that scenario.

Some divers will say that someone shouldn't start a second dive with 1500psi in manifolded doubles because of that but I see it quite often. To be fair, most people are using 100's or 130's around here so that gives them more gas for dive two.

For someone diving a buddy team having more gas available for the dive may be more important (because the buddy provides redundancy). I solo so having an adequate reserve is more important to me. Wookies comment on closing the isolator would achieve the same effect but most manifolded doubles only use one spg on the left post thus leaving one blind as to the volume on the right post. I know some people open and close the isolator to equalize the tanks but for me , by that time, I might as well just do the reg switches.

Another benefit for me is that my mounting system allows the tanks to be positioned right close together and thus I can use my Apexs 32# single wing for both double and single tank dives. I tested the lift capacity quite carefully and wouldn't suggest that someone else try to use a single wing for doubles unless they also do their own testing.

 

[halocline]

The only time you'd shut down the isolator, which isolates one tank from the other, is if you had a problem and were losing gas from one or the other regulator(s) or tanks valves etc, or if you were doing a valve drill. In the case of having a gas leak, the dive's over, and something needs to be fixed.

Actually, if you have a problem with a valve or regulator, you don't close the isolator, you close the valve on that post, shutting off flow to the regulator and any leak after the valve seat. You leave the isolator open and thereby have access to the gas in both cylinders.

Independent doubles may have some theoretical advantages over isolation, but real-world reliability and redundancy is not one of them. The VAST majority of gas leaks will come as a leaky regulator or valve (post seat).

Regarding the shutdown of the isolation valve, it's very easy to just leave this valve barely cracked open and still get full equalization between the tanks, and allowing for an immediate closure with a twist. As far as task loading, let's be honest here, if you're diving doubles in a technical, i.e. non-OW environment, and closing the iso valve represents a challenge, you should probably re-think your training level.

 

[DaleC]

Independent doubles may have some theoretical advantages over isolation, but real-world reliability and redundancy is not one of them. The VAST majority of gas leaks will come as a leaky regulator or valve (post seat).

I don't argue with anyones decision one way or the other but I think this is not correct. With independant twins any gas leak (whatever the source) will only result in the loss of one tank. If you follow your gas plan you should always have enough gas to exit/surface. With manifolded doubles a gas leak (whatever the source) could result in the loss both tanks. That is just an objective fact. Whether someone feels they have the skill to say they will "always" be able to isolate is subjective.

Not arguing against manifolds, just that one detail.

 

[Sas]

I don't argue with anyones decision one way or the other but I think this is not correct. With independant twins any gas leak (whatever the source) will only result in the loss of one tank. If you follow your gas plan you should always have enough gas to exit/surface. With manifolded doubles a gas leak (whatever the source) could result in the loss both tanks. That is just an objective fact. Whether someone feels they have the skill to say they will "always" be able to isolate is subjective.

Not arguing against manifolds, just that one detail.

Yes it is possible to lose all of your gas in a manifolded system if you can't isolate, which is why I think it suits buddy diving more than solo diving... Your buddy should be diving with enough gas to get you out in the (unlikely) event that you lose all of your gas.

I think both systems have their place.

 

[DaleC]

Yes it is possible to lose all of your gas in a manifolded system if you can't isolate, which is why I think it suits buddy diving more than solo diving... Your buddy should be diving with enough gas to get you out in the (unlikely) event that you lose all of your gas.

I think both systems have their place.

Yes. I would stress that I dive this system primarily because I solo. I also think Wookies system of closing the isolator and occasionally opening it to equalize would work just as well as IT's. I also recognise that there is a transition point at which having access to all your gas becomes more important that true redundancy (long penetrations or deco obligations). At that point the benefits of IT's diminish IMO.

The best advice is to know the pros and cons of each system and plan accordingly either way. I always enjoy this topic in particular as a good (healthy) debate can bring out little things I might not have considered before.