ind. dual tanks not using manfold w/ ffm

Question
QUESTION: hi mike, I'm using an ocean reef full-face mask. I've seen a diver in the Caribbean that used 2 1st stage regulator connect together with an armored flexible hose, both on the hi press & lo press sides joined to w/ 3 3-way connectors;

1- for the tanks to balance through the 1st stage hi press port, the other

2nd one was also on hi press side for him to read hi dive computer on both tanks as one, via the 3-way connector/3-way valve, so he can shut off tank or the other, or just uses the 3-way connector/valve open to use air  Simultaneously.

3rd is same as above for the BCD hose connector.

now because I use a ffm, this looks like the way I should go, but a lot of shops said there used to be a set up like that available, but no one has seen it for a really long time.

so is it possible to use to independent 1st stages connected together via an armor flex hose?

and to have a 3-way valves to connected to a Full-Face masks 2nd stage regulator?

so you wouldn't have to rip the mask off in order to use a 2nd stage for normal use.

yes I would have a redundant air supply in not 1, but extra regulators for emergency use only.

thanks

al c

ANSWER: Hi Al

Your description of the setup is logical and would certainly work but I have a couple of problems with it.

First, there are a lot of hoses and connectors involved and each one is a potential point of failure.  Your chances of having a leak failure are multiplied several times with such a rig.

Secondly, it's complicated and could pose problems if you needed to make some emergency bailout maneuver.

Thirdly, why not just get a manifold designed for double tanks?  They are set up for isolation of either tank and many are fitted for the installation of a pressure gauge.  There is a reason why mainifolds are made of solid metal and do not include flex hoses.  It's for reliability and to reduce the possibility of the sort of leak failures that I alluded to in my first point.

Fourth, I don't think you need to have redundant 1st stages since the liklihood of one failing is extremely small.  Cave diving and some other technical diving encourages totally redundant air systems due to the divers extreme isolation from the surface but that's not the case with the type of work you're planning to do.

Fifth, the armored hose is fine but will not protect the hose against the most common point of failure...at the connection between the hose and the end fittings.  Generally, the armoring is removed at these points of connection and the flexion at those points eventually leads to failure.  Hose protectors are designed to minimize this but it still happens.

I suggest going with a standard twin tank manifold.  That's probably not what you wanted to hear but I'm looking at it from a reliability and safety standpoint.

Happy Diving!!

Mike

---------- FOLLOW-UP ----------

QUESTION: I would love to use a manifold but the costs would be hi in both purchase and weight & mounting issues when travelling. I would have to buy at least 3 set of manifolds with R/H & L/H valves to be mounted on tanks that I'll be renting on location, and my luck wouldn't have the correct threads to match.

Do dive shops around the globe all keep twins available for use, if so then my questions would be answered, and i would need to be figuring this out, but do they would probably cost more or not?

i just came back from a trip in Belize, and they didn't have any doubles available, so I'm guessing the same for others.

ANSWER: Hi again, Al

Very few, if any, dive operators will offer a twin set-up.  From your question, I'm assuming that you're looking for a twin set-up that includes the tank valves.  This is not how many doubles are set up.  Each tank will have a standard yolk or DIN valve and the manifold simply connects to the tank valves and binds the two tanks together.  I have a couple of divers who have taken a manifold to our Caribbean destinations and then used two standard 80 cu. ft. tanks to set up as doubles.  Tanks are simply changed out between dives.  The manifold is about 14 inches long, made of nickel plated steel, with appropriate connections (yolk or DIN) on each end and an isolation valve and first stage attachment point in the middle.  Not too expensive but somewhat hard to find.  The set-up also comes with a spacer and a clip which will hold the tanks together at a proper spacing from each other.  This entire package is then mounted to the BC.

Keep in mind that the BC must also be set up for doubles.  It requires a special back plate which will secure the double set-up.  Check with the BC manufacturer to see if a double tank conversion is available for your BC.

Twins can be awkward to handle since the weight is doubled although it's not a problem underwater.

I haven't tried to locate a twin manifold like the one I described above in several years but I'd try Googling "twin scuba tank manifold" or something similar and see what you can find.

One of my suppliers, Trident, offers a twin set-up like the one you described but it requires the use of their own tank valves to match their isolation set up and does not have a centrally mounted first stage attachment point.  The use of this set-up would require replacing both tank valves and I doubt that any dive shop operators will be willing to do that.

As an alternative, look for an oversize single like a 120.  That's only 40 cu ft less than a set of twin 80's and won't require all the set-up problems.  It's something to think about.

Al, I hope this helps.

Mike

---------- FOLLOW-UP ----------

QUESTION: I use to use a manifold that connected two singles together over 20 yrs.ago but haven't seen them at all. I've been googling them for the past 3 months, but with no success. I wish I kept my old setup, but got involved in other hobbies, and now back into scuba.

I even had my tanks mounted inverted-upside down, which made it easy to reach the individual tank valves, but I haven't located the "protected stand" that would mount to the neck of the tanks to protect the valves themselves. is there any specialty shop or manufacturer that is in this field?

Answer
Hi Al,

I'm familiar with the set-up you're talking about.  The inverted arrangement was popular for twins back in the early 60's after the "single-hose" regulator was introduced.  Prior to that we all used double hose regulators.  These could not work with inverted tanks because of the pressure differential with the regulator (first and second stages) mounted just above your butt instead of behind your head.  The regulator would either free flow or not work at all depending you your orientation in the water.  The invention of the "single-hose" permitted the first stage to be mounted remotely from the second stage and eliminated the problems encountered with the double hosed set-up.  

The tanks of the early 60's were primarily steel 72 cu. ft. tanks filled to 2250 psi so having a set of twins really extended your dive time.  Manifolds were common back then.

The protective shell reminds me of the equipment used by Jacques Cousteau back in the 60's!!

Anyway, Al, I'm not familiar with any manufacturer who still makes this stuff.  All I could do is the same internet searches you've already done and I'd probably get the same results.  You could, of course, have a stand custom made for an inverted twin set-up but we're getting into some serious money just to add 40 cubic feet of air to a standard 120.

Something else you might consider is a side mount arrangement used by technical divers.  It involves two separate tanks with regulators mounted on each side of a special BC made for the purpose.  It's much more cumbersome and expensive but it is commonly available.

That's about all I can do for you on this question.  Unless there's something I've missed, I still think your best overall solution to the air problem is a 120 cu ft tank.  Anything else requires a considerable expenditure along with a lot more weight and hassle.

Best wishes for a successful solution.

Mike