Cross Bar on doubles?

[bam540]

Most local divers (I live in New Jersey) who dive doubles leave the tanks isolated from one another. One reg/1st stage, pressure gauge per tank. That seems to me to be the safest method. It gives you two seperate air supplys (run the first one down to about 1500 psi, then switch to the second reg/tank.) The first with 1500 psi now becomes your emergency reserve. Should you encounter an emergency situation such as a free flow, etc. you will still have one tank with enough air to make a safe accent. Ive seen alot of doubles in pictures and online that are connected by a cross bar manifold. Even though there is usually an isolation valve in the middle, having to reach for the valve in event of an emergency seems like something I dont want to have to worry about. Can anybody tell me what the advantages of having the two tanks connected are, and are there any disadvantages to having two stand alone doubles? Thanks.

 

[Aquamaniac]

This subject always stirs up lots of emotions, and no doubt will turn into another long debate.
The most common answer is that a manifold allows you to have access to the gas in BOTH cylinders in the event of a regulator failure.

 

[DaleC]

Read away my good fellow!

http://www.scubaboard.com/forums/technical-diving-specialties/319904-independent-doubles.html

 

[RJP]

Most local divers (I live in New Jersey) who dive doubles leave the tanks isolated from one another.

You must live in a different New Jersey that I do.

In the NJ where I live and dive - and DM at a dive shop and crew on a dive boat - there are very few divers who purposely leave their doubles isolated. In fact of the 2 or 3 I've ever encountered...all were from the UK or elsewhere outside the US where the "independent doubles" approach is common.

It's easy to see how the isolated approach might initially seem to make sense, but all the reasons you cite are exactly why you DON'T want to dive isolated:

• 1,500psi is actually an inadequate reserve if you follow the "rule of thirds" gas planning
• You'd need two SPGs, one for each tank
• Not that you shouldn't be acutely aware of gas consumption/supply, but needing to manage "when to switch" on every dive is an extra task that is unnecessary; but isolated/independent you'd need to "worry about it" on every dive
• the need to actually isolate/shut down occurs rarely, and you train properly such that you don't need to "worry" about it

The "pro" for diving manifolded doubles is that you and your buddy have access to ALL of the gas you have on your back...at all times.

 

[battles2a5]

Most local divers (I live in New Jersey) who dive doubles leave the tanks isolated from one another. One reg/1st stage, pressure gauge per tank. That seems to me to be the safest method. It gives you two seperate air supplys (run the first one down to about 1500 psi, then switch to the second reg/tank.) The first with 1500 psi now becomes your emergency reserve. Should you encounter an emergency situation such as a free flow, etc. you will still have one tank with enough air to make a safe accent. Ive seen alot of doubles in pictures and online that are connected by a cross bar manifold. Even though there is usually an isolation valve in the middle, having to reach for the valve in event of an emergency seems like something I dont want to have to worry about. Can anybody tell me what the advantages of having the two tanks connected are, and are there any disadvantages to having two stand alone doubles? Thanks.

As others have stated it gives you access to all your gas in the event of a regulator malfunction (which is VASTLY more common than a manifold failure). The disadvantage is that you do not have a truely independent gas supply, although this point is largely irrelevant if you use common technical diving practices and gas planning. The process of shutting down an isolator is not/ should be a consideration when choosing between the two. If that is a skill that is too much to master you just shouldn't be doing dives that necesitate doubles. A lot of people have strong opinions about this, but the majoriy of people see the manifold w/ crossbar to be the best system out there all things considered as long as you not diving solo. I have never seen a documented case of a manifold failure, let alone one that resulted in catastrophic gas loss leading to an accident.

 

[Deep South Divers]

Yeah, what he said.

If you dive independent doubles and have, say, a regulator failure (could be a freeflow for example), then you no longer have that gas... Either it leaks out completely or you shut the valve off, robbing yourself of whatever's left in that tank.

Manifolded doubles allows you the most flexibility for handling issues.

Islolated doubles also screws up your trim a little... A 1/2 full tank is 3-4 pounds less negative than a full tank. Yeah, it can be compensated for, but why complicate matters?

It also means that you only have one SPG and don't have to remember to switch at a certain point.

Can you think of a reason why NOT to manifold your doubles, other than "because it's cheaper?"

 

[Aquamaniac]

Manifolded doubles with isolator manifold -
Advantages
- total redundancy
- all back gas available to all regulators in case of regulator failure
- simplified back gas management
Disadvantage
- increased failure points due to isolator valve turned off, broken, etc

Manifolded doubles with no isoator manifold
-Advantages
- all back gas available to all regulators in case of regulator failure
- decreased failure points due to isolator valve turned off, broken, etc
- simplified back gas management
Disadvantage
- not totally redundant in case of neck seal or disk failure

Independent doubles (actually independent tanks)
-Advantages
- total redundancy
- decreased failure points due to sep valve turned off, broken, etc
Disadvantage
- all back gas not available to all regulators in case of a regulator failure
- you have to manage your gas swapping carefully so not to drain one tank down in case the other fails

 

[Aquamaniac]

Just for s**ts and giggles, to highlight the difference between the two…

Let’s say I’m doing a dive in a fairly simple cave, back mount, no stages/deco gas etc.

A typical full cave entry level dive.

I’m diving twin independent 95’s
I start the dive out with 190 cuft, breathing down each side correctly.
I get to 1/3rds, I have used 63 cuft to get to this point, and I have 127 cuft remaining in my 2 tanks.
Just as I turn, I get a free flowing reg, and I have to shut down the left tank.
I now only have 63 cuft remaining in my right tank, and I need 63 cuft to get out.
If I get into a bit of a panic, and my breathing does increase, I will have to turn the free flowing reg back on and feather it to get out safely, or go to my buddy for help. More than likely you would go to your buddy, because access to a left post on Indy’s is somewhat more difficult.


I’m diving with manifolded twin 95’s
I start the dive out with 190 cuft, breathing down both tanks.
I get to 1/3rds, I have used 63 cuft to get to this point, and I have 127 cuft remaining in my 2 tanks.
Just as I turn, I get a free flowing reg, and I have to shut down the left post.
It takes me about 10 seconds to isolate, and I loose about 10 cuft of gas.
I now only have 117 cuft remaining in my tanks, and need 63 cuft to get out.
My breathing rate could almost double, and I would still be OK.
Given that access to the left post is fairly easy, I also still have the option to feather if I needed to.

Whilst neither of these scenarios are dangerous, and unless you were diving solo with no safety’s, neither would kill you.
My point in running this scenario was that to show that for convenience, a manifold can in fact be a better option.
Both of these scenarios assume that the diver is properly trained and can access his isolator valve correctly.

The thing to remember with this scenario is that it’s not unrealistic. Regulator free flows do occur, and usually in the worst possible location.

 

[wedivebc]

Just for s**ts and giggles, to highlight the difference between the two…

Let’s say I’m doing a dive in a fairly simple cave, back mount, no stages/deco gas etc.

A typical full cave entry level dive.

I’m diving twin independent 95’s
I start the dive out with 190 cuft, breathing down each side correctly.
I get to 1/3rds, I have used 63 cuft to get to this point, and I have 127 cuft remaining in my 2 tanks.
Just as I turn, I get a free flowing reg, and I have to shut down the left tank.
I now only have 63 cuft remaining in my right tank, and I need 63 cuft to get out.
If I get into a bit of a panic, and my breathing does increase, I will have to turn the free flowing reg back on and feather it to get out safely, or go to my buddy for help. More than likely you would go to your buddy, because access to a left post on Indy’s is somewhat more difficult.


I’m diving with manifolded twin 95’s
I start the dive out with 190 cuft, breathing down both tanks.
I get to 1/3rds, I have used 63 cuft to get to this point, and I have 127 cuft remaining in my 2 tanks.
Just as I turn, I get a free flowing reg, and I have to shut down the left post.
It takes me about 10 seconds to isolate, and I loose about 10 cuft of gas.
I now only have 117 cuft remaining in my tanks, and need 63 cuft to get out.
My breathing rate could almost double, and I would still be OK.
Given that access to the left post is fairly easy, I also still have the option to feather if I needed to.

Whilst neither of these scenarios are dangerous, and unless you were diving solo with no safety’s, neither would kill you.
My point in running this scenario was that to show that for convenience, a manifold can in fact be a better option.
Both of these scenarios assume that the diver is properly trained and can access his isolator valve correctly.

The thing to remember with this scenario is that it’s not unrealistic. Regulator free flows do occur, and usually in the worst possible location.

I have been teaching tech diving for 8 years and have not yet seen a diver here in cold water who can do a complete valve drill in 10 seconds
More like 30 and on a high performance regulator that equals more like 40-50 cuft.
But no argument with the rest of your post.

 

[Gombessa]

I have been teaching tech diving for 8 years and have not yet seen a diver here in cold water who can do a complete valve drill in 10 seconds
More like 30 and on a high performance regulator that equals more like 40-50 cuft.
But no argument with the rest of your post.

He didn't say valve drill in 10 seconds, he said isolate in 10 seconds. In general, woe be the diver who responds to a free flow by performing a full valve drill?