Doubles Tank Band Question

[mahjong]

Just embarking on understanding doubles and piecing together my first doubles rig. I notice that the center flat space between the two rounded portions of a tank band might have a different width on different bands. I'm guessing that this is the issue when I read that the bands and the particular manifold one is using must be compatible--is this correct? If the spacing is too wide (or too narrow, I suppose) then some manfolds might not reach out far enough (or collapse in narrow enough)--is this correct?

Also, given the angle on most bands between the rounded portion and the flat center portion, it would seem that the spine of most backplates could not actually be flush with the center portion because the tanks themselves would hit the wide flat portions of the backplate before the spine would reach the center flat piece--is this correct? If so, how do you lock down the tank bands/tanks to the backplate? Lots of washers? Or is the tension all on the contact points between the tanks and the wide flat portions of the backplate (with the wing sandwiched in the middle)?

Many thanks!

 

[Jimmer]

The flat center section could be different lengths because of how far offset they are from the centerline of the tanks. The center to center dimensions of the bands is what specifies which manifold can be used. Most current bands are built to a 215mm C-C specs.

The backplate never touches the bands on any of my doubles. It rests tangent to the walls of the tanks, and tension on the bolts holds it in place.

Jim

 

[mahjong]

Many thanks! I didn't know there was a C-C spec, so that makes sense now.

Do you mean to say that the "spine" of your BP never touches the center section of the bands (ie, there is a gap that remains between the BP spine and the center section of the bands)? If so, then the tanks must press up against each side of the BP, with the center section of the wing sandwiched in between, no?

The flat center section could be different lengths because of how far offset they are from the centerline of the tanks. The center to center dimensions of the bands is what specifies which manifold can be used. Most current bands are built to a 215mm C-C specs.

The backplate never touches the bands on any of my doubles. It rests tangent to the walls of the tanks, and tension on the bolts holds it in place.

Jim

 

[cool_hardware52]

Many thanks! I didn't know there was a C-C spec, so that makes sense now.

Do you mean to say that the "spine" of your BP never touches the center section of the bands (ie, there is a gap that remains between the BP spine and the center section of the bands)? If so, then the tanks must press up against each side of the BP, with the center section of the wing sandwiched in between, no?

That's how it works. You want the wing to be clamped between the flat part of the plate, near the outer edges and the cylinders. That's why doubles wings have a center section ~9+" wide (215 mm = 8.46") That keeps the inflatable portion of the wing from being trapped between the plate and cylinders.

If the center channel of the plate "bottoms" on the center section of the tank bands the plate will wobble on the tanks.

Tobin

 

[mahjong]

Thanks, Tobin. Makes sense. The tanks have to "bottom" on the flat part of the plate or there would be wobble. And so the center section of the wing has to be wide enough to avoid a pinch with the bladder.

It's starting to come together now.

On a different note, Tobin: I would be purely recreational diving the doubles--no overhead environments or wreck penetration. Am I nuts to be leaning towards a J valve manifold with a single regulator attachment valve--and then simply open the J valve from the start eliminating the reserve function? I just can't see two regs and three knobs for recreational diving--so cumbersome. Or is there some other downside to the J valve manifold?

That's how it works. You want the wing to be clamped between the flat part of the plate, near the outer edges and the cylinders. That's why doubles wings have a center section ~9+" wide (215 mm = 8.46") That keeps the inflatable portion of the wing from being trapped between the plate and cylinders.

If the center channel of the plate "bottoms" on the center section of the tank bands the plate will wobble on the tanks.

Tobin

 

[cool_hardware52]

Thanks, Tobin. Makes sense. The tanks have to "bottom" on the flat part of the plate or there would be wobble. And so the center section of the wing has to be wide enough to avoid a pinch with the bladder.

It's starting to come together now.

On a different note, Tobin: I would be purely recreational diving the doubles--no overhead environments or wreck penetration. Am I nuts to be leaning towards a J valve manifold with a single regulator attachment valve--and then simply open the J valve from the start eliminating the reserve function? I just can't see two regs and three knobs for recreational diving--so cumbersome. Or is there some other downside to the J valve manifold?

Conventional manifolded isolated doubles offer true redundancy, the trade off is more failure points.

These additional failure points can be largely mitigated with proper training in the use of dual outlet manifolded doubles.

Single orifice manifolded doubles seem to present many of the added failure points with none of the added redundancy.

If you need more gas my first thought would be the largest single available (130)

Doubles provide enough gas to get you into trouble (deco) where the redundancy is valuable without ever entering an over head.

Tobin

 

[mahjong]

Sounds sensible, Tobin, thanks.

Being short, I'm most comfortable diving short tanks (I dive AL80's often enough, but it's a length that's far from ideal for me). 120's and 130's are big tanks, tall and heavy, with a lousy weight distribution for me. I was thinking that twin 72's, (the 20" shorties), might allow me to add air while keeping a nice weight distribition right where I can comfortably manage it--across my back (not over the back of my head and down over my butt). Yes, there's more air to get you into deco trouble, but not much more than the largest tank. And of course there is the enjoyment presented by the challenge of learning and mastering a different gear configuration. (I might note that I used to dive the Faber steel 95; the slightly shorter length [~24"] really felt nice, but they were 8" in diameter and heavy hanging off an STA and I'm not sure the small amount of added air was worth it--I had two and man were they a @#$% to carry around in freezing cold temperatures in the NE). The twin 72's would be heavier still, but quite a bit more air and in a package that would be mounted closer to my back and more than 3" shorter than the 95.)

As for the manifolds, I see your point. Again, being short, and esp if I'm decked out in thinsulate underwear and a drysuit, it really isn't easy for me to be dialing down or dialing open turn knobs that are behind me. I can slip out of my harness and attend to it that way, but in certain situations that might not be easy either. It seems that the dual isolation manifold adds real failure points (regulators, orifices, hosess) while also allowing for the possibility to recover if a failure occurs, but successful recovery, as you said, requires training--as well as circumstances that don't inhibit your ability to do what you were trained to do. I don't see quite the same categorical kinds of failure points in a single outlet manifold, given only one reg attachment and one open/close turn knob. There is the reserve switch, which if need be in a pinch is easier to manage (one half turn), I would think, than a turn knob that has to be dialed several times. The manifold fixture elements (including the 3/4" thread valve-tank attachment) I would think are otherise the same in both types of manifolds. In short, I somehow still sense that the real added failure points of the dual isolation manifold are only reasonable to take on if you are doing real technical diving. But again, this is only an impression; I not only might be missing something, I've no direct doubles diving experience behind it.

But, yes, overall life would be easier simply with a larger single tank--and it would be cheaper too!

Conventional manifolded isolated doubles offer true redundancy, the trade off is more failure points.

These additional failure points can be largely mitigated with proper training in the use of dual outlet manifolded doubles.

Single orifice manifolded doubles seem to present many of the added failure points with none of the added redundancy.

If you need more gas my first thought would be the largest single available (130)

Doubles provide enough gas to get you into trouble (deco) where the redundancy is valuable without ever entering an over head.

Tobin

 

[fire_diver]

Am I nuts to be leaning towards a J valve manifold with a single regulator attachment valve--and then simply open the J valve from the start eliminating the reserve function? I just can't see two regs and three knobs for recreational diving--so cumbersome. Or is there some other downside to the J valve manifold?

Only if you are doing it as a "retro" gear dive. J-valves went out of use because they were prone to failure. If you are setting up doubles, then set up modern doubles that includes double-oring barrel manifold.

You don't have to worry about "all those knobs". Just make sure everything is open prior to diving.

I agree that if all you are going for is a simple, large gas supply then look at something like an HP 120 or HP130 tank. Simple and doesn't require changing your current setup.