AL40s Anyone?

[rhuntley]

I have not been in this situation YET (it's coming up fast though) but I did read on another thread that what some people do is bring three tanks. On the first dive they breath one down more than the other, then swap the low tank for a full one the second dive.

Wouldn't breathing one tank down more than the other be the opposite of what you want to do? If something happens to the tank [or reg of the tank] with more air, your dive would be limited to the tank with less gas, or feather breathing if possible.

 

[James R]

Hey...What a great idea!
Cheaper than buying more tanks.

A proper transfill whip isn't exactly cheap...around $160 USD

 

[Rich Keller]

I think the idea is brilliant! This makes more sense to me then using full size tanks on a charter boat. I would probably want to use steel 45s instead but thats just my opinion. As for a trans-fill whip I would use what we called a suicide bar, it was meant to connect two single tanks on a doubles back pack before twin manifolds were invented, you can find them on ebay for about $25.

 

[CamG]

Two AL 40's / Steel LP 45's have been used for cave exploration.
They trim a bit different and one should practice before diving off a boat.
They swim very nicely and are able to be added in addition to bigger cylinders if needed in the future.



CamG

 

[NAM001]

Dont laugh but i have a pair or lp45's doubled up.

Steel lp45s are dreamy to dive too and hold a lot more gas when properly filled.

 

[Ken T]

A proper transfill whip isn't exactly cheap...around $160 USD

$175 CDN at my LDS.

Still a little cheaper than more tanks, and I already have the HP100s.

 

[OkByMe]

Originally Posted by rjack321
Steel lp45s are dreamy to dive too and hold a lot more gas when properly filled.

..............and they have the added benefit of the weight of two boat anchors.

With aluminum you put it on your belt and/or the tank - with steel you take it off, more of it actually. With steel, your actual gross weight could be less depending on the type of steel.

 

[shoredivr]

I've used a mismatched Al40 and Al30 on a shallow little bimble, tanks disappear but the amount of lead needed for drysuit and undies is unreal.

 

[DaleC]

I have not been in this situation YET (it's coming up fast though) but I did read on another thread that what some people do is bring three tanks. On the first dive they breath one down more than the other, then swap the low tank for a full one the second dive.

Wouldn't breathing one tank down more than the other be the opposite of what you want to do? If something happens to the tank [or reg of the tank] with more air, your dive would be limited to the tank with less gas, or feather breathing if possible.

This was my strategy when I dove independent doubles. I had a band set up that would allow me to swap out the right tank.

• Start the dive on the left and breath it down to rock bottom volume
• Switch to the right and breath down to rock bottom volume
• Ascend on right tank, depleting remaining volume
• Swap out right tank during surface interval
• Begin dive two on right with rock bottom volume in left and full volume in right
• Breath down right to rock bottom value
• Ascend

My only concern with using Al 40's is that you have little bottom volume when reserving rock bottom in each tank (which you should do). If you are diving shallow this is fine but not for deeper dives. For example: If you calculate your RB at 20 cuft that means you have 20 cuft in each tank for BT. Total bottom gas 40cuft. If you calculate a conservative 40cuft for reserve you have 0 bottom gas.

 

[jcaplins]

more math than I want to right now, but a third tank like an HP100 or an HP80 and a transfill whip, and you are diving again with those "little bottles" at near full charge for dive #2.....

The math doesn't make it worth it:

HP100:
start PSI = 3442
volume = 100 ft3

Al40:
start PSI = 500
Volume = 6.7 ft3

end PSI = 2503
end volume Al40 = 33.4 ft3
end volume HP100 = 72.4