Comparing ST doubles to Al doubles

[DaleC]

Can someone point me towards a link or help me figure out the weighting difference and lift requirements between the two.

Twin Al80's vs twin HP ST 80's or HP ST 100's

In my case:

With a single Al 80 (mid 80's US Diver) I need 36lb's of lead to remain slightly negative at the end of the dive (drysuit/winter under garments). If I twin Al 80's I assume I will need 42lb's (36+6 for buoyancy swing in tank two). At the beginning of the dive I will be 12lb's negatively buoyant (approx.). Is my thinking correct at this point?

How would the numbers come out if I twinned comparable ST tanks (80's for volume or 100's for size). Am I humping more weight (over all) with the Al's when the tank weight and lead is tallied and which system leaves me more negatively buoyant at the beginning of the dive?

I hope that makes sense. What I am trying to determine is the overall difference ST tanks would make over Al tanks (of comparable size) both in rig weight and needed lift capacity (wing size).

 

[evil_xander]

You can check the discussion I had in my post here (notice I was doing it for steel tanks):
http://www.scubaboard.com/forums/tanks-valves-bands/258057-bts-steel-tanks-2.html

Basically, per other thread cuft in liters, I dive steel 85/98 CF HP tanks, depending on the fill, for the reference.

This is as far as steel goes. Haven't dove AL tanks, the principle should be pretty much the same, but the benefits are on the steel doubles side (when talking about drysuit diving), less weight, less buoyancy swing.

People who dive/did dive AL doubles should chime in...

 

[halocline]

There definitely will not be less of a buoyancy swing if you are carrying more gas. For example 10lbs negative to 2 lbs negative is still 8 lbs, more than 2 lbs neg to 4 lbs pos.

I'm interested in this thread too, as I'm putting together my first set of doubles and it will either be LP72s or AL80s, for cost.

I think your calculation going from single to double AL80s is not quite right. It's true you'll have a potential 12 lbs swing instead of 6. But, if you use 36 lbs with a single, you won't need an additional 6 lbs with doubles because of the manifold, bolts, bands, and extra reg. Those parts, minus the weight of the 2 valves, will probably come pretty close to 6 lbs. Just a guess, but I bet 38lbs would do it for you with AL80 doubles.

For carrying weight, think 63 lbs for the tanks, 12 for the gas, maybe 12 for the bands/manifold, 38 in lead, total weight 125 lbs. (right?)

E7-80s are about 28 lbs, so 56+12+12=80. All other things equal, the empty is 13 lbs more negative (-5 for the two steels, +8 for the ALs) so 25 lbs lead, total carry weight 105 lbs. I think the numbers are close for the worthington X-7 80, and maybe 10lbs heavier for the HP 100s.

Take my calculations with a grain of salt, I'm just guessing and I'm certain someone will jump in to correct me.

 

[evil_xander]

Just keep in mind that for sizing a doubles wing, dry land weight is not of much use/not a concern, only buoyancy.

I use a 45lbs wing for my set.

 

[sclakediver]

Try this link. Scuba Cylinder Specifications from Tech Diving Limited - 928-855-9400

You should be able to drop 8 lbs from your ballast by switching to DOT E 100's. (commonly and incorectly called HP 100's)

As mattboy stated, you are probably overdoing it with an extra six pounds for the doubles. Toss the empty set of AL80's with regs in a pool, and watch them sink.

I dive with double LP85's, nice size tanks for me, but I usually use DOT E 100's for single tank dives. Nothing like having more air than an 80 with an overall lighter rig.

Good luck with your decision.

 

[ianr33]

You would not need any extra weight for the double Al 80's.

If you sucked the tanks dry then the extra Al 80 will be 4 pounds positive,but that will be offset by the extra reg,manifold and bands. You could probably get by with a couple of pounds LESS.

If I was using 36 pounds lead with Al tanks then I would much prefer to dive steel tanks.

As regards wing size: I have a 40 pound doubles wing. Works fine with Al doubles,even with a couple of deco/stage tanks. Using it with double 104's though is pretty marginal. A 55 pound wing would be much better with big steel doubles.

 

[tgsmith]

As regards wing size: I have a 40 pound doubles wing. Works fine with Al doubles,even with a couple of deco/stage tanks. Using it with double 104's though is pretty marginal. A 55 pound wing would be much better with big steel doubles.

Be wary of this train of thought. Yes, of course the bigger tank you get the heavier it weighs, but as another member pointed out it's bouyancy that matters. Look at Faber tanks vs PST or especially Worthington, and you'll find they're very light in water.

Faber LP121's are monster sized tanks, yet they're only -5 lbs full and actually QUITE positive when empty. TF-121Pro [TF-121Pro] - $504.95 : Tech Diving Limited, a subsidiary of Scuba Training and Technology Inc.

Check that out, and scan around at the buoyancy differences between the other fabers, both LP, HP(exempt series tanks at 3442psi, since it apparently matters to some) and then look at the differences with the PST's and Worthingtons. You'll find such generalizations that a bigger tank requires more lift is not really true. It's all dependent on the alloy, determined by the series of tanks, much more so than size. I talk to so many people that don't realize how light they are at 500psi with doubles. They think cause they're wearing steel doubles they don't need weight, which if you're diving in cold water simply isn't true. Thankfully, our equipment is reliable enough that these divers have not had to find out the hard way how light they are, by holding a deco stop in contingency gas or worse having a deco obligation, being in contingency gas, and being forced into drift deco.

Good on you for wanting to figure out the weighting.

 

[Doc Intrepid]

"What I am trying to determine is the overall difference ST tanks would make over Al tanks (of comparable size) both in rig weight and needed lift capacity (wing size)."

Dale,

Remember that everything you have with you is a system - its a cumulative effect. IOW, I suspect you wear a drysuit. I also suspect you likely use a fairly substantial undergarment for it, based on where you're located.

I do also.

I use two sets of tanks - one of steel PST E8-130s, and the other of Luxfer aluminum neutral 80s (the tanks that pump to 3300 psi and are supposedly neutral rather than bouyant when empty).

In terms of OVERALL difference, though, if I compare and contrast two different dives on the same day, one with each set of tanks, here is the difference:

With the Aluminum 80s I need to add a V-weight onto the doubles to ensure that with only 200 psi remaining in the tank I can hover effortlessly at any depth between 10' and the surface (7', 5', 3', whatever).

With the steels I can eliminate the V-weight to achieve the same level of performance.

(Obviously, with the two steel 130s I'm starting out with ~260 cu ft of backgas, and with the 2 aluminum 80s I'm starting out with ~160 cu ft of backgas...almost 100 cu ft of gas difference (or the volume of a 100 cu ft slingtank).

But holding all else equal, (including performance,) that is the sum tangible variance to my diving rig between a set of large steel doubles and a set of aluminum doubles: one V-weight, and ~100 cu ft of gas.

I use the same Halcyon 55 lb wing with both twin sets. (In fact, I use the same set of equipment nearly every dive, unless I'm doing something radically different that requires something unusual.)

Like evil_xander said, I don't count the weight difference of the tanks on land, because submerged the weight difference is zero. There is only one above-water circumstance where weight matters (at least to me,) and I confess that I often prefer to use the 80s in this circumstance. When doing shore entries with lots of tennis-ball sized rocks, if I fall down wearing the steels getting back up could be interesting. The aluminum tanks - even with the V-weight - are a little easier to handy on rocky shore entries.

FWIW.

YMMV.

Doc

 

[DaleC]

Thanks all so far.

Doc, that was exactly the kind of comparison I was looking for.

I'm not independently wealthy (3 kids and a mortgage) so I have to buy gear gradually. After a wing I will have everything to dive with Al 80's but I want to know if it would be feasable to create such a set for heavy drysuit diving or would I want to wait till I can buy 2 more ST tanks.

And yes, I dive a drysuit and the weighting is for my thickest winter undergarments. At 36lb's I can hold a SS with 500psi and with 34lb's I am fighting buoyancy. Ugh!
I can't complain about the undergarments though. Yesterday we were in 39F and only my hands were cold.

 

[DA Aquamaster]

A set of AL 80's is a great starter set for doubles. They are comparatively floaty and have comparatively little swing weight as doubles go (160 cu ft / 10 pounds swing weight compared to 260 cu ft / 16 lbs swing weight for double 130's or cave filled double 95's.)

7.25" bands work on any E7 or X7 series doubles and the manifold will work on just about any set of doubes from 7.0" to 8.0" in diameter. The AL 80's then make great stage bottles as singles.

Much like doc, I dive pretty much the same configuration anytime I dive doubles and I dive doubles anytime I do not have to fly to get somewhere. With double LP 95's or double HP 100's I use just a steel plate (- 6 lbs), with floatier double steel 72's (+4 lbs bouyant when empty, compared to -3 lbs bouyant when empty) I add an 8 lb v-weight. With double AL80's I'd add an 11 lb V-weight as the double AL 80's are about 8 lbs positive when near empty.

In short, compare the relative bouyancy of the tanks and then just add or subtract weight to give you the same net buoyancy when the respective tanks are empty. This link gives a pretty fair idea of relative empty buoyancy traits for most commonly encountered tanks.

Tank chart

When switching from a single to double 80's I agree the added 4 lbs of empty tank buouyancy is offset by the bands and manifold so it is basically a wash. But be aware you will be about 5 lbs heavier at the start of the dive with doubles than with a single AL 80 due to the 10 lbs versus 5 lbs of gas you are carrying.