Here at our local dive shop there seems to be an ongoing debate on the virtues of steel vs. alum tanks to use for doubles. There are a great many locals that are cave divers and therefore seem to be more in the steels camp. ( fresh water, no current, no waves.) One of the interesting lines of thought for the alums is the bouy. they give you at the end of the dive. Down current, big waves, lots of gear, drop weights to get more bouy. Steel tanks, BC better be enough to get the job done or you might have to shuck considerable investment to stay afloat. Assume no dry suit is involved to add bouy., just typically a 7mm wet suit, in salt water. This is the bebate in a nut shell. Any thoughts? I'm looking to set up a rig for some deeper wreck dives.
Alum vs. Steel
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lamont
Contributor
the big difference between steel and aluminum is going to be the amount of extra lead required to sink the aluminum. if you already don't require a lot of extra weight to sink the aluminum then the steels in the same configuration may be inherently overweighted (overweighted at the end of the dive) and/or you might need an al backplate. if you are using less than 10# of lead with aluminum-80s you'll probably want to stick with aluminums.
also, diving with really large steels can give you almost twice as much swing from the gas which makes you twice as overweighted at the start of the dive. if you are diving nitrox rather than rich helium mixes this can be really significant. so you probably want to be looking at double hp-100s (or lp80s or lp85s) rather than double hp-130s.
also it could be argued that drysuits are a much better choice once you've hit the need for 7mm wetsuits in the ocean and you start deep wreck diving. there's a lot of buoyancy swing caused by that 7mm suit when going down to 160+. that alone is going to potentially cause issues.
also, diving with really large steels can give you almost twice as much swing from the gas which makes you twice as overweighted at the start of the dive. if you are diving nitrox rather than rich helium mixes this can be really significant. so you probably want to be looking at double hp-100s (or lp80s or lp85s) rather than double hp-130s.
also it could be argued that drysuits are a much better choice once you've hit the need for 7mm wetsuits in the ocean and you start deep wreck diving. there's a lot of buoyancy swing caused by that 7mm suit when going down to 160+. that alone is going to potentially cause issues.
I agree with Lamont that diving wet with doubles using anything other than aluminum 80's or some of the only slightly negatively bouyant steel tanks in the 72-80 cu ft range can leave you badly over weighted at depth. A deep descent to 150 plus feet in a wet suit with full steel tanks of 100 plus cu ft each can leave you with a wing that has to be very full to get you neutral and if it fails, it is a lot of negative bouyancy to swim back up to the surface unless you have some form of redundant bouyancy.
In that regard, I am not a fan of double bladder wings due to the complexity and interfence issues involved. Stacking two wings is an only slightly better option and using a lift bag works in a pinch (assuming it is properly designed), but doing that safely reqiures practice and it is not really any option inside a wreck or cave. A dry suit is by far the best option for redundant bouyancy.
Ditchable weight is an option only if you are in a no decompression situation and can ascend all the way to the surface with no deco or safety stops - and if you are diving doubles, the odds are good that deco stops will be required, and if you are in a wreck or cave, being positive at depth is very problematic. So ditchable weight is not really a workable plan under water for a technical diver who will most likely be in an overhead situation.
It is true that on the surface dropping ditchable weight will quickly increase your bouyancy and I would argue that is the only proper use of ditchable weight - even in a recreational situation - and any other use is just a crutch that fails to address a greater underlying configuration issue.
On the other hand, if I am on the surface and need more bouyancy due to a wing failure, I can inflate a lift bag, remove the tanks/BP/wing and then hang them on the lift bag and keep them tethered on a jon line rather than just letting them sink so any potential disadvantage compared to dropping perhaps 10 lbs of ditchable weight is minimal.
Either way, I am pretty sure a coast guard helicopter is not going to rescue both me and my tanks, etc in rough seas anyway so whether I remove them now or later, I suspect they will be removed and may well end up on the bottom if I am recovered by helicopter.
In that regard, I am not a fan of double bladder wings due to the complexity and interfence issues involved. Stacking two wings is an only slightly better option and using a lift bag works in a pinch (assuming it is properly designed), but doing that safely reqiures practice and it is not really any option inside a wreck or cave. A dry suit is by far the best option for redundant bouyancy.
Ditchable weight is an option only if you are in a no decompression situation and can ascend all the way to the surface with no deco or safety stops - and if you are diving doubles, the odds are good that deco stops will be required, and if you are in a wreck or cave, being positive at depth is very problematic. So ditchable weight is not really a workable plan under water for a technical diver who will most likely be in an overhead situation.
It is true that on the surface dropping ditchable weight will quickly increase your bouyancy and I would argue that is the only proper use of ditchable weight - even in a recreational situation - and any other use is just a crutch that fails to address a greater underlying configuration issue.
On the other hand, if I am on the surface and need more bouyancy due to a wing failure, I can inflate a lift bag, remove the tanks/BP/wing and then hang them on the lift bag and keep them tethered on a jon line rather than just letting them sink so any potential disadvantage compared to dropping perhaps 10 lbs of ditchable weight is minimal.
Either way, I am pretty sure a coast guard helicopter is not going to rescue both me and my tanks, etc in rough seas anyway so whether I remove them now or later, I suspect they will be removed and may well end up on the bottom if I am recovered by helicopter.
My personal opinion is use the tanks that are going to meet the requirements of your diving. There are going to be pluses and minuses to every scenario. Consider the diving you are planning to do and gear up for that type of diving. If you're going to be fairly deep for considerable times dbl al 80s are probably not going to serve you well simply because they don't hold much back gas. Shops don't generally overfill aluminum tanks, but some will overfill LP Steel tanks. Faber LPs, overfilled to 3000 psi (and that's not a big overfill) hold 97.5 cf of gas each, or 195 cf of gas doubled. Dbl al 80 are going to hold 160 cf.
As far as the buoyancy question goes consider this: a Luxfer al 80 has a buoyancy swing of 5.8 lbs, but ends up 4.4 lbs. positive empty. An OMS LP 108 has a buoyancy swing of 7 lbs, and ends up 1 lb negative when empty. As you can see the difference in the buoyancy swing between the two is nominal. The big difference is the positive buoyancy of the al 80 when empty. Double that and your almost +9 lbs buoyant. Buoyant is not something you want to be while doing a long deco, so weighting, while always critical, is an even more important consideration with aluminum.
Let's look at your scenario: Can you drop your weight at the surface and remain buoyant with dbl al 80s? Definitely should be able to do that. Can you drop weight and remain buoyant with dbl LP 108s? Certainly should be able to...as long as you're properly weighted. Check out this tank specifications chart, it's a pretty good one: Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan
As far as redundant buoyancy is concerned. You shouldn't be tech diving without it...double bladders, stacked wings, drysuit, lift bags, etc. I consider a good lift bag and a dry suit to be my best bet. Depending solely on stacked wings or a dbl bladder wing is risky IMHO.
As far as the buoyancy question goes consider this: a Luxfer al 80 has a buoyancy swing of 5.8 lbs, but ends up 4.4 lbs. positive empty. An OMS LP 108 has a buoyancy swing of 7 lbs, and ends up 1 lb negative when empty. As you can see the difference in the buoyancy swing between the two is nominal. The big difference is the positive buoyancy of the al 80 when empty. Double that and your almost +9 lbs buoyant. Buoyant is not something you want to be while doing a long deco, so weighting, while always critical, is an even more important consideration with aluminum.
Let's look at your scenario: Can you drop your weight at the surface and remain buoyant with dbl al 80s? Definitely should be able to do that. Can you drop weight and remain buoyant with dbl LP 108s? Certainly should be able to...as long as you're properly weighted. Check out this tank specifications chart, it's a pretty good one: Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan
As far as redundant buoyancy is concerned. You shouldn't be tech diving without it...double bladders, stacked wings, drysuit, lift bags, etc. I consider a good lift bag and a dry suit to be my best bet. Depending solely on stacked wings or a dbl bladder wing is risky IMHO.
The principle is right but the math is a little off.
A Faber LP 80 only holds 78 cu ft at 2640 psi and holds 88.6 cu ft at 3000 psi (which is still a 25% overfill).
An AL 80 only hold 77 cu ft. so double AL 80's only offer 154 cu ft while the overfilled Faber 's at 3000 psi will hold 177 cu ft.
154 cu ft is not much if you plan on having a 1/3 rd reserve (and you also need to ensure that 1/3rd is also adequate gas to do the deco off the backgas in the event you lose a deco gas.)
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Realistically, if you have decent gas consumption, you are limited to about 20-25 minutes at 150 ft. If that is enough bottom time, then an AL 80 will probably work for you.
Being a hoover, going deeper and/or longer requires the same1/3 reserve, but the 1/3rd reserve on double 80's begins to become inadequate for deco on the reserve backgas as a contingency if you lose a deco gas. Consequently, a larger reserve, a larger set of doubles, a second deco gas, a stage bottle, or a combination of two or more of the previous options begin to be needed to ensure adequate bottom gas, deco gas and a reserve with adequate lost gas redundancy.
An AL 80 stage is not hard to carry in addition to an AL 40 deco bottle - and both are ditchable early in the dive if you need to ascend early (while you have adequate backgas for the deco over and above the reserve) so using all of the AL 80 stage first and then switching to back gas at depth - with 1/3 rd of the TOTAL stage plus doubles gas maintained in the doubles (77 cu ft/1500 psi, rather than 51 cu ft/1000 psi) can be an effective and fairly inexpensive way to extend the range of a set of AL80's or other small doubles.
The above probably also highlights the obvious - when you move to doubles with the intent of really using them to expand your range of depth or bottom time, the level of training, gas planning and contingency planning goes up considerably. So there is a lot more to it than strapping on double tanks and jumping off the end of the boat.
A Faber LP 80 only holds 78 cu ft at 2640 psi and holds 88.6 cu ft at 3000 psi (which is still a 25% overfill).
An AL 80 only hold 77 cu ft. so double AL 80's only offer 154 cu ft while the overfilled Faber 's at 3000 psi will hold 177 cu ft.
154 cu ft is not much if you plan on having a 1/3 rd reserve (and you also need to ensure that 1/3rd is also adequate gas to do the deco off the backgas in the event you lose a deco gas.)
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Realistically, if you have decent gas consumption, you are limited to about 20-25 minutes at 150 ft. If that is enough bottom time, then an AL 80 will probably work for you.
Being a hoover, going deeper and/or longer requires the same1/3 reserve, but the 1/3rd reserve on double 80's begins to become inadequate for deco on the reserve backgas as a contingency if you lose a deco gas. Consequently, a larger reserve, a larger set of doubles, a second deco gas, a stage bottle, or a combination of two or more of the previous options begin to be needed to ensure adequate bottom gas, deco gas and a reserve with adequate lost gas redundancy.
An AL 80 stage is not hard to carry in addition to an AL 40 deco bottle - and both are ditchable early in the dive if you need to ascend early (while you have adequate backgas for the deco over and above the reserve) so using all of the AL 80 stage first and then switching to back gas at depth - with 1/3 rd of the TOTAL stage plus doubles gas maintained in the doubles (77 cu ft/1500 psi, rather than 51 cu ft/1000 psi) can be an effective and fairly inexpensive way to extend the range of a set of AL80's or other small doubles.
The above probably also highlights the obvious - when you move to doubles with the intent of really using them to expand your range of depth or bottom time, the level of training, gas planning and contingency planning goes up considerably. So there is a lot more to it than strapping on double tanks and jumping off the end of the boat.
Thanks for the correction. You're right on the math.
As an aside, I was simply making a comparison of steel 80's to aluminum 80's to show the potential steel has for carrying more back gas. I wasn't suggesting using them for extended range diving. Sorry if I wasn't clear.
Not sure what/who the "So there is a lot more to it than strapping on double tanks and jumping off the end of the boat" comment is all about unless it's a general comment about tech diving. In which case, I think any commercial or rec tech diver would agree with you 100%. I do.
lamont
Contributor
Its obvious that steel tanks carry more backgas though. The problems with steels, however, are when you start out 10# overweighted inherently, add 20# of nitrox/air onto your back, and then lose another 10-20# due to neoprene crushing at depth. Now you're 40-50# overweighted on the floor of the ocean and potentially having a really bad day...
Exactly. Even a 55 lb wing with normal production tolerances (maybe only 50-53 pounds of lift) and/or slightly squeezed between the tanks and wing may not be enough lift to get the diver neutral in that situation.
That's exactly what it was - pointing out the obvious that double tanks bring with them a greater level of complexity due to the greater opportunity the additonal gas creates to get a diver in trouble.
Well, I think we're getting off topic here for the OP's original concern, but since we're heading down this road anyway...
First, it may be obvious to you and me that you can get more back gas in steel tanks, but the OP is asking questions about the comparison between aluminum and steel tanks. I don't know what s/he knows or doesn't, so I expanded the explanation a little for the OP.
As far as being overweighted goes, I've never found myself in a situation where I was face down on the bottom at 175' with my wing fully inflated...and the only aluminum tanks I have are stage bottles. A capable diver keeps a good record of their weighting in different situations with different gear configurations and adjusts weighting accordingly. Sometimes...gasp...I don't use any additional weight at all.
Of course, this scenario could NEVER happen with AL80 doubles if we stuck to your Rock Bottom philosophy, right? Here's a quote from the Rock Bottom web page: The rule of thumb is that you should not dive to a greater depth in fsw than your tank capacity in cu ft. This works reasonably well for most beginning and intermediate recreational divers and caps the depth that an Al80 should be dove to to 77 fsw (Al80 == 77.4 cu ft). Since double Al80s have a tank capacity of only 154.8 cu ft our depth limit would be 154'-155'...barely beyond recreational limits in the U.S. and just a few feet beyond recreational limits in the UK (BASC).
If an individual can't properly weight his or herself for a dive they shouldn't be doing the dive IMHO. Clearly how critical weighting is depends on the dive. It's probably not as important during a 30' recreational dive as it would be for a dive in the 200' range.
I'll stick to my original thought on this and say use the equipment best suited for the dive. Aluminum may be fine on one dive for a certain individual while steel may be the optimum choice for another.
OP
Thanks guys for all the good input. Most of the local debate seems to be one group trying to one up the other with no real factual information coming from either side.
Most of the diving I'm interested in will be in the 100-200 ft. range. I had already, pretty much, made up my mind on a set of HP STL 100, alum 40 for O2 and alum 80 nitrox mix. HP STL 100's are about the limit for this old back. LP95's or LP104's seem to be so much heavier than the HP100's or is this just my imagination? The dry suit for plan B situations is the reason I'm trying to find the proper glue for my dry suit seals.
Most of the diving I'm interested in will be in the 100-200 ft. range. I had already, pretty much, made up my mind on a set of HP STL 100, alum 40 for O2 and alum 80 nitrox mix. HP STL 100's are about the limit for this old back. LP95's or LP104's seem to be so much heavier than the HP100's or is this just my imagination? The dry suit for plan B situations is the reason I'm trying to find the proper glue for my dry suit seals.
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