Tank change=more weight?

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Scubadoo72

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Ok, so I am considering changing my twin steel 72's out to twin aluminum 80's...(yes, I am an air hog..lol) In my line of thinking I am going from -7.4lbs (empty tank) to +8.8lbs (empty tanks) in buoyancy. To me, this means I would have to add 15.4lbs to my current 31lbs that I regularly use?

If this is so, would it also be safe to add that weight in the form of a "in between tanks" weight? So it is permanently attached to the bc/tanks..and I would be able to surface quickly if I ever had to dump my bc weight and weight belt..

For my configuration I currently use 18lbs in my Zeagle, 10 on a belt, and 1.5 on each ankle. In this case I would quickly be able to dump 28 of my 46lbs..
 
The calculated addition of weight is correct as you figured it if you have the right specification for your steel cylinders, I recognize the Aluminum number. Be sure to check the exact brand and model of the aluminums. Catalinas would save you .8 pounds!

As for adding all of that as non-ditchable weight I'm thinking that it goes back to you being OK with your original configuration. The dynamic buoyancy of consumed air is only slightly greater. The net specific properties of the empty cylinders and weights are both static and will not be changing with depth so that can't get you into trouble.

My only thought is that adding all of that as V weights may make you a little top heavy. the buoyancy will be well placed but the heavier cylinders combined with the added weight puts a lot of mass up above you.

BTW, I have never used doubles. :confused:

Pete
 
Have you considered LP Steel 95's ? It would be better than adding 15 more lbs of weight, that will help by carring more gas, plus you will be able to take some weight off :palmtree: Bob
 
Bob, I like that idea, but I don't really want to spend that kind of cash for new tanks. I have currently 2 alum 80's, 2 steel 72's (2250psi) and 2 alum 50's..Ultimately, I would love twin steel 100's.

Spectrum, yeah, I think those are the correct figures as far as I can tell to the tanks I have...but you confirmed my thoughts on used exact correlating numbers vs some unknown formula for figuring the difference..lol.. The twins are already heavy..lol..I don't know that the extra 15lbs on there would make a noticeable difference.. I guess as long as my BC has the lift capacity all should be good..and yeah, I see what you mean that the extra 15 wouldn't make a difference in ditchable weight if my current situation is already good...
 
I used to dive generic steel 72's and in my experience, I had to add 4 lbs per tank when switching to AL80's to end up with the same bouyancy. -7.4 lbs negative strikes me as a bit much compared to +8.8 for Al 80's. Faber 3000 psi 72's are a bit more negative and would be in the range you are quoting.

The bad news is still that for twin AL 80's you ended up adding about 6 lbs in tank weight and 8 lbs of lead for not much more gas. That said, an 8 lb V weight should get you in the ballpark.

The suggestion to go with steel tanks is a good one, although LP 95's would not be my choice. For the same weight and dimensions you could get PST HP-119's or Worthington X8-120's and get a lot more gas at the rated fill pressure (and the LP 95's hold the same as the 120's when pumped to 3400-3500 psi).

I agree with you that 3442 psi 100's would be a good choice as well with about the same weight as a set of AL 80's but with better bouyancy and less lead weigth required. In fact, I can dive my steel 72's and X7-100's interchangably and both also have the same tank factor which is an added bonus.
 
Hate to be the first to ask, but why aluminum doubles? I've been thrilled recently not having to dive Al tanks! Just curious to see what everyone else is thinking, always nice to see if anyone has an idea I can steal :)
 
As there is very little info in your profile and about your configuration hard to answer the question.
I think adding 13lbs is a bit much, I find switching from steel to aluminum I normally add about 8 lbs to my system for doubles.

I would be looking at a v-weight or tail weight to add the extra lbs as it does not need to be ditchable. I would also look at losing the ankle weights and repositioning your weight as this might help your air consumption. If you are diving dry, practicing with the suit and getting your trim right (gaiters are a good learning tool) will eliminate the need for ankle weights- doing this will make finning easier and your air consumption will decrease.

The difference in air in the 72's and 80's is very minimal, if you are an air hog this change will do little to solve the problem. Better trim in your current set-up all your weight is on your waist or ankles which I can almost guess is pulling you out of the best hydrdynamic position in the water- correcting this problem will do more to help your air problem than adding a few extra cuft of air. I would suggest moving a bunch of that weight up to your back- v-weight or SS backplate are both good suggestions. Switching to a BP would also lose a few lbs of weight due to the inherent buoyancy of your BC- you can also strip the Zeagle down to the bare essentials to eliminate some of that buoyancy- take the lumbar pads and other unnecessary padding off this will help.

Fine tuning your weighting and trim is a better solution than going to al 80's. If after fixing your trim and weight and you still need more air, moving from steel 72's the next step would be 95's or 100's which is enough extra air to actually make a difference.
 
Thanks for the input...

DA, my 72's are 2250's..so maybe not as negative as I read on the chart(guessed which ones they were)

My current configuration is a Bare dry suit with lots of underwear (last dive was 39 degrees)..lol, I am running twin steel 72's (2250) with independent first and second stages. I could likely use better fins to help cut some air usage...my buoyancy control is pretty good (for a noob) and I use only enough in my suit for comfort/warmth...

In my personal inventory I have 2 steel 72's, 2 alum 80's, and 2 alum 50's. I don't really want to buy any new tanks right now...unless a good deal comes up on some steel 100's or the like..

I would like the most air possible to increase my bottom time, as I plan on staging a third bottle. So, my thought pattern was to twin the 80's..80'cu at 3000psi is better/more than my 2250x72'cu.
 
I would like the most air possible to increase my bottom time, as I plan on staging a third bottle. So, my thought pattern was to twin the 80's..80'cu at 3000psi is better/more than my 2250x72'cu.

A few more points to consider.......

First of all this is an air supply issue so whether you have the air at 3000 PSI or 2250 does not really matter, that's just a packaging detail.

Are your 72's + rated? It's only with the + rating and permitted overfill to 2475 that you will realize the true capacity. Without the + they are more like 65.5 CF at 2250.

Remember that the AL80 at 3000 PSI contains 77.4 CF, not 80.

That being said going from a + rated 72 to the AL80 only nets you 10.8 CF for the pair. If as a relative newbie with a good sac means .5 CF/MIN then at 66 feet this only buys you 7.2 minutes of bottom time as an example, and the rig will be a pig. In fact between lugging the extra lead and pushing the bigger cylinders through the water it may be close to a wash for all your trouble.

If you do not have the + rating on the 72s then you may add 15.8 minutes of bottom time at 66' with the AL80s before considering the efficiency.

Pete
 
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A generic steel 72 holds 71.2 cu ft at 2475 psi and 64.7 cu ft at 2250 psi. As stated above an AL 80 really only holds 77 cu ft at 3000 psi and at an average LDS 2800 psi under fill (a 3000 psi hot fill after the tank cools) it will hold 74.6 cu ft so the difference between the two really is pretty small if you can get the steel 72 filled to 2475 psi (and at 2600 psi the volume of the steel 72 is the same as the hot filled AL 80).

I'd continue to use the steel 72's as back gas and then on dives where you think you may need part of it, sling an AL 80 as a stage. They are bulky on the surface, but are barely noticeable in the water. The procedure here would be to use the gas from the AL 80 first and keep your reserve in the redundant doubles. On a day trip with two recreational dives, this doubles plus stage approach will let you do your first dive on the stage and a small portion of the gas in the doubles and then do the second dive on the remaining gas in the doubles. Realistically, many non-tech 6 pack boat operators get crabby with two sets of doubles but will probably be ok with 1 set of doubles and a stage and unless you have a tech inclined buddy who also has doubles (and you are first off the boat) you won't have the bottom time to require two sets of doubles.

That approach would give you a much lighter and friendlier configuration on your usual dives and it would give you an extra 77 cu ft when you feel you need it - more than enough gas to get into all kinds of trouble. Said another way, if you need more gas than what is in a pair of steel 72's, it's really time to start thinking about taking advanced nitrox and deco procedures courses. An investment in a third regulator and an AL 40 for a deco bottle is not all that much if you shop around. I recently bought a new in the box Aqualung Conshelf 20 for $60. (I don't use Aqualung regs, but it was still too good a deal to pass up).
 
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