Specs on the old 72cf steel tanks?

[engdiver]

Looking at most other tanks I noticed that the majority tend to be neutral +/-3 lbs when empty. It would seem that a more negative buoyancy tank would be beneficial in eliminating lead required. Is there a downside to this thinking?

 

[Nemrod]

If you have a set of for real LP 72s such as the ones that Sparticle displays you are in luck. Those from most of the 60s and all of the 70s should have the same thread at the neck as any standard aluminum tank or commonly available steel tank of today. You can use any manifold on these tanks you wish and custom bands can be made for them such as a set I have made for me by Vic from ebay. Excellent. He actually made them for a set of 50s but they work perfect on my steel 72s as well. They balance well and provide nearly 140cf or more if I pump them up. We used to cave fill these things back in the early 70s. The steel 72 is an excellent tank to dive.

"Looking at most other tanks I noticed that the majority tend to be neutral +/-3 lbs when empty. It would seem that a more negative buoyancy tank would be beneficial in eliminating lead required. Is there a downside to this thinking?"

Yes there is, not all diving takes place in cold water where heavy exposure protection is needed and therefore there may be no "lead" to eliminate. In such a case heavy tanks result in overly inflated BCs which affect the darling of scubaboard--> trim and balance and all in all are a PITA. Not everybody dives in a dry suit. In a 3/2 suit I may have only three or four pounds of lead in saltwater and in fresh--NONE. I think you can see how a heavy, negative tank would then be detrimental or at least not optimal.

In my avatar there you might notice a steel 72, I am wearing a rash guard tee shirt and swimsuit. I am carrying no lead and I have no BC and I am virtually neutral throughout the dive within the limits of breath control to control bouyancy. With these modern heavy steel tanks this would not be possible. N

 

[engdiver]

I did not mention that I was diving with a dry suit and approx 26 lbs of lead. I understand that warm water diving requires much less lead due to less of a wetsuit or none at all. I remember diving in Cozumel with only 4 couple pounds of lead and a single aluminum 80.
I am looking strictly at reducing lead for cold water dives. I forget how broad an audience scubaboard really has!

 

[pescador775]

OK, I will repeat myself. Dry suit divers sometimes use doubles with net negative buoyancy. Among some groups, although the tanks themselves have neutral characteristics, the effective buoyancy due to heavy steel plates and ballasted vests creates a situation as if they were diving with tanks filled with sand. Is that heavy enough? Sho' is but it alters their perspective when responding to posts like yours. In other words, a Faber HP tank suddenly feels godawful heavy.

 

[royalediver]

Looking at most other tanks I noticed that the majority tend to be neutral +/-3 lbs when empty. It would seem that a more negative buoyancy tank would be beneficial in eliminating lead required. Is there a downside to this thinking?

Be careful when doing your calculations. You want to be positve without lead. A few divers have entered the water negative without a weightbelt and have died because the could not acheive positve buoyancy.

 

[SparticleBrane]

Then they were stupid for not inflating their BCs or being able to swim up their rig.
I enter the water on every dive, negative, without a weight belt. There's no need for me to have one; I can hold my last stop without a weight belt. In fact, I'm usually still slightly negative at the end of the dive, even if the tanks don't have much left inside.

 

[pescador775]

Be careful when doing your calculations. You want to be positve without lead. A few divers have entered the water negative without a weightbelt and have died because the could not acheive positve buoyancy.

Engdiver. That is absolutely correct, BE CAREFUL. If you don't know how to subtract 13 from 26 we have a problem, Houston.

 

[SparticleBrane]

Well I suppose it depends on what you mean. When I walk into the water, I'm positive due to my BC, but when I empty it I'm negative. Were you talking about just jumping off the boat and directly descending--if so, I don't do that.

 

[engdiver]

Sorry about getting off the original post here but it brings up something that still confuses me a little. Divers that use large doubles seem to be able to remove all lead from there belts and be negative throughout the dive. Looking at differnet tanks I can see the ability to do this when the tanks are full (eg. two LP121's are 9 lbs negative full x2 = 18 lbs negative). When the tanks are nearly empty they are 1 lb positive each. That changes there buoyancy by 20 lbs during the dive. How do they stay negative at the end of the dive?

 

[dsteding]

Sorry about getting off the original post here but it brings up something that still confuses me a little. Divers that use large doubles seem to be able to remove all lead from there belts and be negative throughout the dive. Looking at differnet tanks I can see the ability to do this when the tanks are full (eg. two LP121's are 9 lbs negative full x2 = 18 lbs negative). When the tanks are nearly empty they are 1 lb positive each. That changes there buoyancy by 20 lbs during the dive. How do they stay negative at the end of the dive?

Your particular example may be at the extreme end of the scale, but when I went to double 119s I dropped from 23 pounds of ballast (diving dry, steel 98s/119s) to just a 10 pound plate.

Math on the reason why (my rough numbers)

2-3 pounds for the extra reg, ~4 pounds for the bands, ~3 pounds for the extra manifold/valve, 2 pounds for the extra 119 and call it a pound or two for the argon system.

Considering you are about ~8-9 pounds negative with a balanced rig in a singles setup, this drop in weight corresponds almost exactly with the additional weight of gas (assuming it is nitrox) that you are carrying. Therefore, you should still have a balanced rig.

Your example of the 121s is a bit different, they do go positive when empty, but at 500 psi they are just about neutral, so roughly the same analysis should apply (maybe you have to have few more pounds as compared to 119s, for instance).

I used to think about all this a fair amount (as the above demonstrates), now I just try to get it in the ballpark and then go out and do a weight check.