Steel vs Ally Tank?

[TSandM]

gee13, if you are carrying the correct amount of weight for the equipment you are using, you will have the same amount of air in your BC at a given depth, no matter what tank you are using. The total weight you will carry will vary with the buoyancy characteristics of the tank, as people have described above, but since you are weighting yourself to be neutral at a given depth, with a given amount of gas in the tank, the compensation your BC will be doing will be the same -- compensating for the gas you are carrying and intend to use, and compensating for the compression of your wetsuit.

Getting correctly weighted IS important, because being significantly overweighted makes your buoyancy issues more difficult, as has already been described. But ending a dive too light is both uncomfortable and potentially dangerous, so if you are going to err, it is better to err a little on the heavy side. Therefore, I would not recommend Tobin's approach to a brand new diver.

You can do a weight check either of two ways: Either at the end of the dive, purge a regulator until your tank is at 500 psi and check that you are neutral there, or check with a full tank and add one pound for each 13 cubic feet of gas you are carrying. (Sorry, I don't know what that is in liters off the top of my head.)

In general, in cooler water, using steel tanks will result in carrying a lower total weight of gear. You are also essentially putting some of your ballast on your back, which encourages better trim.

 

[halocline]

Most people suggest you set your initial buoyancy so that you float eyeball high with an empty BC and an empty tank. This has been the standard approach for many years.

Tobin, at Deep Sea Supply, has a different approach and it may apply more specifically to 7mm wetsuits. He suggests eyeball high with an empty BC and a full tank.

While I'm not sure that Tobin would totally agree with your assessment of his weighting technique, the simple fact is that if you are neutral at the surface with a full tank, you're going to be 5 lbs light with an empty tank.

For virtually everyone, being 5 lbs positive at the surface is going to make it very difficult to hold a safety stop at 15 ft. If you have a particularly buoyant suit that's getting compressed at 15 ft, I suppose that theoretically it could lose a few pounds, but I really doubt 5.

It's true that some divers overweight at the surface due to a little trapped air in the wetsuit or BC and that air can get squeezed out during a dive. But probably not 5 lbs.

A good way to check your weight is at the safety stop with 500 PSI in your tank. If you can, hand off weights to your buddy until you have trouble holding the stop. That's your minimum weighting. It might be best to do this at the end of the stop so if you go too light and head for the surface you won't blow your stop.

 

[Teamcasa]

Too many people just tend to overthink it.
For me, I use both but my first choice is my HP100 vs my AL80

HP100 = more gas, 5 less lbs on my belt.
When I use the AL80, I just add 5 lbs of ballast and go, albeit a little less bottom time. Simple.

Does the added cost of a steel tank make up for the added bottom time, for me - yes.
AL80 vs HP80, is worth it for my wife to take the extra 5lbs off her belt, for me, nope.

 

[LeadTurn_SD]

...Therefore, I would not recommend Tobin's approach to a brand new diver....

I'll second that. I have dove using Tobin's approach in the pre-BC days, and I still dive a couple pounds "lighter" than the traditional end-of-dive buoyancy check would have me at.

Most new divers are overweighted by their instructors, and do need to shed some ballast almost immediately, but I think that at the level of bouyancy control most newly certified divers have, it is safer to be a couple pounds on the heavy side of "perfect-weighting" rather than a couple pounds light. But just a couple pounds heavy, not 5, not 8!!, etc.

Once buoyancy control is no longer an issue, then you can dive a few pounds lighter than the traditional weighting procedure and still hold a safety stop just fine (even in a 3mm suit), I just do it closer to 20' if I'm feeling a bit "floaty"

Best wishes.

 

[rstofer]

While I'm not sure that Tobin would totally agree with your assessment of his weighting technique, the simple fact is that if you are neutral at the surface with a full tank, you're going to be 5 lbs light with an empty tank.

I think he will be satisfied. It is EXACTLY the way he continues to explain it, over and over, on the BCs forum. Eyeball level with a full tank and empty BC.

I'm not saying that I have achieved this or that I have tried to hold 15' at the end of the dive. But I am saying that this is EXACTLY what he recommends.

For virtually everyone, being 5 lbs positive at the surface is going to make it very difficult to hold a safety stop at 15 ft. If you have a particularly buoyant suit that's getting compressed at 15 ft, I suppose that theoretically it could lose a few pounds, but I really doubt 5.

His point of view is that the traditional 7mm Farmer John (not necessarily my 7/8mm SolaFX) does, in fact, lose about 8# of buoyancy at 15'. Well, he would certainly know more than me.

His other point is that the wetsuit will not be as buoyant at the end of the dive as it is at the beginning because the neoprene cells will still be compressed and they won't decompress any time soon.

Again, I suspect his recommendations are based on actual observations. He doesn't seem to be the type to just do things willy-nilly.

It's true that some divers overweight at the surface due to a little trapped air in the wetsuit or BC and that air can get squeezed out during a dive. But probably not 5 lbs.

But have you actually measured it? Have you tried it? Apparently Tobin has worked with this procedure long enough to make it his standard recommendation.

A good way to check your weight is at the safety stop with 500 PSI in your tank. If you can, hand off weights to your buddy until you have trouble holding the stop. That's your minimum weighting. It might be best to do this at the end of the stop so if you go too light and head for the surface you won't blow your stop.

There is nothing wrong with this approach. It is the standard in the industry. However, if the wetsuit does compress and doesn't decompress very fast, there will be a different result if you are returning to 15' from 100' versus just diving a 500 psi tank from the surface to 15'. Considering Tobin's approach, you will need a lot more weight if you just start from the surface because the wetsuit was never compressed.

It's probably worth discussing alternate concepts. Otherwise we could just point to page such and such of the PADI manual and call it a day.

Personally, I am still on the heavy side at the beginning of the dive. As I have to trudge that extra weight, I plan to work toward reducing it. If I were boat diving, I wouldn't care.

Richard

 

[divingTim]

I have 3 aluminum 80,s and 1 hp steel 80, and the only reason for the steel is, thats its my wifes tank and its about 7" shorter. The aluminum tanks are half as long as she is.

 

[halocline]

But have you actually measured it? Have you tried it? Apparently Tobin has worked with this procedure long enough to make it his standard recommendation.

You're referring to the amount of trapped air in a suit and/or BC at the beginning of a dive, I just didn't know how to include my comments in the quote.

My answer is, I don't know exactly how you would measure that, but I have definitely helped many beginning students and typical OW vacation divers with weighting. I can tell you from experience that if they are weighted neutral at the surface with a full tank, they're probably going to have problems holding a stop with 500PSI. It's true for me too, and I have decent buoyancy control. Now, most of these people, including me, are wearing at the most a 5 mil suit. With, say, a 7 mil farmer john, sure it's going to lose more buoyancy at 15 ft, but I still would very strongly bet that most people, in most cases, are going to have trouble keeping a stop at 15 ft if they're 5 lbs or more positive on the surface.

You can't argue with the physical fact that the gas in the tank weighs 5 lbs or more if you're carrying a bigger tank. You have to account for that one way or the other when figuring your weighting.

 

[rstofer]

As you point out (as did I), the recommendation applies to 7mm wetsuits. I don't know whether it is recommended for 5mm wetsuits.

It is worth considering that conventional wisdom (eyeball height with a 500 psi tank) may not be the optimum.

I still have some more experimenting to do.

Richard