Aluminum 63 buoyancy?
- Thread starter Gombessa
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Rich (UTAH)
Registered
Cave Diver,
I have a related question to Gombessa's. I 've been wanting to pick up another AL80, this one for my son that is in the process of his open water diver cert. Some have told me that I should be looking at the HP steel tanks instead of aluminum due to the buoyancy problems associated with the AL80; it becomes +4.4 lbs buoyant at the end of the dive when you're often preforming a safety stop in the 10' range.
This confuses me because as I perform a buoyance check at the beginning of my dive and apply the proper amount of weight to my BC pockets, I become 'neutrally' buoyant. If I am diving with an AL80 my buoyancy with change approximatly +5.8 lbs between the start and end of my dive. With a LP Steel 95, I have a buoyancy change from -8.0 lbs when full to -1.0 lbs empty, a buoyancy change of +7 lbs. With a HP Steel 100, I have a buoyancy change from -10.0 lbs to -2.5lbs, a similar change of +7.5 lbs.
My question is this, isn't the buoyancy change between start and finish the determining factor to being too 'buoyant' at the end of a dive? It seems to me that if I have a buoyancy pre-dive check and get properly weighted, I would have a bigger problem with being too buoyant with either of the steel tanks since they have the bigger change.
I would also like to ask the opinion of those of you out there that have used the LP and HP steel tanks and compare them to the standard AL80. (I do recreational diving, but the boats I typically dive from require you bring your own tank) Your opinions are appreciated.
Rich (Utah)
I have a related question to Gombessa's. I 've been wanting to pick up another AL80, this one for my son that is in the process of his open water diver cert. Some have told me that I should be looking at the HP steel tanks instead of aluminum due to the buoyancy problems associated with the AL80; it becomes +4.4 lbs buoyant at the end of the dive when you're often preforming a safety stop in the 10' range.
This confuses me because as I perform a buoyance check at the beginning of my dive and apply the proper amount of weight to my BC pockets, I become 'neutrally' buoyant. If I am diving with an AL80 my buoyancy with change approximatly +5.8 lbs between the start and end of my dive. With a LP Steel 95, I have a buoyancy change from -8.0 lbs when full to -1.0 lbs empty, a buoyancy change of +7 lbs. With a HP Steel 100, I have a buoyancy change from -10.0 lbs to -2.5lbs, a similar change of +7.5 lbs.
My question is this, isn't the buoyancy change between start and finish the determining factor to being too 'buoyant' at the end of a dive? It seems to me that if I have a buoyancy pre-dive check and get properly weighted, I would have a bigger problem with being too buoyant with either of the steel tanks since they have the bigger change.
I would also like to ask the opinion of those of you out there that have used the LP and HP steel tanks and compare them to the standard AL80. (I do recreational diving, but the boats I typically dive from require you bring your own tank) Your opinions are appreciated.
Rich (Utah)
You're weighting should be determined by the change in bouyancy from the start to the end of the dive.
If you are neutral at the start of the dive and your tanks have a +7 lb swing from full to empty, you'll have add additional weight to compensate for this at the end of the dive.
The main benefit to steel vs. al is that often the steel tanks are heavier.
For example, if you need 10lbs to be neutral with an AL tank at the end of the dive, you might only need 4lbs with the steel tank due to its negative characteristics.
If you are neutral at the start of the dive and your tanks have a +7 lb swing from full to empty, you'll have add additional weight to compensate for this at the end of the dive.
The main benefit to steel vs. al is that often the steel tanks are heavier.
For example, if you need 10lbs to be neutral with an AL tank at the end of the dive, you might only need 4lbs with the steel tank due to its negative characteristics.
The buoyancy CHANGE from full to empty depends on the cu. ft. of gas used PERIOD. The tank material or pressure rating does not effect the CHANGE in buoyancy at all.
If you are neutral at the start of a dive, your buoyancy at the end of the dive will depend on the amount of gas that you have used, not the tank material, size, or pressure rating.
If you are neutral at the start of a dive, your buoyancy at the end of the dive will depend on the amount of gas that you have used, not the tank material, size, or pressure rating.
You need to look at it in terms of the entire system.
Regardless of the tank, you should be neutrally bouyant at 10' at the end of the dive with 500 psi or less in the tank and no air in the BC (and if using a dry suit, with just enough air in the suit to loft the insulation and avoid a squeeze.)
If you need to add weight to acheive this neutral bouyancy (for example if you wear a 7mm wet suit or drtysuit with heavy underwear), then a steel tank that is perhaps -2.4 lbs bouyant when empty compared to an AL80 that is +4.4 lbs bouyant makes some sense as it would allow you to wear 6 lbs less on your weight belt. When comparing steel to AL tank weights and lead weight requirements in this situation, you may find that the steel tank may actually result in you carrying 10 lbs less total equipment weight up the ladder at the end of the dive - which is why everyone recommends steel tanks.
However this is not always the case as if on the other hand you dive with a 3mm shorty or even just a T-shirt, you may not need any weight at all to be neutral at the end of the dive with an AL80 and the same steel tank would leave you 6 lbs negative at the end of the dive and 11-12 lbs negative at the start. This would mean you are saving nothing in total weight and the extra gas in the BC will make precise bouyancy control a little more difficult and labor intensive throughout the dive.
Regardless of the tank, you should be neutrally bouyant at 10' at the end of the dive with 500 psi or less in the tank and no air in the BC (and if using a dry suit, with just enough air in the suit to loft the insulation and avoid a squeeze.)
If you need to add weight to acheive this neutral bouyancy (for example if you wear a 7mm wet suit or drtysuit with heavy underwear), then a steel tank that is perhaps -2.4 lbs bouyant when empty compared to an AL80 that is +4.4 lbs bouyant makes some sense as it would allow you to wear 6 lbs less on your weight belt. When comparing steel to AL tank weights and lead weight requirements in this situation, you may find that the steel tank may actually result in you carrying 10 lbs less total equipment weight up the ladder at the end of the dive - which is why everyone recommends steel tanks.
However this is not always the case as if on the other hand you dive with a 3mm shorty or even just a T-shirt, you may not need any weight at all to be neutral at the end of the dive with an AL80 and the same steel tank would leave you 6 lbs negative at the end of the dive and 11-12 lbs negative at the start. This would mean you are saving nothing in total weight and the extra gas in the BC will make precise bouyancy control a little more difficult and labor intensive throughout the dive.
Rich (UTAH)
Registered
Thanks everybody for the replies. This actually makes much more sense. Thanks for clarifying this.
Spot on Aquamaster! I have used this method of determining optimum weight with over 100 divers. The only difference, we check for neutral buoyancy at 15 during our safety stop. I have never had an ascent problem from the safety stop.
This gives us an opportunity to practice things like hovering, trim and free flowing regulators. When we hit the 500 psi mark, no air in the BCD you should be totally neutral. If not add or subtract a pound or two until you hit the sweet spot.
Mark
tommy.okc
New
The 80 cubic foot aluminum has 1.4 pounds negative buoyancy full and the 63 cubic foot tank has a 2.2 negative buoyancy full. At 500 psi the 80 has 3.4 positive and the 63 cubic foot tank is 1.7 positive. This is based on the aluminum cylinders distributed by XS Scuba per their catalog.
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