In general, it makes sense for a cold water diver to dive a negatively bouyant tank given that with a dry suit or thick wet suit, he or she will need to add lead anyway to acheive neutral bouyancy. And in most cases the resulting total weight is significantly less.
For example an X7-100 weighs 33 lbs but is 2.5 lbs negative when empty compared to an AL 80 that weighs 31 lbs and is 4.4 lbs positive when empty. So you gain 2 lbs in tank weight (but another 20 cu ft in capacity in what is a smaller package) but you can take 6.9 lbs off the weight belt compared to what you would need with an AL80 to be neutral at the end of the dive so the net loss of total weight you lug up the ladder is 5 lbs.
On the other hand, for a warm water diver things can quickly get out of hand. A Faber HP 80 for example is 7.25 lbs negative when empty so unless you currently require 11.5 lbs of lead to be neutral at the end of a dive with an AL80 and your warm water exposure suit, you would be badly overweighted with an HP 80. This would impact your ability to precisly control bouyancy as the larger bubble is harder to manage and it could also make an ascent diffilcult if your BC were to fail (if for example the inflator assembly broke off). This is particularly true early in the dive when the Faber HP 80 would be 13.25 lbs negative.
When you consider doubles, things quickly get worse by a factor of 2 plus the weight of the manifold and bands. Given the potential to dive dry in the winter and to dive in a 3mm in the summer, I tend to prefer tanks that are fairly close to neutral when empty. In the winter with a pair of X7-100's (-5 lbs empty), I will need a SS plate (- 6 lbs) and a v-weight (-8 lbs) to sink a well insulated dry suit, but in the summer I am not over weighted in a 3mm wet suit with an AL back plate (- 2 lbs) and no v-weight.
In comparison AL 80's would require the 8 lb v-weight in the summer as well and would require another 8 lbs of weight in the winter with a dry suit - an awful lot of lead. At the other extreme, a very negative tank like a Faber HP 80 or HP or HP 100 would let me take off 10 lbs in the winter, but would leave me 10 lbs over weighted in a 3mm in the summer - meaning that if my wing failed early in the dive, I would be 24 lbs negative - a lot of weight to swim up from the bottom.
So my advice is to consider the diving you will be doing as well as the suit related bouyancy and weight requirements and then select your tank(s) as part of the overall system. The nice part is that there is enough variety in tank capacity and bouyany to find one that will meet your needs almost perfectly.
For example an X7-100 weighs 33 lbs but is 2.5 lbs negative when empty compared to an AL 80 that weighs 31 lbs and is 4.4 lbs positive when empty. So you gain 2 lbs in tank weight (but another 20 cu ft in capacity in what is a smaller package) but you can take 6.9 lbs off the weight belt compared to what you would need with an AL80 to be neutral at the end of the dive so the net loss of total weight you lug up the ladder is 5 lbs.
On the other hand, for a warm water diver things can quickly get out of hand. A Faber HP 80 for example is 7.25 lbs negative when empty so unless you currently require 11.5 lbs of lead to be neutral at the end of a dive with an AL80 and your warm water exposure suit, you would be badly overweighted with an HP 80. This would impact your ability to precisly control bouyancy as the larger bubble is harder to manage and it could also make an ascent diffilcult if your BC were to fail (if for example the inflator assembly broke off). This is particularly true early in the dive when the Faber HP 80 would be 13.25 lbs negative.
When you consider doubles, things quickly get worse by a factor of 2 plus the weight of the manifold and bands. Given the potential to dive dry in the winter and to dive in a 3mm in the summer, I tend to prefer tanks that are fairly close to neutral when empty. In the winter with a pair of X7-100's (-5 lbs empty), I will need a SS plate (- 6 lbs) and a v-weight (-8 lbs) to sink a well insulated dry suit, but in the summer I am not over weighted in a 3mm wet suit with an AL back plate (- 2 lbs) and no v-weight.
In comparison AL 80's would require the 8 lb v-weight in the summer as well and would require another 8 lbs of weight in the winter with a dry suit - an awful lot of lead. At the other extreme, a very negative tank like a Faber HP 80 or HP or HP 100 would let me take off 10 lbs in the winter, but would leave me 10 lbs over weighted in a 3mm in the summer - meaning that if my wing failed early in the dive, I would be 24 lbs negative - a lot of weight to swim up from the bottom.
So my advice is to consider the diving you will be doing as well as the suit related bouyancy and weight requirements and then select your tank(s) as part of the overall system. The nice part is that there is enough variety in tank capacity and bouyany to find one that will meet your needs almost perfectly.
