Pony Setup

[Pinecube]

Weight you carry on land is supported by your back. Weight you carry in water is supported by the bouyancy of your BCD and exposure protection. What might have changed is your trim. Different tanks will affect your trim differently. With the 15L tank you may have unknowingly been arching your back to compensate for the trim which can give you a sore back.

The instructors on this board could probably explain it better, but I found this out the hard way on a trip to the carribean. I was wearing rental gear and I didn't take the time to properly weight/trim myself. In my case the BCD was probably too loose as well and I ended up with a sore back later that evening.

 

[tbone1004]

I don't know what the 12l is because we don't have a faber equivalent in the US, but the 15l is either a LP95 or FX117. Neither of which are particularly heavy ~4lbs heavier than an AL80, but roughly the same rig weight on land with the adjustment for buoyancy.
The back pain could have been from back arching due to improper tank positioning. The picture showed a standard jacket bc which usually has the tanks much lower on the body than a bp/w and while the 15l's are short, it appears the OP is as well.

So looking at quick/rough SAC numbers assuming the average depth is accurate, the SAC rate is somewhere between 0.55 and 0.7cfm or 15-20lpm. This is considered well within the normal range and while the high side could be considered a bit high, it's not bad at all. I would look at streamlining your gear and diving because the addition of a bottom stage, especially when hung like it is in the picture is only going to increase your SAC rate because you are not going to be able to move as efficiently through the water

 

[BoltSnap]

I don't know what the 12l is because we don't have a faber equivalent in the US, but the 15l is either a LP95 or FX117. Neither of which are particularly heavy ~4lbs heavier than an AL80, but roughly the same rig weight on land with the adjustment for buoyancy.
The back pain could have been from back arching due to improper tank positioning. The picture showed a standard jacket bc which usually has the tanks much lower on the body than a bp/w and while the 15l's are short, it appears the OP is as well.

I have and use both, faber 15L and the short 12L tanks here and don't have a problem with back. They aren't that much different in weight. I think his positioning of the tanks on his back is the issue or perhaps his torso is unusually short cause an issue with longer tanks.

 

[rx7diver]

I think his positioning of the tanks on his back is the issue or perhaps his torso is unusually short cause an issue with longer tanks.

I have the opposite experience: If my tank is too short, then, depending on which plate or BC I wear, my lower back can ache BIG TIME. If the tank rests on, or comes into contact with, my lower back, then backache often results.

A longer tank will rest on my derrière. No backache!

And, aside from the safety reason for NOT having a weight (on your weight belt) positioned in the small of your back, another big reason pertains to backache: My tank resting on, or coming into contact with, a weight which is positioned in the small of my back will almost certainly cause backache!

EDIT: Also, wearing my weight belt on my waist--instead of lower, on my hips--will cause backache, too, depending on how much lead is on the belt.


Safe Diving,

rx7diver