is there a rule of thumb for air consumption?

[Rhone Man]

my sac while cruising is ~28 and hunting is ~40

This doesn't tell us much without a depth to relate it too.

If it is cu ft/min at surface, it is an impressively low SAC. I think mine are low, and they are 0.42 and 0.58 respectively. Careful you don't start bumping into NDLs with a 120.

 

[fire_diver]

Yeah, a .28 SAC is incredibly low. I think thats about the lowest I have ever had. I average about .45 which is still considered low in the diving world.

I think you made an error in your math.

 

[elan]

good point, the depths were the same thats why i didnt reference it. around 40' deep for both.

and my units for sac are psi/minute.

how many dives do you have ? 40psi /min on AL80 gives about 1cuf/min depending on your shape you might get an improvement with time. If your rate is stabilized already then just get a bigger tank

 

[Graeme Tolton]

A bigger tank is a great option. However, I don't think that 120's make good doubles... If you are looking for more bottom NOW, you will be looking for even more later (typically). I dive double 119's now, was diving them single for a few years... They are a good all round tank. Some say the 130's trim better as doubles, but I am 6'6" and have no issues whatsoever. That being said, I dive dry with BP/W setup. Are the 120's popular where you are, do your buddies dive them? If so, try one out before buying it. You might love it, you might hate it. My advice would be to try as many tanks as you can before buying anything.