D_B:
I am still undecided on the LP / HP tank as to which one to get ... I like the smallness of the HP (new 3450psi) tank.
This information is from Pressed Steel, but generally it should come pretty close to applying to cylinders from other manufacturers.
The High Pressure E8-130 cylinder has the same internal volume as the Low Pressure 104. What this means is, if you were to fill the E8-130 to 3442 PSI you would have a tank with approximately 130 cubic feet of air in it, but if you filled the exact same tank to 2640 PSI (2400 PSI with the allowable 10% overfill) you would have 104 cubic feet of air in the tank. Following this same comparison the tanks below have the following capacities.
E8-130 at 3442 PSI = 130 cu. ft., at 3000 PSI = 117 cu. ft., at 2640 = 104 cu. ft.
E8-119 at 3442 PSI = 119 cu. ft., at 3000 PSI = 107 cu. ft., at 2640 = 95 cu. ft.
E7-120 at 3442 PSI = 120 cu. ft., at 3000 PSI = 107 cu. ft., at 2640 = 96 cu. ft.
E7-100 at 3442 PSI = 100 cu. ft., at 3000 PSI = 89 cu. ft., at 2640 = 80 cu. ft.
E7-80 at 3442 PSI = 80 cu. ft., at 3000 PSI = 72 cu. ft., at 2640 = 64 cu. ft.
E7-65 at 3442 PSI = 65 cu. ft., at 3000 PSI = 58 cu. ft., at 2640 = 52 cu. ft.
If you follow what I am showing, you can buy a high pressure tank and fill it to high pressure when you need the extra volume, but when you do not need the extra air or if a high pressure fill is not available you can always use it as a mid pressure tank, or even a low pressure tank.
If you own a low pressure tank you are limited to the low pressure fills and the volumes associated with those fills.
I own both Faber 95's (low pressure) and PST E8-130's (high pressure). At my next annual inspection I will switch things around a bit and have the Fabers as single tanks and the PSTs as doubles. Low pressure steel tanks can be found at fairly reasonable prices while high pressure steel tanks are still pretty expensive.
Mark Vlahos