Calculating SCUBA Cylinder Capacity | Dive Gear Express®
LP85: Steel BS85 (Blue Steel/Faber) † 13.0 L 2640 psi 82.5 ft3
HP100: Steel X7-100 HDG (XS Scuba/Faber) 12.9 L 3442 psi 106.7 ft3
These are the real numbers. The important one is the capacity. The LP85 is 13.0 liters. The HP100 is 12.9 liters That is a difference of 0.77% which is less than 1%
Three things confuse the issue; the inaccuracy of labeled capacity, the ideal gas laws and the compressibility of gas being non linear which creates a diminishing return.
You have to look at the actual numbers rather than the labels. The LP85 is rated at 81.1 cu ft at it's rated pressure of 2640 due to the Z factor of 1.0173. The hp100 holds 101.3 cu ft at 3442 psi. The Z factor of air at 3442 is 1.0532. The Z factor of air at 3,600 psi is 1.059.
The LP85 ideal gas capacity is 82.5 cuft. at its service pressure of 2,640. If you fill it to 3,600 psi the equation goes like this. 3,600/2640*82.5=112.5/1.059(Z factor)=106.23 cuft.
The HP100 ideal gas capacity is 106.7 cuft. at its service pressure 3,442. If you fill it to 3,600 psi the equation goes like this. 3,600/2,640*106.7=111.6/1.059(Z factor)=105.38 cuft.
You get less than one cuft additional gas into the LP85 at 3,600 psi than the HP100 at 3,600 psi.
The Z factor is an expression of the fact that at higher pressures gas molecules don't compress as tightly and so the higher you compress the less each additional 100 psi adds to the volume of compressed gas.
Here is a link to a handy chart.
BAUE Z Factor Table
Of course if you cave fill your LP85 you will get much more than 85cuft but you won't get more than your HP100. This matters if you are really counting on having 15 additional cuft and really have only .85 cuft additional.
I hope I didn't explain this in a clunky fashion.