Now for medical tanks with the CGA 870 valve:
We recommend that all straight thread valves be installed in Luxfer high-pressure aluminum scuba cylinders with a torque recommended by the valve manufacturer for aluminum cylinders.
Without a recommended torque from the valve manufacturer,we would recommend a torque of 50 lbf-ft, plus or minus 10 lbf-ft. All torquing should be done such that valve, valve components and cylinder are not damaged.
Basically - this is a cylinder manufacture who gives a baseline range for valves from other manufacturers - with a plus/minus 10 ft-lb range and then proceeds to give an exact value for a valve/tank combination they have and describe the ramifications for failing to adhere to it. The SCBA tanks luxfer makes out of Al have the 50ftlb ±10ftlb torque spec - or 40-60ftlb range and the 'non-critical' comment I have made above. Conversely, the 870 tanks have a specific 75ftlb requirement.
Luxfer has extensive experience with new aluminum and composite cylinders, as well as new valves of the standard 870 post-type valve design for oxygen. For such new equipment, Luxfer recommends a torque value of 75 ft.-lbs. However, under certain circumstances described below, this torque value may not apply.
Thus, Luxfer’s current recommended torque value of 75 ft.-lbs. applies only to the installation of a new standard brass, straight-threaded, nickel-coated 870 post-type valve with an approved o-ring into a new Luxfer oxygen cylinder. The o-ring makes a seal in a seat that Luxfer has machined into the cylinder opening. Since Luxfer seat dimensions may differ from those used by other aluminum cylinder manufacturers, Luxfer’s recommendations only apply to Luxfer cylinders. The o-ring material and size are critical for a leak-tight seal—not just any o-ring will work properly with a Luxfer cylinder. In fact, with valve types that use a metal o-ring, if the o-ring is installed incorrectly, gas will leak from the cylinder no matter what torque is applied.