Alkin Airetex W31 has been 10' tall and bullet proof over the last 5 years for me. Oil, filter cartridges, vent tube lines and some elbow grease is all it has cost me in upkeep. I keep a meticulous run log and have my air tester 4 times per year. Mine maintains an 8 bottle bank at 3500psi.
For two years I routinely used it for continuous blend NITROX fills when I was doing public air fills.
Owner of the company is a great guy and will answer any questions you might have. I understand from talking to them at DEMA the new W31 is supposed to be even tougher than mine.
I would not hesitate to buy another one if I live long enough to wear this one out.
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Doing it right should include some common sense, not just blindly following specs and instructions. .Gary D, AWAP on SB