well I'll start this one off since the other guys took the last few, but the air you are breathing is at ambient pressure not interstage pressure, and interstage pressure is x+ambient not just straight x. If you were breathing straight interstage pressure your lungs would explode quite quickly, that is why there is an air chamber in the second stages. Without it it would be quite dangerous.
Pistons are also not any cheaper to service or purchase once you are at the high end of the spectrum, seen Atomic and Scubapro's pricing lately?
1. I usually don't do that with used regs, preferring to have them taken to a technician first *usually my kitchen table, but I'd rather have a shop bench test them, usually costs $10 to have them check all of the things you need to know*
2,3,4,5 see 1
6. If you know how to do that, you wouldn't be reading this article
7. doesn't really matter because they aren't going to remember that regulator and most of the time they won't have service records
8. 9. valid
10. doesn't matter, regulators should be service when they ask for it which is usually much longer than that service interval. At that interval take it to the tech and ask them to bench test it for IP and cracking pressure, if it checks out, then adjust as needed and keep diving. Service intervals exist only to make dive shops money, I have quite a few of your 1/100 type regulators with well over 500 dives on them, and most of my dives are well over an hour long in caves or wreck penetration, so there's that.... The key is to use them regularly, they don't like sitting
11. probably most important thing you mentioned
12. I have a Poseidon Cyklon made in 1959, I can still buy every part for it, it was the first of the "modern style" 2 stage regulators. I have buddies with Dacors from 10 years ago that while they can be serviced, they don't make parts for them any more, age has nothing to do with it, it's all about what is still supported.
Dust cap is for dust, not water. Leave the reg connected to the tank and pressurized when dunking, the valve needs rinsing too
Don't ever leave a regulator out in the sun, it destroys rubber hoses very quickly and makes the second stage housings brittle. Hanging it up in a decently well ventilated place works just fine, i.e. draped over the bathroom curtain rod. Don't leave it near refrigerators or in the garage unless it is locked in a rubbermaid since ozone doesn't play nice with rubber and plastic either
Second most expensive piece to what? If paying new prices regs are much more expensive than BC's, and other than a drysuit I can't think of anything that comes close.... I also don't care about seeing service histories because it doesn't really matter. All soft parts can be replaced during a normal service and last I checked metal doesn't degrade. If the gear looks like it is going to work, it probably is....
