Lou...a "fast fill" generates heat, heat generates condensation of any moisture in the air used to fill. Granted there should be no moisture in the air you fill your tank with, but with that thinking I should have blue eyes, thick black hair, and exceptional good looks....life ain"t like that!
As fast fill hot air cools, pressure goes down, requiring the need to wait and then add air.
In the end, a "slow fill" is the best way to exchange air from a fill tank to a smaller pcp cylinder.
To my knowledge, the hydro people will tell you if the tank cannot be re-hydro'd. Steel and aluminum tanks are hydro'd many years over their anticipated life expectancy. Carbon fiber DOT certified tanks can be hydro'd more than once...if there's a problem, expect them to advise you it is at it's life's end.
Further, frequency of use isn't necessarily a factor in life expectancy, unless the tank was repeatedly "cave filled", or filled past it's rated maximum pressure. The standard used to be 3/5ths maximum hydro pressure was how a tank was rated for psi fill maximum. Commercial scuba tanks are used thousands of times in a season at a busy resort. Use doens't necessarily mean "wear" in a scuba tank...unless it is abused by no maintenance procedures over it's lifetime.
Things may have changed and use the above info at your own risk!!!
That's what I know...use it how you will. Not everyone will agree, individual mileage always varies!!!
Hoot
I mostly agree with everything in your post but the moisture part. If a tank is filled with confirmed 'dry' filtered air from a compressor, scuba fill shop, or from a large nitrogen tank, then thats it, no water will be introduce to any tanks filled with the large dry' fill tank regardless of speed of fill from that 'dry tank. If the fill tank is not dry, has water vapor in it from a poorly set up compressor, than speed of fill won't change anything, you will get that water in what ever tank you fill with that wet tank
The valve on many tanks humans use to breath with(SCUBA or SCBA) have a short pipe on the tank valve that sticks down into tank so if any water/debris is in the tank, no matter the position of the tank, the water can't get to the end of the pipe as water/debris settles to the lowest point. Water/debris is only able to find the exit pipe if the tank valve is opened fully for many seconds creating so much turbulent flow it carries out water/debris. Something that never happens with a regulated human breathing air tank. But slowly cracking open/regulating the valve of an unregulated tank for a fast fill will not create enough turbulence to let anything on the tank bottom come out that pipe.
Since we don't use these tanks to breath with and tank is used from standing up or laying on its side, I remove this pipe making the valve port flush with the bottom of an inverted/upside down tank. That way I can invert the tank and slightly crack the valve to make sure no water/mist comes out confirming I've got a dry fill. Crack the valve open slowly and just slightly for this check, any water will come right out.
As Always when filling with a compressor keep it dry/clean by using appropriate filters/desiccants/etc... Or best yet, I love my nitro tank for pcp, imo dry/clean inert nitrogen is by far the best way to fill pcp/pcp tanks hands down. If like most, not using a regulator on the big nitro tank, just have to not fall asleep at the wheel letting your pcp gun/tank get filled to 6000psi. Thats the reason some manufactures recommended against nitrogen, its a liability for asleep at the wheel fill to 6000psi types...
Always have tanks hydro tested as required, never use outdated tanks. Always keep an eye on fill station hoses and connections, etc,, for wear, abrasion, kinks, leaks etc. Many hydro shops will check these for you also if you're not confident of doing it.
jmo