this topic has been around forever.
here is the real poop on this. there is a deesignated wp working pressure that you are not supposed to exceed. DO NOT OVER FILL is related to that number. People refuse to accept that you can gage wise over fill a tank and it is acceptable SO LONG AS when the pressure is corrected to I think 70F it is working pressure or less. here are examples (for reference 1 degree F is about 5psi) your tank is geting filled it is a hot day and no matter how slow you fill the tank the tank is 110F that is 40 psi over the spec temp which the spec PSI is mated with. if you fill it to 200 psi over WP it is not an over fill, so that a 3000 psi al80 can be filled to 3200 if the tank is 110F. Now lets look at the reverse problem which was not really considered at the time as diving goes but is in fire fighting. you tank is 40F so that if you fill it to 2999 psi at 40F you are legally over filled by 150 psi if the temp were corrected to 70 degrees. If someone tells you other wise they lack understanding. Now if you filled a tank to wp of 3k at 70F and you take the tank to the lake and the tank heats up to 100F... You put the reg on and the psi reads 3150 psi,,,, are you now in violation of working pressure being exceeded. NO.
The water bath issue is also a BS thing. It is a rule to prevent water from getting into the valve and into the tank causing rust and long term wall thickness reduction. This is a common sense thing. put the fill hose on the tank than set the tank in the bath instead of having water in the tank valve and then putting on the fill hose and pushing the water into the tank. The disdane about wet filling is that some believe it will keep the tank cool while filling. it does so-so depending how fast you fill. if you fill fast the inside is hot and the out side is cool until the heat makes it through the tank wall making a bath useless. I wet bath fill. it cools the tank proior to fillng back down to 70-80 instead of being at 100F + temp rising because of gas compression. I also set the tank in a area of wet grass and that cools the tank. You can put a towel on the tank and pour water on the towel and do the same thing. HOt fills for many divers is a sore spot. fillers dont have a clue when they fill . they fill an already hot tank in less than a minute and will not exceed 3000 psi,,,,, by the time you get it to the dive site it only has 26-2800 psi in it because of the tank cooling. I have tried to do a slow fill from a compressor and it is not easy. A 6 cuft compressor filling a al80 cuft tank takes about 10-12 minutes. the rule for filling rate is 600 psi per minute or less. that means that a al80 can be topped of in 6 minutes and yet when filled form a compressor it gets hot. not only because of the compression but from the hot gas being put into the tank. Coming form a cascade where the temps of gas drops to the 70's is a totally different situation than off a compressor. Businesses that fill call many times > 2700 a full tank because of the way the filler is cutting off filling at 3000 and letting the tank cool. YOU can fill off a compressor if you are say filling a dozen tanks at once. fill psi rate is slow but there is minimal tank heat generated form compression. In my fill station I can tell you tanks get hot with out something to speed draw to heat out. That is filling 3 tanks at once on a compressor and not using a cascade for air source.
