Consensus on Overfilling Tanks?

[rob.mwpropane]

Well call me a nerd, but I find this all pretty fascinating.

@abnfrog how do you fill tanks?

 

[MichaelMc]

@tbone1004, for + rated tanks it would seem to be
131F: 5/4 * "10 percent in excess of its marked service pressure"
so 5/4 * 1.1 * 2,400 = 3,168

But cooling to 70F: 1.1 * 2400 = 2,640

By the specs quoted above.

 

[abnfrog]

I fill to tank specs but after it cools I don't do cave fill (we don't have any ) 10 bucks is not worth my life that being said I don't know what the tank burst disk is solid doubled etc ......but I have the luxury to fill slow , we used to teach a hot fill is very bad now we TEACH a hot fill (in specs for psi per min) is ok , air that is blending is something else in my early career (70s) I filled a 72 that had a boot on it turns out after I lifted it out (2250) the boot came off and I saw the bolt someone had put in (the tank was condemned and drilled ) I became hyper sensitive of tanks ever since , as I could have be killed filling that tank . now its all about me ,,,,my compressor my rules , I have owned 2 dive centers and 4 compressors im still here so for me my rules work

 

[rhwestfall]

S-L-O-W

 

[Caveeagle]

If you need more gas......
Why can't you just buy bigger tanks ??

Larger tanks increase dimensions and drag. I pick the size and type of tank I am going to use and then fill to have max limit of available gas for the given dive I have planned. If you have ever swam a pair of double 85s into a high flow cave like Little River and then compared that to the resistance of a pair of 104 or 108s, you would see the relative advantage of the smaller tanks.
..I don’t always dive big double 8” diameter tanks. ...But when I do, you can bet they are pumped to around ~3600 psi too.

 

[Jwoobs]

Excuse my ignorance, but is there chart somewhere (or maybe a calculation) for what tank pressures can be at certain temps? It would be nice to carry around something that I can show a shop. If they say no, at least I can say I tried and had fundamental science to back it up.

I'm terms of just pressure, legal requirements aside, there's the ideal gas law: PV=nRT

volume(V), moles(n), and the gas constant (R) all drop out, so pressure is proportional to the absolute temperature (Degrees rankine or Kelvin): P ≈ T

To get rankine from Farenheit, add 460.

So 130*F (590 R) to 68F (528 R) would be 89% of the the pressure when hot. 528/590 = 0.89

 

[rob.mwpropane]

Really appreciate everyone's responses. I've learned a ton and things that I didn't even think to ask that were made aware.

So the general consensus is it's ok to fill if warm to 5/4 working pressure or less so the correct pressure is in the tank when cooled off. If possible just fill slow, but that's not always possible.

 

[rjack321]

The problem with the conversion table is knowing what temp the gas in the tank is at. You know the ambient temp, you can measure the metal of the cylinder. But if the tank is being filled or was recently filled you don't really know the gas temp. The only practical way to determine that is to let it sit until its at ambient. It's almost always warmer in there than the cylinder, hence the gradual heat loss.

You're never going to get consensus on overfilling 3AA steel tanks, this exact same thread has been on dozens of forums over the years, someone is always going to say "it's illegal" or "it shortens their life" or "it's risky" (probably more reasons but those are the top 3 over the years).

There is a consensus on overfilling 3AL, and SP tanks - don't.

 

[rjack321]

So the general consensus is it's ok to fill if warm to 5/4 working pressure or less so the correct pressure is in the tank when cooled off. If possible just fill slow, but that's not always possible.

5/4 is completely nuts in a 3AL cylinder. That's 3750psi in an Al80.

 

[tbone1004]

5/4 is completely nuts in a 3AL cylinder. That's 3750psi in an Al80.

it is, but if it's actually 130F when you stop filling, then it should cool back down to 3000. I usually set the regulator at 3500 if they're coming up from low pressure and they almost always settle out a hair under 3000, and yes the regulator is calibrated and accurate to within 1%