Packhorse
Contributor
In NZ OSH limit air fills to 227 bar (I think ). Any such limit across the ditch?
Max Pressures for Regulators
- Thread starter adza
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adza
Contributor
Well, I'm no expert, but I've had plenty of tanks filled over 227. (In some cases, where the operator has been in a rush, I've seen it filled to over 250 bar quickly, and then as the tank cools, it drops back to around 220bar).Packhorse:
As for what the law/regulations state - I have no clue. As for practise, I've seen plenty of tanks filled over 227.
jeraldjcook
Contributor
adza:
Here, overfilling is alright as long as pressure at room temperature isn't above the cylinder's maximum. Section (f) states that during a hot fill (130*), you could fill an AL80 3000psi tank to 3750 and still be within the regulation. Credit goes to Lamont for this.lamont:
227 bar = 3337 psi
250 bar = 3675
Max psi using 5/4 rule = 284 bar or 4171 psi
Pumping a 227 bar tank up to 250 bar would be fine in the states, assuming a really hot fill.
Actually using ideal gas laws, to meet the rated pressure at 70deg F, the pressure at 130F will be 11% overpressure.
(130F + 460F) / (70F + 460F) = 1.113 = 111.3 %
or 11.3 % over the rated pressure.
Example for a 3000psi tank:
3000psi * [(130F + 460F) / (70F + 460F) ] = 3340psi
So to meet the maximum temperature of 130F and the 3000psi at 70F, the maximum pressure should not exceed 3340psi.
Note:
130Farenheight + 460F = 590 Rankine (absolute temperature)
70F + 460F = 530R
(130F + 460F) / (70F + 460F) = 1.113 = 111.3 %
or 11.3 % over the rated pressure.
Example for a 3000psi tank:
3000psi * [(130F + 460F) / (70F + 460F) ] = 3340psi
So to meet the maximum temperature of 130F and the 3000psi at 70F, the maximum pressure should not exceed 3340psi.
Note:
130Farenheight + 460F = 590 Rankine (absolute temperature)
70F + 460F = 530R
Some shops get hung up on this and feel that at no point can they exceed the service pressure stamped on the shoulder. In fact, as indicated above, you can fill to what ever you need in a hot fill situation as long as the final room temp pressure does not exceed the service pressure (or 10% over the service pressure in the case of a current plus rated steel cylinder.)
Obviously you want to be reasonable about it and not slam the air into the tank anyway, but as a practical reality you need to overshoot the tank pressure by 200-400 psi to get both a full fill and a reasonably fast fill rate.
If you are good at it, you can feel the temperature of the shoulder of the tank, and know pretty much exactly how far over the service pressure you have to go to hit the service pressure on the nose once the tank cools to room temp.
The naysayers will decry this procedure as unsafe, but the fact is that the DOT recognizes that tanks will get left in the sun, get left in the trunk, etc and have ensured that there is ample safety margin to accommodate the higher pressure of a tank that may reach 130-140 degrees while filled with it's full rated volume of gas.
You'd need an incredibly hot fill to reach or exceed the test pressure (normally 5/3rds the service pressure, and in any event the burst disc would go and prevent you from exceeding the test pressure.
Obviously you want to be reasonable about it and not slam the air into the tank anyway, but as a practical reality you need to overshoot the tank pressure by 200-400 psi to get both a full fill and a reasonably fast fill rate.
If you are good at it, you can feel the temperature of the shoulder of the tank, and know pretty much exactly how far over the service pressure you have to go to hit the service pressure on the nose once the tank cools to room temp.
The naysayers will decry this procedure as unsafe, but the fact is that the DOT recognizes that tanks will get left in the sun, get left in the trunk, etc and have ensured that there is ample safety margin to accommodate the higher pressure of a tank that may reach 130-140 degrees while filled with it's full rated volume of gas.
You'd need an incredibly hot fill to reach or exceed the test pressure (normally 5/3rds the service pressure, and in any event the burst disc would go and prevent you from exceeding the test pressure.
oxyhacker
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Note that in the current CFR the pressures are given as 21 [deg]C (70 [deg]F) and 55 [deg]C (131 [deg]F).
I think the 5/4s thing is a red herring since a tank filled with air or similar dive mixes cannot be filled to 5/4s and still be legal at 21 [deg]C (70 [deg]F). That is to say, 131 and 5/4s are given as maximums, not as a mathematically correct relationship, and it never says you can fill it to131/ 5/4s, just that the tank can under no condition exceed that.
It's hard to determine the temperature of a tank during a typical fill cycle so I think most shops/fillers that bother to compensate for heating do it empirically - you learn pretty quickly what temperature you got to fill a tank to so it ends up right once it has cooled. Which works just fine. I can't blame a shop too much, though, that doesn't want to "overfill" a tank (even though it really isn't overfilling a tank to compensate for temperature, since the regs specifically permit it), especially since there are elements in the dive community promoting the notion it is illegal to do so or even that for safety all tanks should be deliberately UNDERFILLED. But I believe that any shop that fails to compensate for cylinder heating should at least offer the customer the option of either a slow fill or letting the tanks cool then topping up, so that the customer gets the full rated capacity of the tank.
I think the 5/4s thing is a red herring since a tank filled with air or similar dive mixes cannot be filled to 5/4s and still be legal at 21 [deg]C (70 [deg]F). That is to say, 131 and 5/4s are given as maximums, not as a mathematically correct relationship, and it never says you can fill it to131/ 5/4s, just that the tank can under no condition exceed that.
It's hard to determine the temperature of a tank during a typical fill cycle so I think most shops/fillers that bother to compensate for heating do it empirically - you learn pretty quickly what temperature you got to fill a tank to so it ends up right once it has cooled. Which works just fine. I can't blame a shop too much, though, that doesn't want to "overfill" a tank (even though it really isn't overfilling a tank to compensate for temperature, since the regs specifically permit it), especially since there are elements in the dive community promoting the notion it is illegal to do so or even that for safety all tanks should be deliberately UNDERFILLED. But I believe that any shop that fails to compensate for cylinder heating should at least offer the customer the option of either a slow fill or letting the tanks cool then topping up, so that the customer gets the full rated capacity of the tank.
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