Looking for a good fill on my HP steel 100's

[D_B]

you tell them the rated pressure of the tank is at the rating temperature ... if the temp of the tank is higher than that, the pressire is higher .... it does not mean that your 3442psi tank suddenly somehow becomes overfilled if it warms up to 80f (it also means that if the tank is colder than rated, then the pressure will be lower than 3442 as well )

 

[Aqua-Andy]

This is not true. The temperature has to do with the steel, not the gas. As you heat steel it's strength decreases so in reality the rated pressure would be lower at higher temperatures.

No. The rated pressure on our so called high pressure tanks is 3442 @ 70 degrees. That rating is with the tank and gas at the same temp, if the tank and all the gas is at 50 degrees and the pressure is at 3442 PSI then the tank is over filled because when the temp rises to 70 degrees the pressure will be above 3442 PSI. This will also hold true when you get a hot fill, the rated pressure will actually be higher and it should settle to 3442PSI when it reaches 70 degrees.

 

[halocline]

The only thing the water immersed fills do is allow heat to dissipate from the steel more quickly. Since it's the air heating up the steel from the inside, if you fill a tank quickly in water and then take it out, the steel will just heat up after the fact. It's the temp of the air that determines the pressure, of course. The water could be helpful if the tank is filled and then left in the water until the air inside cools down to the water temp. Of course, with several tank in a small water bath, like most dive shops have, the tanks will soon just heat up the water.

Dive shops need to realize that because the tank's test pressure is so far above anything even close to what they need to "overfill" in order to have the tank cool to rated pressure, there is no real-world risk in overfilling a few hundred PSI to allow the tank to cool to it's rated pressure. Unfortunately some shops are just not willing to do that, and since they own the compressor, they make the rules.

So, I guess the real answer to your original question of "how do I make sure I get good fills" is to buy a compressor....sorry!

I've found that my LDS routinely underfills, but as soon as I NICELY ask them to top it off, they get it full.

 

[D_B]

the LDS terminology also needs correcting ... filling a tank to a couple of hundred pounds over it's stamped pressure is not "overfilling it" - if that tanks temp is higher than 70f so will the, perfectly legal, safe, normal fill, also be

 

[zimmo]

No. The rated pressure on our so called high pressure tanks is 3442 @ 70 degrees. That rating is with the tank and gas at the same temp, if the tank and all the gas is at 50 degrees and the pressure is at 3442 PSI then the tank is over filled because when the temp rises to 70 degrees the pressure will be above 3442 PSI. This will also hold true when you get a hot fill, the rated pressure will actually be higher and it should settle to 3442PSI when it reaches 70 degrees.

Let's be clear:
The rating on the tank has to do with the ratio of the stresses in the steel to the yield (or ultimate) stress of the steel. We call this a safety factor. If the steel heats up, the yield (and ultimate) stress will decrease, and therefore if the pressure inside were still 3442 psi, the safety factor would decrease and you would be technically overfilled. Now in the case you mentioned, if you filled the tank to 3442 psi at 50F and then let the tank + air heat up to 70F, the pressure would now be 3577 psi (ideal gas law) and you'd be exceeding the rating of the tank like you said. If you get a hot fill you're just doing this process backwards.

In practical terms this is all nonsense though. You have to heat steel quite a bit more than 20 or 30 degrees F to get appreciable decrease in strength, and going 1 or 2% over the rating isn't really going to hurt anything.

 

[Shftrdog]

It is not good to over pressure a tank and let it cool down. Everytime you over pressure a tank fill it to fast you are decreasing the life of the tank. Also everytime you over pressure a tank you have a great risk of it rupturing. Ask any PSI instructor and they will tell you the same thing. An average tank should get you 100,000 air fills before the tank wears to much and is no good. Everytime you over pressure or fill a tank to fast you are cutting the life of the tank almost in half. An example, a shop in FL had a tank that they filled to the correct pressure and at the correct fill rate and everything was fine. Come to find out that the tank had a crack across 12 of the 15 threads. If that tank was over filled to cool down it would have rupptured. Your LDS is doing the correct thing.

 

[D_B]

Question ... so just what temp is OK for a tank fill ?
... you know that it's going to get hotter than 70f after that at times, sometimes much hotter

 

[aquaregia]

Just fill the tank at a reasonable rate. I hit AL80s to 3100 and they're still at 3000 on the surface.