Air Fill Protocol

[jmneill]

Tanks do not burst from being overfilled... ever. A simple google search would prove that, and this discussion would now be over.

+1 !!! Reading this thread was painful.

 

[Superlyte27]

Tanks do not burst from being overfilled... ever. A simple google search would prove that, and this discussion would now be over.

If you are talking about LP Steel Scuba tanks in good VIS and Hydro, then you are absolutely correct.

 

[dfx]

Tanks do not burst from being overfilled... ever. A simple google search would prove that, and this discussion would now be over.

Oh! Well in that case... 10,000 psi fills here I come! And no need for those pesky hydro tests any more either, sweet!

 

[dpaustex]

Most probably don't remember their "gas laws" from their classes. Who was that Charles guy? Boyle? Huh?

Shops, for the most part, fill their tanks, with the tanks submerged in water. Any gas temperature rises, when the gas is compressed. When the tank is "on the rack" to be filled, there is a pressure gauge. This gauge is reading a "hot gas". So if the filler stops at 3000 psi, that is full - at that temperature.

Now, you take that tank, sometimes stored outside in 85 degree heat, in the sun, and it "reads" 3000 psi. True, enough, for that temperature (tank is probably 90+ degrees in the sun). But when you hit water that is 10-15 degrees cooler, the gas law kicks in that say the pressure is related to volume and temperature. The volume is the same (the 80 cubic foot tank), but the temperature has dropped. With the drop in temperature comes the drop in pressure.

Most shops (including the one where I teach), fill the tanks to around 3400-3500 "hot". The tanks then cool down to around 75 degees (over time, in the A/C), and read around 3000 to 3100 when renters pick them up.

Many "cattle car" boats will hot fill their tanks to only 3000 psi (if you were to read the temp of the air during a fill, it's over 120 degrees). But when the tanks cool down (when you hit the water), the pressure drops, often significantly. Part of this is by design (less time in the water).

Most liveaboards fill their tanks to around 3200-3300, directly out of the whips (nothing like a mad customer, over a period of days). These generally cool down to well over 3000 psi.

What I recommend if you do get a "short fill" is to simply swap the tank. If they are all short filled, I'd complain...because you didn't spend all that money to get to a dive location, then dive site, to have your u/w time cut by 20-25% because of a short filled tank. No excuse for that - ever.

 

[Lead_carrier]

Since aire pressure will increase by 5-6 psi per degree temperature increase, if I increase temperature by say 100 degrees, I still only increase air perssure by 500 psi. Which for a standard AL 80 tank should be absolutely no problem. To get the temperature high enough to really cause a tank problem, you're getting the tank so hot that it's structural integrity is going to be put in jeapordy anyway.

---------- Post added ----------

Correct. A long slow fill will gie you a better fill than a short hot fill, as long as both tanks are filled to the same pressure. And this is true whether the tank is filled wet or dry. A hot fill in a water tank will do very little other than put water on the floor when it is removed since the temperature transfer isn't quick enough to really matter.

 

[boulderjohn]

My experience in filling is a bit different I guess.
That one has been debunked according to this: http://www.fillexpress.com/library/dryfills.pdf
Basically it states that the tank doesn't sit in the water long enough for it to have an impact on the temperature inside the tank.

It says a lot more than that, and what it says is different from that in intent. Here is a key quote:

Scuba tanks should leave the dive store warm and not over-filled. Those conditions will produce a safe situation where the only way for the tank’s pressure to go is “down” (which is good), rather than “up” which is bad).​

The article argues vigorously that tanks should NEVER go above their rated pressure, so they should essentially be underfilled at the shop so that if the diver puts the tank into a warm car, a 3,000 PSI tank won't accidentally get to 3,001.

I'm not a tank monkey, but from whatI've read an AL80 properly filled to 3000psi at 70f will raise to 3342 psi at 130f. This is quiet acceptable.

Here's some really good reading material on the subject:

http://www.fillexpress.com/library/dryfills.pdf

As mentioned above, the article says repeatedly that a tank must never go above its rated pressure.

Now, it is interesting to me that the article is located in the Fill Express library. I got my Advanced Gas Blending certification there. They routinely fill LP tanks (like my 108s) to 3,000 PSI, which is an overfill. My AL 40s and 80s were routinely filled just above 3,000 PSI. They do believe that there is no benefit whatsoever to filling in a water bath.

I am also a certified tank inspector. In the course I took to get that certification, we were given rough guidelines by the instructor on how much to overfill tanks to get them to the rated pressure when they cooled.

When I fill tanks, I do overfill to a degree intended to have it cool to the pressure I want.

 

[dfx]

Since aire pressure will increase by 5-6 psi per degree temperature increase, if I increase temperature by say 100 degrees, I still only increase air perssure by 500 psi. Which for a standard AL 80 tank should be absolutely no problem.

Why aren't AL80s filled to a "cold" 3500 psi to begin with then?

 

[supergaijin]

Ahhhh sarcasm. Doesn't translate well in black and white.

Originally Posted by supergaijin
Tanks do not burst from being overfilled... ever. A simple google search would prove that, and this discussion would now be over.​

Shops, for the most part, fill their tanks, with the tanks submerged in water. Any gas temperature rises, when the gas is compressed. When the tank is "on the rack" to be filled, there is a pressure gauge. This gauge is reading a "hot gas". So if the filler stops at 3000 psi, that is full - at that temperature.

Now, you take that tank, sometimes stored outside in 85 degree heat, in the sun, and it "reads" 3000 psi. True, enough, for that temperature (tank is probably 90+ degrees in the sun). But when you hit water that is 10-15 degrees cooler, the gas law kicks in that say the pressure is related to volume and temperature. The volume is the same (the 80 cubic foot tank), but the temperature has dropped. With the drop in temperature comes the drop in pressure.

Most shops (including the one where I teach), fill the tanks to around 3400-3500 "hot". The tanks then cool down to around 75 degees (over time, in the A/C), and read around 3000 to 3100 when renters pick them up.

Many "cattle car" boats will hot fill their tanks to only 3000 psi (if you were to read the temp of the air during a fill, it's over 120 degrees). But when the tanks cool down (when you hit the water), the pressure drops, often significantly. Part of this is by design (less time in the water).

Most liveaboards fill their tanks to around 3200-3300, directly out of the whips (nothing like a mad customer, over a period of days). These generally cool down to well over 3000 psi.

What I recommend if you do get a "short fill" is to simply swap the tank. If they are all short filled, I'd complain...because you didn't spend all that money to get to a dive location, then dive site, to have your u/w time cut by 20-25% because of a short filled tank. No excuse for that - ever.

Personally I have never worked in a shop or visited a shop where they fill in water. The myth of being able to cool tanks in room temperature water is an oldie but a goodie. It's an idea often thrown around by learned divers but doesn't actually make a difference in real life... unless said tank was filled slooooowly over an hour or so. Again a quick google search should sort this out. (seriously type in keywords like "water cooling scuba tank filling" and see how many pictures you find of this operation.

Shops, for the most part, fill their tanks, with the tanks submerged in water.

'Cattle boats' purposely under-filling to limit divers' time underwater?
There is actually an easier way to control people's air consumption than that.

Having your dive time limited by 20-25% because of a short fill is seriously one very short fill indeed! That would be a fill of what?...somewhere around 2400psi when one expects something like 2800-3000. I would also be complaining about that! I have never had 'hot fills' reduce to that much, even in Djibouti where evening-time temps are still just shy of 100F going in to summer.

The only way to reliably fill a tank rated to 3000psi without intentionally overfilling is to top it up after the tank has cooled for a couple of hours. If there are empty tanks to fill, then the top up takes a couple of mins and a quick hand (nothing dirty intended).

I wish there was a special code between tank makers and the fillers where tanks were made to be filled to 3300psi but were written as only allowed to 3000psi. There used to be one- it was the + sign.... but then word got out and then divers were complaining that they weren't getting that extra 300 psi. Maybe they could come up with an even craftier signal? (sarcasm intended).

 

[D_B]

That's assuming the tank ratings are designed to allow for the additional pressure caused by heat. How do you know that's the case? And if it is, how much heat is not too much heat? Shouldn't that be in the specs too then?

---------- Post added ----------



I do have technical knowledge about this. It's called high school physics. But just in case, they also teach Charles' Law in OWD courses, for the sole reason of trying to tell people not to let tanks heat up.

as far as I can tell, it is and even if not, high school physics would show that they would have to ..

If we're going by fill pressure only, and not taking temperature into account, then if I cool my tank to 35f and have them top it off, it's not overfilled is it

 

[dfx]

If we're going by fill pressure only, and not taking temperature into account, then if I cool my tank to 35f and have them top it off, it's not overfilled is it

Correct. Problem is, it's gonna warm up and then it will be. Unless you go ice diving or w/e, then you're good.