Overall volume & heat

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Posted by Ken Kurtis on February 23, 2007 at 14:24:40:

In Reply to: Re: Re: Air fill in under 30 seconds ? posted by HoloD on February 23, 2007 at 13:04:34:

Not to beat a dead horse (but I actually DO like the physics discussion) . . .

HoloD said: "The point you are missing is that storage tanks are huge and when an empty tank is hooked up to a storage tank and becomes one system the TOTAL volume is almost unchanged (because your tank is so much smaller than storage tanks)."

I don't want to seem rude but what you've stated is totally wrong.

Your scuba tank is NOT part of the overall fill system so the relationship to the volume of your scuba tank and the bank system is moot. You might be able to make the argument that your tank becomes part of the overall system once it's been filled with air and equalized to the pressure of the banks, but that's not the case when you START the fill.

(And if the banks are at 4K and your tank's at 3K, then you have NOT equalized the tank to the pressure of the system and the tank would NOT be part of the overall system volume.)

Basically, we're taking a tank that has no (or little) air in it and jamming in 80cf or so. We're doing it through a teeny little opening in the tank valve. And we're putting more air in the tank than the actual internal volume of the tank at one atmosphere of pressure (which is about 0.4cf).

Boyle's Law tells us that if you put in 200 times (80cf) the volume of the tank, and the internal volume of the tank doesn't change, then the pressure will go up. That's why, when you jam in so much air regardless of how long you take to do it, the pressure of the tank will rise by a factor of 200, from roughly 15psi to 3000psi.

In addition, as mentioned many times previously, Charles' Law states that the tank will get hotter as the air is jammed in, and - from personal experience - the faster the fill, the hotter it gets. (The slower the fill, the less hot it gets or the heat dissapates you don't really notice the temp change.)

You say it was not filled in water so we can rule out water compensating for any heat increase, let alone in a 30-second fill.

As far the temperature of the tank, there are only two possibles. Either it DID get warmer/hotter, or it did not. If it did get warmer, you're simply mistaken either about the length of time the fill took or the increased temperature of the tank.

However, if you're right about the tank NOT getting warmer (and I'd be willing to say that you probably got this one correct), there are also only two possibilities:

1. Either your tank was already full (or nearly so) when you brought it in and no air was added to the already-full tank, resulting in no rise in the temperature.

. . . or . . .

2. The tank-filler never opened your tank valve. What you saw with your 30-second fill was not the tank being filled, but simply their lines beng pressurized. The tank didn't get warmer because no air was ever added to it.

As I said earlier, the laws of physics are the laws of physics. There's simply no way you can fill a tank in 30 seconds and not have it get hot. So . . .

Have you gauged your tank since the miracle-heatless-fast-fill to see if it's really full? If it's not full, there's your answer to the lack of heat.

If it is full, is there any chance it was already full when you took it in and you really didn't need a fill afterall?

If either of these two scenarios are correct, it would explain everything because what you're describing is simply impossible.

Ken Kurtis
Owner, Reef Seekers Dive Co.
Beverly Hills, CA

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