Tank volume calculations..what was the temp used?

[Big-t-2538]

Guys...tank temp is a factor, but it is NOT seriously significant unless you're really hot-filling your tanks. Tank ratings are given at a standard room temp...which is somewhere between 60 and 70 degrees depending on who you ask.

If you do the calculations, you will find that 1 degree F is approximately equivalent to about 5.5psi.

BTW...this all stems from Charles's Law BTW. Which states that pressure is directly related to temperature.

 

[pescador775]

Tank temp is a big factor. A smallish, 3.5 cfm compressor will heat a tank enough to require a 10% overfill just to bring the capacity to its normal level. Some philosophers have been trying to make a case for abandoning water cooled fills. These fills are made from banks or large compressors and completed in a matter of a few minutes and do require water cooling, no matter what they say. The move away from water cooling is for the shops' convenience only.

Guys...tank temp is a factor, but it is NOT seriously significant unless you're really hot-filling your tanks. Tank ratings are given at a standard room temp...which is somewhere between 60 and 70 degrees depending on who you ask.

If you do the calculations, you will find that 1 degree F is approximately equivalent to about 5.5psi.

BTW...this all stems from Charles's Law BTW. Which states that pressure is directly related to temperature.

 

[Big-t-2538]

Tank temp is a big factor. A smallish, 3.5 cfm compressor will heat a tank enough to require a 10% overfill just to bring the capacity to its normal level.

Let me re-phrase....

Tank Temp SHOULD NOT be a big factor. Fill slower.

So 10% of a 3000psi fill is what....300psi..

To be honest, I'm not buying the fact that your 3.5 CFM compressor heats a tank up that much and you need to fill it to 3300psi to ensure a good fill?

So what is 3.5cfm per psi in an AL80 as in psi/min. Well, Let's find out. 3.5 / 77.4 * 3000 = approximately 135 psi / min.

So if you're tank needs 2000 psi to be filled...or 2300 to overfill and it takes approximately 15 to 17 minutes to fill your tank....you're telling me that it will rise 55 degrees in that amount of time? (300 / 5.5 = approximately 55 degrees) So you're telling me that your tank raises it's temp to 125 deg F? I just don't buy it.

 

[pescador775]

Geez, don't you know who I am? I'm the guy who has made dozens of posts regarding tests and service data on MIL surplus compressors. I have jammed many SCUBA tanks over the years and reported on the consequences of heating. Just throw away your calculations, I've done it.

For others, I say this. Although there is some variance, steel tanks being the 'worst', any normal size SCUBA tank will heat up significantly when filled by a portable compressor. Never mind a large, shop compressor, a 3.5 cfm compressor will heat up a steel tank enough to require top off of 10% and an aluminum tank will require about 8%. On a typical 85F day a steel tank will heat up enough to be very noticable to touch. A 4 cfm compressor will heat up a tank to the point of being uncomfortable to touch. After a couple hours, a 3000 psi tank which has been filled without water cooling will register 2700 psi. When the diver enters 75F water it will register 2600 psi.

Let me re-phrase....

Tank Temp SHOULD NOT be a big factor. Fill slower.

So 10% of a 3000psi fill is what....300psi..

To be honest, I'm not buying the fact that your 3.5 CFM compressor heats a tank up that much and you need to fill it to 3300psi to ensure a good fill?

So what is 3.5cfm per psi in an AL80 as in psi/min. Well, Let's find out. 3.5 / 77.4 * 3000 = approximately 135 psi / min.

So if you're tank needs 2000 psi to be filled...or 2300 to overfill and it takes approximately 15 to 17 minutes to fill your tank....you're telling me that it will rise 55 degrees in that amount of time? (300 / 5.5 = approximately 55 degrees) So you're telling me that your tank raises it's temp to 125 deg F? I just don't buy it.

 

[Big-t-2538]

I have jammed many SCUBA tanks over the years and reported on the consequences of heating. Just throw away your calculations, I've done it....<anip>....A 4 cfm compressor will heat up a tank to the point of being uncomfortable to touch.

As have I and if you fill slow enough, it's not an issue.

Simple solution....Stop Jamming tanks and Fill Slower. No need for a water bath which I think cause more issues than they solve.

 

[pescador775]

The people who have published those essays on the net are clueless. There is no practical way, other than water bath, to fill a SCUBA tank without raising temperatures and pressures and reducing true air volume in the tank.

Let me give you some info on elementary physics. This is just for funsies. Temperature and heat are not the same thing. Air is a poor conductor. Aluminimum is a good conductor. Steel is a fair conductor. Water is an excellent conductor. The heat capacity of water is higher than any of the above. The heat capacity of air is smaller than any of the above, even when compressed.

It is possible for the temp of air inside a tank to be higher than the surface temp of a tank. This is due to the fact that the transference of heat from air to metal does not necessarily cause a balancing of temperatures between the air and the metal, not for a long time. Air, which is much hotter than the tank's metal can continue to transfer heat while maintaining a higher temperature than the tank. This is due to two factors. Air is a poor conductor and metal is a good to fair conductor. More importantly, metals have much larger heat capacities than air resulting in a LOWER temperature rise than would be expected from the heat transfer causing the tank to remain cooler than the air inside it. As the temperature of the tank rises, the heat transference declines due to the lessening temperature differential between the gas and the metal. This effect is more noticable with steel than with aluminum which is a better conductor and in either case can last a long time unless the conductivity of metal and water come into play. Therefore, it is likely that the temp inside a tank is higher than the temp on the tank surface regardless of the fill time to full pressure unless the tank is water cooled thereby increasing differentials and transference. In any case, tank fills that are performed slow enough to avoid this effect of temp and pressure rises woud take hours and be impractical. For best results, water cooling and a slight overfill are needed to get the job done.

As have I and if you fill slow enough, it's not an issue.

Simple solution....Stop Jamming tanks and Fill Slower. No need for a water bath which I think cause more issues than they solve.

 

[Big-t-2538]

The people who have published those essays on the net are clueless.

some are...some aren't...have you spoken with them?

There is no practical way, other than water bath, to fill a SCUBA tank without raising temperatures and pressures and reducing true air volume in the tank.

I have filled my share of AL tanks. I fill them to a pressure, and they happen to stay there even though they're not filled in a bath. No, I'm not filling them in the snow either. I too am speaking from personal experience.

Let me give you some info on elementary physics.

I know my share of physics, let's just agree to disagree. Besides filling tanks is only a small step in diving, so it's not worth getting fussy about. When ya hit the water whatcha got is whatcha got. It's all a matter of enjoying the dive and getting back to the surface from there.

 

[rab]

<snip>
It is possible for the temp of air inside a tank to be higher than the surface temp of a tank. This is due to the fact that the transference of heat from air to metal does not necessarily cause a balancing of temperatures between the air and the metal, not for a long time. Air, which is much hotter than the tank's metal can continue to transfer heat while maintaining a higher temperature than the tank.
<snip>
In any case, tank fills slow enough to avoid this effect of pressure rises woud take hours and be impractical. For best results, water cooling and a slight overfill are needed to get the job done.

By your own analysis, the difference in temperature is due to the air inside and the tank itself. The water is only going to reduce the temperature of the tank and have little effect on the air inside.

-Rob

 

[SubMariner]

This discussion is becoming a bit overheated (pun intended).

Please, let's keep it civil!

Just a reminder from one of your friendly neighbourhood Moderators.

 

[pescador775]

not to quote you or anything, but that's a crock of hooey in it's own. I have filled my share of AL tanks. I fill them to a pressure, and they happen to stay there even though they're not filled in a bath. No, I'm not filling them in the snow either. I too am speaking from personal experience.

There is only a tiny shread of truth in this statement. That is, aluminum tanks do have a slightly less problem than steel. So, maybe one could bow out of this with a sleight of hand. Otherwise, the statement is false or misleading, take your pick. Also, dive shop owners who try to schmooze customers with statements like 'enjoying the dive is more important than the air fill' may not appreciate that divers do notice when they get short fills, especially when they've driven long distances and taken long, expensive boat rides to get where they're going.