Cold vs hot cylinder

[Scubadoo72]

Just a thought running through my head today as I have a bunch of cylinders to fill today, and it is going to be quite warm for the next few days..

I am thinking, if fill my cylinders (for example steel 72's rated 2250) to say..1500psi, while they are sitting in a bucket of cold water...would the expansion be enough to fill them? Or would it be safe? I am thinking this would save not only time, but be easier on the compressor, as well as extend the life of my filtration (yes, I know minimally..but still)..

Just curious on thoughts here...

 

[JeffG]

I'm thinking that it would have pretty much no effect. Its not the temperature of the tank that matters, but the temp of the gas. The speed of temperature exchange between the tank and gas would be too slow to have any real effect.

 

[akscubainst]

They should be full at 2250psi and 70 degrees F.

Hypothetically, even if this worked,and you had them down to 32 degrees the swing would only be about 190psi.

DM

 

[CraigAClark]

If I botch this, Pesky will probably correct what I got wrong. Pressure read on a gauge is nothing more than a measure of force exerted by the movement of molecules of the gas. Colder gas has less movement while warmer gas has more. If you fill to 1500 psi in an ice bath, the pressure (force) will increase as the gas warms up. How much, I have no clue, since the parameters aren't being controlled. Since temperature also has a relation to pressure and volume, you can get more gas molecules in a given volume at lower temperatures. These are not exact, but let's say you can get 2 cu ft at 80F to occupy the same space as 1 cu ft at 40F. As the temperature increases the pressure and volume will also increase. Does that make sense to you?

Craig

 

[pescador775]

Scubadoo,
The temp of a tank increases when filled from a compressor and somewhat less so if filled at equal rate from a bank. Once the tank cools it may experience a pressure drop varying between 200-300 psi depending on rate of fill. Originally, the tank heated up because heat produced in the air by compression is transferred to the tank walls. This will continue as long as the differential temperatures exist between the two. The differential is fed provided that more energy is released by compression and the tank cools to the environment. At this point, the metal by virtue of its weight holds a lot more heat than the air. Anything which can cause the metal to shed heat will drop the temperature of the air inside and cause the pressure to drop. Water baths are a common method. Metal to water causes heat to leave very rapidly due to physics of heat transfer. As was mentioned, the capacity potential of a big chunk of metal is considerable. The heat which is left in the relatively small mass of air will be sucked into the metal although at a slightly slower rate. If the tank is removed from the bath very quickly, this continuing heat transfer can be felt as a slight warming of the metal which continues for a minute or two. A slight warming of the metal translates to a big drop in air temp due to the different capacities. IOW, a big drop of the air temp at this point will only cause a slight warming of the metal because the residual amount of heat in the air is very little despite the higher temp. A bath is considered beneficial for purposes of one stop filling. If the bath has enough water and if some flow is ducted into the bath, the tank can be filled from the compressor without the necessity of overfilling to compensate for pressure drop. Alternately, the tank does not have to be topped off after a lengthy cool down period.

The relationship between heat and temp is not clear to everyone. One way to visualize this is to consider the Tokamak fusion core. It can create temps hotter than the sun but not enough heat to boil a glass of water. As Craig mentioned, it's all about molecular speeds.

 

[Scubadoo72]

Ahh..ok, fair enough..well, I am filling an alum 80 right now..in cold water...I will note the pressure when I quit...and if there are any increases in pressure as the cylinder warms up to normal air temp.

 

[rjack321]

Ahh..ok, fair enough..well, I am filling an alum 80 right now..in cold water...I will note the pressure when I quit...and if there are any increases in pressure as the cylinder warms up to normal air temp.

Its doubtful you will see any rise. And even if you do see say 50psi greater, as soon as you put the cylinder into your ~52F PNW waters to dive that pressure will be "lost" again anyway.

If you are filling 3AA +rated steel 72s just fill them to 2600 "hot" and they will cool down in a couple hours to 2400ish.

 

[pescador775]

Scubadoo, aluminum is a pretty good heat sink. Steel, due to its lower specific heat and lower conductivity, may take longer to cool.

 

[CraigAClark]

I guess what hammered home the physics lesson about temp and pressure relationship for me was during my chamber operators course in college. Nothing drives home how heat builds up during compression and heat loss during decompression, even at a rate of ascent/decent of 60'/min, better than taking a chamber ride to 165' and back. Sweat on the way down and cold on the way up. Not to mention the well learned lesson of narcosis effects at that depth. Give me that 80/20 HeO2 mix anytime! Sur "Ds" anyone?

Craig