Air fill question:

[texdiveguy]

To bath or not to bath those tanks.

Just for the record, most shops, charters and DIY's dry fill in my experiences.

It's personal preference.....just fill at a 'slow rate' and all will be good.

I have experience a few out of calibration gauges in shops....they even admit to it once brought up.

 

[richerso]

Just my opinion (and I could run through some thermal analysis / heat transfer / thermodynamics calculations if someone wanted); but my thoughts are:

Slow fills are always better. Gas law physics will predict this. Look up isothermal compression and adiabatic compression for more details.

A water bath makes everybody feel good; but I am not so convinced that the heat transfer rates are high enough to make any real practical impact on the process. This heat transfer rate is perhaps the most difficult to gauge by intuition alone; but I will stick with my intuition until somebody is really motivated to perform a heat transfer analysis.

I suppose there could be some remote chance of introducing water to your cylinder. Make your own decision here. A water bath does add some level of operational safety to the fill station. If a tank should rupture, it would be better to have this rupture occur while in a water bath than if the tank is standing upright on the floor.

In terms of any high temperatures damaging the tank, your tank doesn't care what the temperature is. If the temperature were to get high enough to damage the tank material, 400 deg F, then no one would want to touch the tank and you could roast your favorite hot dogs using the tank radiation as a heat source. So, if you can't roast hot dogs from the glow of your tank, don't worry about the temperature.

Hot, quick fills?
Nothing about this makes any sense. This process will gaurantee that you compress hot gas into your cylinder. Absolutely. Then, minutes after you leave the fill station, all of that heat (evidenced by high gas temperature) will then absolutely migrate into the cylinder (and eventually move through the cylinder to reach equilibrium with the ambient environment). All this does is gaurantee that your gas cools to a lower temperature and a requisite lower pressure (compared to the pressure measured while standing at the fill station).

 

[Thalassamania]

We did extensive testing on water baths, both in terms of safety and cooling. Cooling simply does not happen there's not enough heat transfer to the water. In terms of safety, it depends on the containment. The water transfers most of the force to the container which turns into shrapnel, metal and concrete are not a good idea unless shaped to direct the force upward, plastic trash cans worked well because of their low mass.

 

[SteveAD]

Just my opinion (and I could run through some thermal analysis / heat transfer / thermodynamics calculations if someone wanted); but my thoughts are:

Slow fills are always better. Gas law physics will predict this. Look up isothermal compression and adiabatic compression for more details.

A water bath makes everybody feel good; but I am not so convinced that the heat transfer rates are high enough to make any real practical impact on the process. This heat transfer rate is perhaps the most difficult to gauge by intuition alone; but I will stick with my intuition until somebody is really motivated to perform a heat transfer analysis.

I suppose there could be some remote chance of introducing water to your cylinder. Make your own decision here. A water bath does add some level of operational safety to the fill station. If a tank should rupture, it would be better to have this rupture occur while in a water bath than if the tank is standing upright on the floor.

In terms of any high temperatures damaging the tank, your tank doesn't care what the temperature is. If the temperature were to get high enough to damage the tank material, 400 deg F, then no one would want to touch the tank and you could roast your favorite hot dogs using the tank radiation as a heat source. So, if you can't roast hot dogs from the glow of your tank, don't worry about the temperature.

Hot, quick fills?
Nothing about this makes any sense. This process will gaurantee that you compress hot gas into your cylinder. Absolutely. Then, minutes after you leave the fill station, all of that heat (evidenced by high gas temperature) will then absolutely migrate into the cylinder (and eventually move through the cylinder to reach equilibrium with the ambient environment). All this does is gaurantee that your gas cools to a lower temperature and a requisite lower pressure (compared to the pressure measured while standing at the fill station).

Considering the amount of energy in a full tank, the only difference if it were to rupture in a waterbath vs rupturing dry is that anyone around it would get blown up...AND WET

 

[richerso]

That's simply not a true statement. Due to the design shape of air tanks, a high pressure rupture appears as a large, long material tear oriented in a longitudinal direction (with respect to the cylinder axis). It is a classic hoop stress failure for pressure vessels. This type of failure sounds as if the world is coming to an abrupt end. Aside from the sound, this type of rupture is not fatal.

Here some secondary effects become important. A hoop stress rupture will send a tank tumbling about the room. A high speed impact from a tank can be dangerous. Here's where the water tank becomes important. Such a rupture while sitting in a tank of water will simply blow water out of the tank. Furthermore, the water tank serves to limit the dynamic motion of the air tank to the confines of the water tank. As long as you are not bathing in the water tank, you are not likely to be struck by the moving air tank. The water also serves to viscously dissipate any motion of the air tank. Additionally, the water serves to muffle the acoustic emission from the event so you don't just completely scare the pee out of everyone in the vicinity. See? It's all good. Nothing bad about any of this. Granted, you are correct. Anyone in the room will get wet. I suspect most scuba divers don't care if they get wet.

 

[texdiveguy]

Were does the tank valve, and fill whip valve and hose go?

Just fill them SLOW and dry (as you agreed in one of your earlier post!)!

Just SO RARE in the real world.

But interesting to discuss.......

richerso you being an engineer really lends body to this thread---

That's simply not a true statement. Due to the design shape of air tanks, a high pressure rupture appears as a large, long material tear oriented in a longitudinal direction (with respect to the cylinder axis). It is a classic hoop stress failure for pressure vessels. This type of failure sounds as if the world is coming to an abrupt end. Aside from the sound, this type of rupture is not fatal.

Here some secondary effects become important. A hoop stress rupture will send a tank tumbling about the room. A high speed impact from a tank can be dangerous. Here's where the water tank becomes important. Such a rupture while sitting in a tank of water will simply blow water out of the tank. Furthermore, the water tank serves to limit the dynamic motion of the air tank to the confines of the water tank. As long as you are not bathing in the water tank, you are not likely to be struck by the moving air tank. The water also serves to viscously dissipate any motion of the air tank. Additionally, the water serves to muffle the acoustic emission from the event so you don't just completely scare the pee out of everyone in the vicinity. See? It's all good. Nothing bad about any of this. Granted, you are correct. Anyone in the room will get wet. I suspect most scuba divers don't care if they get wet.

 

[PvilleStang]

I'm sorry, but have any of you seen what happens when you take a hot tank out of your summer car trunk, then dip into the lake? I can almost watch the guage drop 100 PSI just at the surface. It's a fact that water absorbs the heat much faster than the air (thus the reason we wear wetsuits and drysuits). I'm a firm believer in slow and steady fills. Bleed the air in there. But, I try, whenever possible, to do wet fills with the tank nearly submerged, with the tank valve and whip above water.

 

[Zinc]

I favor water bath

Even with your RIX?

I've never bothered storing the water needed. Slow is my preference, but practicality plays it's role, case by case. I've made many hot fills that will cool more than a few hundred psi. which is estimated to balance out. To me, water is for IN/OUT operations like you'd see in Florida.

My preference is dry, nice and slow... exceptionally slower w/ high O2 - & keep your tanks & valves 02 clean

 

[ReefHound]

I'm sorry, but have any of you seen what happens when you take a hot tank out of your summer car trunk, then dip into the lake? I can almost watch the guage drop 100 PSI just at the surface. It's a fact that water absorbs the heat much faster than the air (thus the reason we wear wetsuits and drysuits). I'm a firm believer in slow and steady fills. Bleed the air in there. But, I try, whenever possible, to do wet fills with the tank nearly submerged, with the tank valve and whip above water.

Not to mention try draining the water from your aluminum block car engine and go for a drive - you'll get a quick lesson in just how much heat is dissipated by water. Granted there are some huge differences, such as the water is circulating and being cooled slightly by a radiator. But is there any reason why a water bath fill system couldn't circulate the water and cool or replace the water? All else equal, filling in water has to be more efficient than filling in air.

That said, if you just fill at a slow rate it is not an issue to begin with. And the problem is that all else is usually not equal. A water bath is not a substitute for slow fills and if you use it to fill faster you will do more harm than good.

 

[texdiveguy]

Even with your RIX?

The last I heard his RIX was sick...