Filling scuba tanks

[Jonny Wishbone]

Water is a just over 800 (depends where in the world you measuring) denser than air. According to PADI water absorbs heat 20 times faster than air, NAUI states 25 times faster, USN states 30 times, and Mythbusters determined the factor is 28.

I forget where I read this (either through PSI/PCI or the paper from the University of RI), it takes a tank, filled out of water, just under three hours to cool to ambient temperature after it has been filled. I haven't seen a report indicating how long it takes for the gas inside a scuba tank to cool to ambient temperate if it is filled in a cooled/cold water container.

Yes 800 times denser. I knew there was an 800 something or other.lol. it appears that one eyes are not the only thing to go later in life.lol

 

[DA Aquamaster]

from this link: PSI-PCI - Filling Cylinders In Water - Time to Review
it overall doesn't matter (but dry is better) as long as the person doing the fills understands that it's best to slow fill: cylinders, when filled at the industry recommended fill rate of 300-600 psig/min, do not get hot
The article mentions several issues with wet filling which makes it harder to get a proper/correct fill.
Happy reading.

I read the article and there are some thing said that need to be properly qualified.

For example, if you are filling a tank directly off a compressor, the tank will indeed get hot at a 300 to 600 psi per minute fill rate. I have a 4 CFM compressor and it takes about 20 minutes to fill an AL 80 from empty to 3000 psi. That's just 150 psi per minute and the tank will get hot, because the freshly compressed air is itself hot.

As noted above the options are to use a water bath to keep the tank at close to room temperature, or to fill the tank to around 3200 psi so that it cools off to 3000 psi at room temperature.

If you crunch the numbers you'll find that a tank filled to 3200 psi at a temperature of 106 degrees F will cool to 2996 psi at 70 degree F. That means the tank is not in fact over filled. But some shops will insist that doing the initial fill to 3200 psi with the tank heating to 106 degrees F is "overfilling". Those same shops will slow fill a tank to 3000 psi at 70 degrees F and then insist it is not overfilled when it is sitting in the sun on the boat deck at 106 degrees F.

PSI suggests that a water bath is not effective unless it's an ice bath, and then they insist that this will result in over filling. If the tank really does chill all the way down to 32 degrees F, then yes a 3000 psi fill will be about 3230 psi at 70 degrees. But think about this for a minute. What drives cooling efficiency is the temperature gradient - the higher the difference in temperatures, the better the heat transfer. If you are filling a 106 degree tank in 70 degree water, PSI suggests that 36 degree temperature gradient is not sufficient. Yet they insist the 38 degree temperature gradient between 70 degrees and 32 degrees is enough to cause a over fill. It's basically the same gradient so their logic fails.

In my experience, if I use a water bath on a 70 degree day, I can reduce my initial fill pressure to about 100 psi over the target pressure and be pretty much on the money once the tank cools to 70 degrees. If I am filling tanks on a 95 degree afternoon, a fresh water bath with 70 degree water is a real help as it means I can fill to about 100 psi over the target pressure instead of around 300-400 psi over the target pressure to get the proper service pressure at 3000 psi. In that context, a water bath on a hot day eases the strain on both my compressor and my tanks.

Similarly, on a 95 degree day in N FL, if I want a 3600 psi fill once I'm actually in the 70 degree water in a cave system, I do in fact need to fill the tank to 3800 psi (3771 psi to be exact). That's assuming the tank is actually at the ambient temp of 95 degrees during the fill. That won't happen unless I'm both slow filling and using a water bath (where the water will be close to the ambient air temp) to improve the heat transfer. I not, I am once again needing a 3900-4000 psi initial fill pressure to get a 3600 psi fill once I'm in 70 degree water - and that is really pushing the test pressure of the tank. In that regard a water bath makes a great deal of sense and that's why you see almost every diver shop in N FL using a water bath.

Obviously I am a fan of cave fills. PSI is correct to the extent that DOT considers a tank to be over filled if it exceeds the service pressure when it is at 70 degrees F. Yet every dive shop in cave country has been doing 3600 psi cave fills in 2400 psi service pressure 3AA steel tanks for at least the last 2 decades and DOT has not shut any of them down. Why? Partly it's a jurisdictional issue as DOT's jurisdiction applies to tanks used in interstate commerce. DOT inspectors can try to insist that a scuba tank filled in N FL can potentially be taken across a state line, and thus be used in interstate commerce, but it's weak argument when divers are getting fills that morning for a dive in N FL that same day. Part of the reason DOT ignores the practice however is the fact that 2400 psi steel tanks that are cave filled on a regular basis to 3600 psi are not exploding, and in fact are not even failing requalification every 5 years.

PSI also cites concerns for water getting in a tank due to being filled in a water bath. However, provided the tank monkey cracks the tank valve to ensure there is no water lurking in the valve, and likewise ensures there is no water in the fill whip, no water will get in the tank. Water just does not seep in against greater tank pressure. The same threat exists on boats, where spray can get in a valve or in the fill whip just before the connection is made and the tank is filled, An even greater threat is posed by compressors that do not have adequate moisture separation and/or inadequate desiccant in the filter stack. If there is excessive moisture, it will condense in the tank once it cools to its dew point.

 

[papaouiee]

You can hang out in shorts and a t-shirt in 50f all day. You can even work up a sweat and need to cool down. In 50f water you will be hypothermic in a short period of time.

When you are rejecting heat through convection, the mass of the fluid(air and water are both fluids) is quite important. Movement of the air or the water will greatly increase the movement of heat. What you are doing is employing the second law of thermodynamics. The molecules in the air or the water become more excited and the molecules in the tank become less excited. The water has much greater density of molecules available to accept the heat energy from the tank than the air does. Movement brings more molecules into contact with the surface of the tank and keeps the delta T of the molecules in contact with the tank at a higher level.

Greater deltaT (temperature differential) equals greater heat exchange
Greater density of the cooling fluid equals greater heat exchange
Greater movement of the fluid equals greater heat exchange
Longer dwell time equals greater heat exchange

If you fill the tank in a water bath but do it in 5 minutes and then pull it out then the benefit of the water will be limited by the low dwell time. If you do it in water that is room temperature then the effect will be greater than if you do it in water that is 90f but 90f water will still cool a 130 degree tank faster than 70f air. If you have a small volume of water and fast fill lots of tanks the water will get very warm. It has accepted a huge amount of heat but it's capacity to continue doing so is now diminished.

I disagree strongly with anyone that claims it isn't faster to cool a tank down in water than in air but the amount of benefit is dependent on the above factors.

I am ignoring radiant cooling in my description. It is a small factor with it's own set of variables but it does have a greater impact in the air cooled side of this equation. Actually because air cooling is SO ineffective, radiation does play a significant role in that version.

I apologize for the lecture. I'm not good at this.

I'm glad someone else here understands Physics. By the simplest laws of physics, a tank will always be cooler filled in water than air. It's also safer because if it explodes the container and the water will absorb SOME of the energy, thus reducing the blast. In the case of a near full tank going, the amount they absorb will be negligible; but in an explosive situation, anything is better than nothing.

 

[papaouiee]

OK, lets take a trip back in time for a second. Way back in OW cert didn't we learn that water was a better conductor of heat than air? I think we also learned that water can conduct heat something like 800 times faster than air. So if this is true, why would it not be accepted that the heat from a filling scuba tank would be drawn away from the tank better in water than in air. I would think that keeping the tank temp as low as possible when filling would be the best practice not soley for the PSI but for the health of the tank.

Yup, simple logic. Well stated.

 

[papaouiee]

Water is a just over 800 (depends where in the world you measuring) denser than air. According to PADI water absorbs heat 20 times faster than air, NAUI states 25 times faster, USN states 30 times, and Mythbusters determined the factor is 28.

I forget where I read this (either through PSI/PCI or the paper from the University of RI), it takes a tank, filled out of water, just under three hours to cool to ambient temperature after it has been filled. I haven't seen a report indicating how long it takes for the gas inside a scuba tank to cool to ambient temperate if it is filled in a cooled/cold water container.

Well, I can tell you this. When my compressor was working (a small portable gas Nuvair), I filled my Pressed Steel Tanks in about a 50 gallon tub of water. It took about 20 minutes to fill the 3442 80's. When done, I'd take one out and fill the others. I usually filled four tanks at a time after a week end of diving and each tank, wether it was the first one or the second one, when I got to the lake, they had in them within 50 lbs of what they were when I took them out of the water tank and many times no loss at all. We all know what happens when the local dive shop fills them in three minutes if your lucky; they are practically a stove to cook an egg on. They (some will) pump them up to 3300 or 3400 and still when you get to the lake, you shy by 400-500 lbs or worse. It's been said here multiple times; yet many still deny the reality here.

 

[papaouiee]

I read the article and there are some thing said that need to be properly qualified.

For example, if you are filling a tank directly off a compressor, the tank will indeed get hot at a 300 to 600 psi per minute fill rate. I have a 4 CFM compressor and it takes about 20 minutes to fill an AL 80 from empty to 3000 psi. That's just 150 psi per minute and the tank will get hot, because the freshly compressed air is itself hot.

As noted above the options are to use a water bath to keep the tank at close to room temperature, or to fill the tank to around 3200 psi so that it cools off to 3000 psi at room temperature.

If you crunch the numbers you'll find that a tank filled to 3200 psi at a temperature of 106 degrees F will cool to 2996 psi at 70 degree F. That means the tank is not in fact over filled. But some shops will insist that doing the initial fill to 3200 psi with the tank heating to 106 degrees F is "overfilling". Those same shops will slow fill a tank to 3000 psi at 70 degrees F and then insist it is not overfilled when it is sitting in the sun on the boat deck at 106 degrees F.

PSI suggests that a water bath is not effective unless it's an ice bath, and then they insist that this will result in over filling. If the tank really does chill all the way down to 32 degrees F, then yes a 3000 psi fill will be about 3230 psi at 70 degrees. But think about this for a minute. What drives cooling efficiency is the temperature gradient - the higher the difference in temperatures, the better the heat transfer. If you are filling a 106 degree tank in 70 degree water, PSI suggests that 36 degree temperature gradient is not sufficient. Yet they insist the 38 degree temperature gradient between 70 degrees and 32 degrees is enough to cause a over fill. It's basically the same gradient so their logic fails.

In my experience, if I use a water bath on a 70 degree day, I can reduce my initial fill pressure to about 100 psi over the target pressure and be pretty much on the money once the tank cools to 70 degrees. If I am filling tanks on a 95 degree afternoon, a fresh water bath with 70 degree water is a real help as it means I can fill to about 100 psi over the target pressure instead of around 300-400 psi over the target pressure to get the proper service pressure at 3000 psi. In that context, a water bath on a hot day eases the strain on both my compressor and my tanks.

Similarly, on a 95 degree day in N FL, if I want a 3600 psi fill once I'm actually in the 70 degree water in a cave system, I do in fact need to fill the tank to 3800 psi (3771 psi to be exact). That's assuming the tank is actually at the ambient temp of 95 degrees during the fill. That won't happen unless I'm both slow filling and using a water bath (where the water will be close to the ambient air temp) to improve the heat transfer. I not, I am once again needing a 3900-4000 psi initial fill pressure to get a 3600 psi fill once I'm in 70 degree water - and that is really pushing the test pressure of the tank. In that regard a water bath makes a great deal of sense and that's why you see almost every diver shop in N FL using a water bath.

Obviously I am a fan of cave fills. PSI is correct to the extent that DOT considers a tank to be over filled if it exceeds the service pressure when it is at 70 degrees F. Yet every dive shop in cave country has been doing 3600 psi cave fills in 2400 psi service pressure 3AA steel tanks for at least the last 2 decades and DOT has not shut any of them down. Why? Partly it's a jurisdictional issue as DOT's jurisdiction applies to tanks used in interstate commerce. DOT inspectors can try to insist that a scuba tank filled in N FL can potentially be taken across a state line, and thus be used in interstate commerce, but it's weak argument when divers are getting fills that morning for a dive in N FL that same day. Part of the reason DOT ignores the practice however is the fact that 2400 psi steel tanks that are cave filled on a regular basis to 3600 psi are not exploding, and in fact are not even failing requalification every 5 years.

PSI also cites concerns for water getting in a tank due to being filled in a water bath. However, provided the tank monkey cracks the tank valve to ensure there is no water lurking in the valve, and likewise ensures there is no water in the fill whip, no water will get in the tank. Water just does not seep in against greater tank pressure. The same threat exists on boats, where spray can get in a valve or in the fill whip just before the connection is made and the tank is filled, An even greater threat is posed by compressors that do not have adequate moisture separation and/or inadequate desiccant in the filter stack. If there is excessive moisture, it will condense in the tank once it cools to its dew point.

All, excellent points.

 

[clownfishsydney]

We have had a Bauer Junior II for the past 16 years and filled over 4,000 tanks. We have always filled using a water tank and can say that for steel tanks it makes a big difference in final fill pressures. For aluminium tanks it is less of a difference. If you have a small compressor and need to fill in temperatures over 25C (US figure out what that is!), then I reckon use a water tank if you can.

 

[rcontrera]

It's also safer because if it explodes the container and the water will absorb SOME of the energy, thus reducing the blast. In the case of a near full tank going, the amount they absorb will be negligible; but in an explosive situation, anything is better than nothing.

I am afraid that this really isn't true. When a tank fails, it usually splits and lets all the air out at once. If it is being dry filled at the time, the air can escape in every direction equally and looses it's pressure everywhere. However, once you put a tank into a water bath, that failure will have the expanding air pushing against water which is not compressible. So, instead of air rapidly expanding into the surroundings, it is rapidly displacing the water. The water has nowhere to go but up and that is usually not enough to soften the pressure. So the water also expands to the sides causing a drastic failure of the walls of the container. Most dive shops that fill in water use standard horse troughs and those are usually galvanized or heavy plastic. That means some gnarly shrapnel is flying along with the water.

Just my $.02

 

[nadwidny]

I but in an explosive situation, anything is better than nothing.

In the same way that pissing on your house when it's on fire is "better than nothing".

 

[NAM001]

I have never heard a real valid reason for filing dry. The goals were never get water in your tank. dry filling supports that. fill slow, dry filling helps that. filling with hot gas from a compressor. dry is useless, enhancing cool down time after fill , dry filling does nothing. precooling the tank, dry filling does nothing.

I normally fill wet when I can. whether it is in a bath of with wet towels wrapped on the tank. They are not wet till the fill line is attached and the line is not attached till the valve is blown out by cracking the valve. A rix 6 will heat up a lp95 very fast and make it hot. Yes the internals are not cooled by wet filling bu the rate the heat is drawn out is sped up. next putting them in a bath lowers tth tank temp prior to filling. so yo are not filling a 110f tank to 140F. The best thing to do as I can see is to get a cooler for the filliing gas to cool it prior to entering the tank.now you can take a coil of say 100 ft of steel line and put it in a bucket with water and that will cool the gas a whole lot. do that prior to the moisture separator and that is even better. AS IT IS I EITHER WET BATH OR USE A TOWEL WRAPPED AROUND THE TANK AND KEEP ADDING WATER TO IT TILL THE TANK IS FILLED TO 2-300 OVER intended pressure. One day I will be able to afford a cascade.

What I dont do is say that i can fill faster with a wet fill. wet or dry makes no difference in a 30 second fill.