PDA

View Full Version : Filling - to submerge or not to submerge, that is the question



Roughwater
12-22-2008, 18:46
Hi Guys,

I'm just wondering about the practise of filling tanks and whether or not they should be submerged in water.

I've heard some say that this helps to keep the tank cooler, allowing the fill to be more efficient.

I've heard others say that this is not the case, and that it can make things worse by creating greater stress, having a greater difference of temperature between the outside and the inside of the tanks.

However, I see people practise both still, so was just wondering - is the jury still in session regarding this?

DMWiz
12-22-2008, 21:23
Check out this article (http://www.psicylinders.com/library/Current/wetfills.htm) on the subject.

comet24
12-23-2008, 03:06
No cooling benefit for aluminum tanks as they are just too thick to make a difference.

Steel tanks are a different story but I will leave that debate to others.

No Misses
12-23-2008, 09:17
There is no benefit to filling in water. Unless you count having your freshly rinsed tank dunked in warm salt water as a benefit. Do you really think that they change that water?

in_cavediver
12-23-2008, 16:34
There is no benefit to filling in water. Unless you count having your freshly rinsed tank dunked in warm salt water as a benefit. Do you really think that they change that water?

Unless you dive freshwater.....

My opinion is it doesn't matter so long as the whips/valves are kept clean and dry. If not, its a good way to send moisture into the tank. (course once its hooked up, it doesn't matter)

WaScubaDude
12-23-2008, 18:16
Check out this article (http://www.psicylinders.com/library/Current/wetfills.htm) on the subject.

Interesting, but bs in the real world. Every fill I have had, in tub or out leaves my tanks warmer than ambient. I think fill ops fill at a certain rate no matter if the tank is in a tub or not. Most if not all fill faster than the recommended rate. Thus physics being physics the tank is cooled faster in water than in air. The difference may be really inconsequential unless the water is cooled in some manner.

As for over fill. Tanks in and out of water are commonly over filled, in fact I request a over fills. Sometimes get um some times don't. But it is far more common to end up with a short fill by the time the tank cools regardless of in water or out fill.

WaScubaDude
12-23-2008, 18:18
Also, i am always skepticle when some starts their argument with "set aside logic", blah blah blah.

cummings66
12-23-2008, 21:39
Your last two posts make absolutly no sense to me, were there deleted postings you refer to? The link has not even got the words you quoted "set aside logic" in it, nor do any of the other postings I can see.

I also do not see what you're call BS in the article? It seems to be straight forwards and not lying from what I know, nothing to say it's full of BS as you put it.

I'm wondering, did this thread get heated and then postings deleted because something I just do not get your last two postings.

cummings66
12-23-2008, 21:44
At any rate, I'll tell you what I was told at my psi cert course. They claim the cylinders being filled in the water are not left there long enough to transfer the heat generated and so for the most part all that happens is you get a fill, risk water being introduced, and got no benefit of the water. The other point they mention is that after a couple fills the water would be too warm to do any good anyhow so that if it is of benefit it would be a very limited benefit that most would not enjoy and they would find as you stated, a warm cylinder being handed back to them with a possible short fill.

I'm in the group of why bother, just fill at a good rate and you won't have problems anyhow. Overfills, don't go there. Why risk it? A cylinder is good for a certain number of hydro test pressure fills, after that it's scrap. Why hasten the scrapping process?

ektess1
12-24-2008, 00:19
No water. Waste of time, and you can introduce water into your tank. Get a dry hot fill. When it cool, it will be the correct volume.

in_cavediver
12-24-2008, 08:42
I'm in the group of why bother, just fill at a good rate and you won't have problems anyhow. Overfills, don't go there. Why risk it? A cylinder is good for a certain number of hydro test pressure fills, after that it's scrap. Why hasten the scrapping process?

The only point I want to add is the level of 'overfill' and whether it really is an overfill. A fill pressure is stated at a specific temp. Therefore, if the air/tank is at 90 degrees, the fill pressure should be higher and conversely for cold tanks.

Also, a few hundred psi aren't anything to worry about either. You are operating the error of the guage there. If that's not compelling enough, a garden variety AL 80 has a hydro pressure of 5000, 3200psi isn't very close to that.

Now, for cave fills - no argurement there that you are shortening the life of the tank. For those unfamilar with cave fills, its taking a LP (3AA) steel tank that is rated at 2400 or 2640 and filling it to 3600-3800psi. (its hydro pressure is 4000psi). How much shorter, I personnally don't think its significant *FOR THE INTENDED USE* - IE scuba. I don't care if I get 5000 fills rather than 50,000 or 500,000 before failing hydro. Heck, a thousand fills is a good set of dives for me. There is also a pretty good safety history over the last 20-30 years of LP 3AA steels handling overfills without failing hydro or otherwise catastrophically failing. Still, the benefit has to be there in the dive to be worth it to the tanks. For many of my dives, the extra 100cft I get in my 104's are worth it.

cummings66
12-24-2008, 09:16
It is significant if you believe Faber. They rated (memory don't fail me now) their cylinders to I think 10,000 hydro test pressure fills before it'd fail, the number was in the handouts at the inspection course. Now that's a lot of fills which probably is the lifespan of the owner, but it's essentially unlimited for normal fills which means you, your kids, their grandkids, etc could all enjoy that cylinder and by doing the hydro test pressure fills (ie cave fills) you've cut it's life by a large percentage and perhaps your great grandkids can't use it. Assuming the water doesn't rust it out first anyhow.

Now a small pressure above normal fill (which varies per temp) you don't really hurt things. In the inspection course they gave you a formula to figure out how much air is needed based on current temps so that when it cooled you'd have a full fill.

In practical terms, for a steel cylinder, overfills don't cut it's lifespan for the owner. They could probably dive that one cylinder for their whole lifes getting cave fills on a regular basis. Wild guess, 200 cave fills a year would give you perhaps 50 years of diving on that one cylinder, have two of them and you could easily get 100 years of diving out of it.

But, water baths, get a few wet fills without knowing it, store it with a cave fill, and now you've got a receipe for serious rusting to happen, and you could lose that cylinder in less than a year. Rust happens much faster under pressure compared to a low pressure environment, one reason psi likes cylinders stored with almost no pressure in them, I'm talking on the order of 50 psi here. Much less than the 500 psi you hear bandied about. Enough to be easily pressurise it, but low enough to not cause problems in a fire or some such event, and it helps if there's water in there.

None of that is my own conjecture, it's what I was told in a course, whether it's true or not I do not know. There was a university study which shows a cup of salt water in a cylinder would destroy it about a year I think, I don't recall the specifics and it was an old study from I think the 70's.

ReefHound
12-24-2008, 09:46
Your last two posts make absolutly no sense to me, were there deleted postings you refer to? The link has not even got the words you quoted "set aside logic" in it, nor do any of the other postings I can see.

I also do not see what you're call BS in the article? It seems to be straight forwards and not lying from what I know, nothing to say it's full of BS as you put it.

I'm wondering, did this thread get heated and then postings deleted because something I just do not get your last two postings.

I think he was just paraphrasing that they start off acknowledging the physics and saying "we can all pretty much agree that heat transfer is far greater in water than in air" and then proceeds to set that aside. Much of that article is bunk, like the fear of overfilling because putting the tank in chilled water can reduce pressure at fill time and then it will expand to overfill later. Never mind that the article later claims that temperature differential is usually not enough to make a difference, never mind that I have never seen a place that fills in water that uses "chilled" water so cold that it not only absorbs all the heat generated from the fill but additional heat to reduce temperature below ambient.

With all the long winded debates on this topic you would think that someone would have conducted a simple side by side test. Take two tanks at same pressure, one in ambient temp water and one in air, hook them up to whips and fill at same rate, measure surface temp of tanks when full.

My position is that a water bath can be beneficial if proper procedures are used but that it is not really necessary if proper procedures are used.

in_cavediver
12-24-2008, 11:08
Your last two posts make absolutly no sense to me, were there deleted postings you refer to? The link has not even got the words you quoted "set aside logic" in it, nor do any of the other postings I can see.

I also do not see what you're call BS in the article? It seems to be straight forwards and not lying from what I know, nothing to say it's full of BS as you put it.

I'm wondering, did this thread get heated and then postings deleted because something I just do not get your last two postings.

I think he was just paraphrasing that they start off acknowledging the physics and saying "we can all pretty much agree that heat transfer is far greater in water than in air" and then proceeds to set that aside. Much of that article is bunk, like the fear of overfilling because putting the tank in chilled water can reduce pressure at fill time and then it will expand to overfill later. Never mind that the article later claims that temperature differential is usually not enough to make a difference, never mind that I have never seen a place that fills in water that uses "chilled" water so cold that it not only absorbs all the heat generated from the fill but additional heat to reduce temperature below ambient.

With all the long winded debates on this topic you would think that someone would have conducted a simple side by side test. Take two tanks at same pressure, one in ambient temp water and one in air, hook them up to whips and fill at same rate, measure surface temp of tanks when full.

My position is that a water bath can be beneficial if proper procedures are used but that it is not really necessary if proper procedures are used.

First - I agree with Matt a few posts abovr completely. I agree with Faber but for me and my uses - 1000 cave fills for a the life of a set of tanks is OK. The dives that enables are worth it.

Now for the above quotes: Here is the real meat of the question - what does the water bath add - ideally, even if chilled. I'd wager less than 400psi change and more likely in the 100-200psi max range. (I am too lazy to look up the forumula for calculating it). Give this worst case scenario - is it even worth worrying about? Probably not.

Moisture introduction is the worst enemy of the practice.

ReefHound
12-24-2008, 11:30
Now for the above quotes: Here is the real meat of the question - what does the water bath add - ideally, even if chilled. I'd wager less than 400psi change and more likely in the 100-200psi max range. (I am too lazy to look up the forumula for calculating it). Give this worst case scenario - is it even worth worrying about? Probably not.

I thought that was what I was saying in my last sentence.

cummings66
12-24-2008, 11:30
never mind that I have never seen a place that fills in water that uses "chilled" water so cold that it not only absorbs all the heat generated from the fill but additional heat to reduce temperature below ambient.


In our course it was looked at from that standpoint that almost nobody using those fills has any recirculation of the water so in the end it's not doing everybody a favor and has risks. Therefore why risk it?

Here's one reason it's used, from a real life shop.

I know of one shop that uses water baths and IMO they need to due to how they fill. I can tell you for a fact that I routinely end up with about 300 psi less air there than I get from a shop that uses a slow air fill. To give you an idea of how fast it is, they go from 500 to 3000 psi in about 5 minutes or less depending on how many people they have waiting. The lesson is that fast fills such as theirs do benefit from a water bath, and in this ops case they could have several cylinders in there at once being blasted. Their water was warmer than expected, I don't know how warm but when I stuck my hand in there it didn't feel cold to me. Without the water bath they could not do the fills that fast. I never saw them blow out the tank valve, nor the whips so I would assume the possibility of water infiltration existed there as well.

To give you an idea of their setup, it was a large round tank with dividers in it in which you would set your cylinder, each section had a whip above it and they would attach it there. When I say large, I mean it was probably about a 10' diameter tank. They believe in the water bath because in their words, they could not fill as many cylinders as they do without it. They fill twice as many cylinders as anybody else, and they do it using a water bath. I will not do business with them for two reasons, one water infiltration risk, and two I used to always get short fills there.

I stopped being an air customer after I learned of the risks.

ReefHound
12-24-2008, 12:09
The question is whether the water bath encourages fast fills or whether a place like that would be doing fast fills anyway. I hate getting fast fills and is why I prefer to fill my own at my shop where I DM, an option many don't have. If I must get a fill outside the shop I try to do it at less busy hours and emphasis I want a slow fill.

Italianprincess
12-24-2008, 12:58
I guess I don't get out that much...I've never seem anyone fill tanks underwater!

ReefHound
12-24-2008, 19:02
Not under water, in water up to or near the neck. Windy Point used to fill in water, I haven't been in a few years though.

texdiveguy
12-24-2008, 19:06
I 'prefer' my tanking filled dry....the water thing really has no bases of doing anything productive to the fill process.

cummings66
12-24-2008, 21:27
This shop would have the cylinders submerged up the the crown of the cylinder, and for the short ones they would fill them outside the tub so those were no big deal.

I think they just knew in order to fill them that fast they would either be giving you a serious underfill, or perhaps they realized they could damage cylinders over time and so they got the tank. At any rate, they use the tank because it's the only way they can do it that fast and fast is what they want.

I don't deal with them now that I have a bit more knowledge under my belt, I used to like getting a fast fill before I realized it could cause problems.

captain
12-26-2008, 19:15
I have a compressor that will fill a steel 72 from 500 psi to 2500 psi in 20 to 25 minutes depending on ambient temperature and humidity. I have a piece of 8" PVC pipe capped on one end with a garden hose connection on the bottom. With a tank in it the water overflows the PVC pipe at the height of the tank neck. I keep a slow trickle of water running while filling tanks. Without the water bath I have to let the tank cool for 30 minutes and top it off because it will be at least 200 psi low once it cools, with the water bath I don't have to top them off.
It saves me the hassle and time of having to go back and top off a bunch of tanks

cummings66
12-31-2008, 18:32
Yes, but you're also better than the average shop monkey who barely knows how to attach a yoke.

cogrwy
01-04-2009, 14:53
As a DM, when I fill tanks, I don't use the shop's water tank, although some others do. The only time I've found heat to be an issue is when the tank is nearly empty. If necessary, you can always stop the fill to let the tank cool down.

JahJahwarrior
01-05-2009, 10:36
In winter time, with an outside fill station, the water can be quite cold, and I feel it makes a large difference. Why? Feel the crown of the cylinder which is not underwater, and feel the middle of the tank which is underwater. The crown feels hotter than the middle part. Either the water is helping cool the tank, or air heats up as it rises. I know from the fact that jet's wings ice over that air does not usually heat up at higher altitudes, so the water must be helping to cool the tank.

When filled in the bath, I routinely end up with 300psi more than when filled dry.

Even 100psi makes a difference on a cave dive: 3500 vs. 3600. 3500/3 is an ugly number, 3600/3 is 1200. I prefer to have that 100 psi :)

I always open the valve to clear any water from it, and tanks are hooked up outside of the bath if they are not doubles, before being put into the bath, and if they are low, I usually wait until they have 500psi before dropping them underwater to fill.


Fill baths suck when burst disks blow-- water tends to go all over the place :D