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bigman241
01-08-2010, 12:40
I know this was on here before though I tried to look in the search button and the only thing I found was about hot fills not a water soak when filling. I was on the lds website I plan to use when doing my cert. I noticed when looking at there price on a hydro they have the air fill hoses laid over a tank of water. Why do they do this? I seem to remember something about allowing them to fill them quick or to contain the tank if it blows a hole in it. Also Do any of you guys check your pressure before you leave the lds.

Panmanmatt
01-08-2010, 12:50
The water helps keep the tanks cool whiling filling, this allows them to be filled to their proper pressure. As tanks are filled they heat up and the gas expands, as it cools the gas contracts thus leaving you with a lower pressure. Keeping them cool during filling helps to alleviate heat build up.

FishFood
01-08-2010, 13:07
The cooling effect is arguable. Seems many these days do it out of force of habbit. My LDS submerges the tanks completely to check for valve leaks.

As for checking pressures - I do if Im picking up nitrox fills, otherwise I generally watch them get filled and chit chat.

EDIT -

Here is Larry's take on the subject:

http://forum.scubatoys.com/190249-post5.html


It is against the tank manufacturers recommendations, as it increases the chance of water getting in a tank (if empty and not careful) and it does nothing except encourage people to fill too fast - which is dangerous. But I guess if they fill slow, and are careful not to get water in the valve - it would not actually hurt anything - but a tiny flow restricting device in the line would do much more for the safety of the fill station and help get good fills.

Noob
01-08-2010, 13:43
what I was told is if you fill too fast its unsafe and can weaken the tank, or rupture it. Plus after the air condenses you have less pressure and an improper fill.

bigman241
01-08-2010, 15:28
Thanks guys. Guess what larry says is properly right. Guess after reading that no need to ask if scubatoys does it.

DevilDiver
01-08-2010, 15:37
Here is a good and breif read explaining some of the points:

Wet Fills (http://www.psicylinders.com/library/Current/wetfills.htm)

bigman241
01-08-2010, 16:21
Thanks very good read. Seems I saw that sometime last year as well.
Here is a good and breif read explaining some of the points:

Wet Fills (http://www.psicylinders.com/library/Current/wetfills.htm)

DevilDiver
01-08-2010, 16:23
Thanks very good read. Seems I saw that sometime last year as well.
Here is a good and breif read explaining some of the points:

Wet Fills (http://www.psicylinders.com/library/Current/wetfills.htm)

It has been posted on a lot of the forums.

in_cavediver
01-08-2010, 16:51
what I was told is if you fill too fast its unsafe and can weaken the tank, or rupture it. Plus after the air condenses you have less pressure and an improper fill.

This is close but not quite.

You use lower fill rates to reduce the friction and resultant heating in the HP gas route. We need to think of hp pressure much more like a liquid rather than a gas. (they are both fluids btw). A turn or restriction will cause localized heating to occur. The faster the gas flow, the more friction and the more heat generated. This is not a linear relationship either. If you fill to fast or more accurately put - get to high a velocity of flow, you run the risk of fire, catastrophic failure due to uneven expansion, catastrophic failure due to metals changing temper (strength) etc.

This is the very same reason when doing PP fills, the O2 fill rate is held very low.

JPL (NASA) did studies on this and one researcher was quoted as stating that they had yet to find something they could not light on fire with HP o2 and the right rates.

The second bit on 'air condensing' - this ties to the basic gas laws. In simple terms, for any given volume of gas, in any given volume, the pressure exerted is proportional to the temperature of the gas. The tank specs are really 3000psi at 70F not just 3000psi. If the tank is at 85F, you should have more than 3000psi. If less than 70F, it should be less than 3000psi. Air does not 'condense' except at very low temperatures - basically when it becomes liquid.

THe temperature pressure relationship can be seen regularly when diving. If you take a nice summer dive in a quarry or the great lakes. Outside temp is 90+F, water temp can be in the 50's. You will see a pressure drop once you enter the water and the tanks equalize.

Now for the most important bit. Don't overly concern yourself with the fill pressure. There is very little difference between 2900psi and 3100psi. The fact is unless you have a calibrated guage, you may be able to tell the difference due to guage error (even digital ones have error - they can be precisely wrong!) THis is not to say to ignore fill pressures but to look at them as a reasonable person. Still - NEVER rush fill tanks. Max recommended fill rates are 300-500psi/min for air. Slower is better.

navyhmc
01-08-2010, 22:30
Thanks guys. Guess what larry says is properly right. Guess after reading that no need to ask if scubatoys does it.

Properly Right (don't you mean probably right?) You dare question the wisdom of the all powerful, Grand Poo-Bah Larry!

(be merciful oh great one! For he surely knows not what he says... And if not, let me get out fo the way before you afix your wrath! :smiley36:)

jugglematt
01-09-2010, 03:47
when i fill my tanks i fill to about 200 bar,remove the first tank
put another tank on the fill take the first tank over to my rinse tub and soak it there while the second tank is filling.

my justification is it helps the tanks to cool and gives me better fills without running my compressor to at higher pressure
and it rinses my steel tanks of all salt water and reduces rust.

even with a fairly slow fill say 20 mins for a 10litre tank the tanks do heat up a bit , cant imaging how hot the tanks would get if your filling in just a few minuites , or is the heating effect les when filling from storage banks.

if i was set up properly i would fill my tanks while in a water bath .

Matty

in_cavediver
01-09-2010, 07:17
even with a fairly slow fill say 20 mins for a 10litre tank the tanks do heat up a bit , cant imaging how hot the tanks would get if your filling in just a few minuites , or is the heating effect les when filling from storage banks.


The 'extra' heat you see is from the compressor itself. When you fill from banks, the gas is at 'room' temperature.

scubadiver888
01-09-2010, 07:39
I know this was on here before though I tried to look in the search button and the only thing I found was about hot fills not a water soak when filling. I was on the lds website I plan to use when doing my cert. I noticed when looking at there price on a hydro they have the air fill hoses laid over a tank of water. Why do they do this? I seem to remember something about allowing them to fill them quick or to contain the tank if it blows a hole in it. Also Do any of you guys check your pressure before you leave the lds.

I'd like to add, if you took a picture of my dive shop you would see the fill hoses draped over a water bath. The reason: the previous owner used to put the tanks in the water bath while filling them because someone told them they could fill faster that way. We never use the thing but the new owner doesn't see the point in ripping out the bath (it is incredibly well built and bolted to the floor; not easy to get out).

Bottom line, just because they have the water bath doesn't mean they use it. :smiley2:

Grin
01-09-2010, 08:20
My previous shop did partial pressure fills and dunked them in water, and it was kind of nice as they could fill the tanks fast, and they would always have little change in pressure from cooling down, and cutom mixes were really not considered custom, as every fill was custom. They would dunk all my tanks at the same time and fill them in 15-20 minutes.

My present shop has banked Nitrox and they fill them dry. The tanks get hot and even though they fill them to 4000 lbs, they cool off to around 3400-3500, which is OK. To get them filled to 3600-3700 (what we call correct) they need to sit there for a hour, or more, to cool off while the fill completes. Here in Florida, virtually no shop has that kind of time to wait around for tanks to cool and/or slow fill them. And who wants to have to wait that long or drop tanks off. They either dunk them or fill them to 4000 or so.
I have heard all the arguments and I don't really care either way. People get all worked up their way is the correct way, when both ways work. The old dunking them method has worked for a long time for many shops. I kind of preferred it done that way, becasue I always got good fills every time and fast.
I wouldn't worry about it too much personally. It isn't like your tank will deteriorate or explode. Although others may try to claim that.

bigman241
01-09-2010, 09:04
Well true but it was filled to the top with water.

I know this was on here before though I tried to look in the search button and the only thing I found was about hot fills not a water soak when filling. I was on the lds website I plan to use when doing my cert. I noticed when looking at there price on a hydro they have the air fill hoses laid over a tank of water. Why do they do this? I seem to remember something about allowing them to fill them quick or to contain the tank if it blows a hole in it. Also Do any of you guys check your pressure before you leave the lds.

I'd like to add, if you took a picture of my dive shop you would see the fill hoses draped over a water bath. The reason: the previous owner used to put the tanks in the water bath while filling them because someone told them they could fill faster that way. We never use the thing but the new owner doesn't see the point in ripping out the bath (it is incredibly well built and bolted to the floor; not easy to get out).

Bottom line, just because they have the water bath doesn't mean they use it. :smiley2:

Just Dave
01-20-2010, 14:14
I use a compressor facility in town for my air fills. I've been told they have a chiller in between the banks and the whips. I don't know too much about it except that my tanks are always at 3000 psi +- 100. Gotta love the consistency!