Filling Tanks - How Hot is Too Hot?

[w ripley]

I've been running one for probably 5 years now. FYI, I've had a lot of problems with the 3rd stage seals, and have to replace at least once per year. The compressor still pumps, but pretty slowly when that seal goes. The head bolts are round heads with internal phillips fittings. Suggest you order new head bolts when if you have to replace your seals, as they go about every other time you loosen and retighten them. Guess that comes with buying a compressor made in Turkey.

I fill my tanks in a tub. They're going to get warm-to-hot filling them out of water. It just saves a lot of time filling that way, especially when blending several gases like O2 and HE (nitrox or trimix). Just takes too long to let them settle down on their own, especially during the summer. I normally pump my tanks to 3,600 psi, more if doing a long push or deep dive, so they're going to get hot. Water is what every tech fill station puts their tanks in to offset this.

The compressor is sending out a hot air mix, and it's going directly into your tanks. You know this. So, your tanks are going to get relatively hot, but not like you are describing. Truthfully, you don't have a 3.5 CF/M compressor, because everytime it bleeds you have to subtract what you're bleeding off from the final amount being pumped. If you never blead it you would have 3.5 CF/M, but that's not how the self bleeders work. So, your tanks shouldn't be getting that hot from 2.5 +/- CFM. It's a pretty small compressor.

I pump into 6 high pressure cascaded storage tanks and maintain them at 4,000 psi. That gives me approximately 2,400 cf to work with. The only time my dive tanks ever get really "HOT" is if I try to put too much in too fast from those storage tanks, not directly from the compressor.

BTW: I have my compressor in a detached garage. It takes hours and hours to fill the storage tanks after a big fill so I have a light connected to the compressor that comes on when it is running. The light is mounted outside the garage so that I can see it from my kitchen. That way I always know whether or not the compressor is on anytime of the year and when it's finished topping off the storage bottles.

Regards,

Bill

 

[realdiver7]

You gotta remember that when filling directly off a compressor, you are starting with MUCH hotter air than a shop that fills from a cascade. No matter how slow you go, hot air is still hot air.

Ray....now that makes a lot of sense! Didn't think about that one, especially being fairly new at pumping my own air.

 

[realdiver7]

Unless someone pulled a fast on and sold you a compressor with considerably more output than you think you have, or you are routinely experiencing combustions within the tanks and not noticing it (highly unlikely), then there is no way your tanks are going to overheat enough to matter with using a 3.5 cfm compressor. Yes, they will get hotter than with a slow fill from a cascade, but no they will not get hot enough to matter, and the fact that they don't get too hot to hold reinforces that. You'd have to heat an alloy tank up to 250 F before it would be time to start worrying, and a steel to over double that.

Most of us with compressors just figure out a fudge factor for how much to overfill the tanks to get the the final, cooled, pressure we want, something which is completely acceptable under CFR something-or-other, as long as you don't get carried away.

If it bothers you, and you are frequently filling mulitiple tanks, one way around it is to add another whip, so you can fill two tanks at a time, so the tanks have more time to dissipate the theat as they fill. This is how many of the mid-sized portable compressors, that will not be used with a cascade, do it.

Vance.....this is technically more of what I was looking for in regards to "how hot is too hot." I'll start looking around for another DIN fill whip just to add to my comfort factor. Anybody know where I can find an inexpensive new or used one?

To everyone else....the replies are great, and I'm learning more each time I read about your personal experiences. Keep them coming!

Thanks!

 

[pescador775]

I dry fill SCUBA tanks from a 5 cfm Bauer Capitano. The steel tanks heat up a bit, so what? Like "hacker" said, overfill it or let it cool and top off. The multiple whip manifold would be nice if more than one tank are to be filled.

 

[tep]

I believe the Navy Dive Manual recomends 300 psi per minute dry, no more than 600 in a water bath, but filling them in a bath is out of fashion now. Good rule of thumb is if you cannot touch them comfortably, you are filling too fast.

Filling in a water bath is out of fashion? I noticed that my "new" LDS has no water bath, but then I found out they plan to add one, and the former LDS had several water baths...

I'm confused here... What's the reason to not use a bath?

 

[captain]

My compressor is 3 to 3-/12 cfm depending on temperature and humidity. Usually in the summer air temps are in the mid 90's.
I use the setup pictured. It is a 26" length of 8" OD PVC pipe with a cap on one end. At the bottom is a connection for a garden hose and at the top are three 1" holes that allow the water to overflow and also act as finger holes to carry it. I turn on the water so there is a small amount overflowing the three holes while filling so there is a continous flow of cool water from botton to top.
As you can see there is no way the valve or filler can get below the water's surface.

 

[realdiver7]

My compressor is 3 to 3-/12 cfm depending on temperature and humidity. Usually in the summer air temps are in the mid 90's.
I use the setup pictured. It is a 26" length of 8" OD PVC pipe with a cap on one end. At the bottom is a connection for a garden hose and at the top are three 1" holes that allow the water to overflow and also act as finger holes to carry it. I turn on the water so there is a small amount overflowing the three holes while filling so there is a continous flow of cool water from botton to top.
As you can see there is no way the valve or filler can get below the water's surface.

That looks very stable, simple and practical. Since I fill inside of my garage while pumping fresh air from outside of the garage, I wouldn't be able to have continuous water flow, but the PVC pipe with inlet/drain on the bottom is a great idea for my purposes.

Thanks for the pics.

 

[captain]

That looks very stable, simple and practical. Since I fill inside of my garage while pumping fresh air from outside of the garage, I wouldn't be able to have continuous water flow, but the PVC pipe with inlet/drain on the bottom is a great idea for my purposes.

Thanks for the pics.

Just put a hose on the top to overflow outside. Something like the corrigated hose used for boat bilge pumps. You can also use a plastic boat thru hull fitting on the top to connect the hose to. If both inlet and drain both are at the bottom you just may cool just bottom with no flow up the side of the tank.

 

[ReefHound]

Filling in a water bath is out of fashion? I noticed that my "new" LDS has no water bath, but then I found out they plan to add one, and the former LDS had several water baths...

I'm confused here... What's the reason to not use a bath?

It's against some manufacturer's recommendation, like Luxfer. And it adds the risk of getting water in the tank or fill whip if the filler is careless. But mostly it's that many fill in water so that they can fast fill.

 

[pescador775]

It's against some manufacturer's recommendation, like Luxfer. And it adds the risk of getting water in the tank or fill whip if the filler is careless. But mostly it's that many fill in water so that they can fast fill.

Yes and no; Luxfer recommend against "hot fills" which translates to jamming an aluminum cylinder in less than five minutes. Typically, this is done in a bath. The negative water bath sentiment came about when some dive shop owners felt that water baths were too much trouble including housekeeping, etc. An association of dive stores, and I forget which one, asked a guy to provide cover by putting together a "white paper" in which every spurious reason that could be imagined were incorporated into this anti-bath opinion and later published on the net. One could probably still locate it. The one defensible argument that I could isolate from the paper is the same as mentioned previously, that a careless operator could blow droplets into the Scuba tank. The rest was bull, aimed at gullible readers who lacked understanding of physics.