Dry Fill?

[Web Monkey]

Hello,
I just ordered a x7-100 hp steel tank. the shop tells me they dry fill the tanks to like 5000 psi, but they cool to 3400.
is this ok?

If they filled it properly (slowly) they wouln't need to fill it to 5000PSI to get 3440 PSI.

Legally, it's not OK because the tank is only rated for whatever it's rated for. It's not likely to explode, but they're not doing you any favors either.

From your point of view, it shouldn't be OK because the huge overfills shorten the life of the tank, and weaken the burst disk, so the fast fills cost you money in the long run by making your tank fail hydro earlier.

the description she gave made some sense.... that a tank in water would tend to absorb condensation.

Your tanks should not be "absorbing" anything. The air inside the tanks should be absolutely clean and dry, and the outside of the tank comes from the factory ready for use in SCUBA.

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also, she said do not paint the tank... (is this also true?)
I was thinking of some day glow spray paint... to help visability underwater.
what can I use if I dont paint it?
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I'd recommend not painting it also. The tank doesn't require any painting, so unless you're doing it for looks, you're wasting your money.

Also, I'm not sure what you would be able to paint it with anyway, since the tank expands and contracts as it fills, empties, warms and cools.

Terry

 

[jonnythan]

The 100 is, IMO, the perfect singles tank. The 120 is too long, and the 119 is much heavier. 100cf is plenty of gas and a perfect-size tank.

To do a "hot fill" and have it settle back down to ~3400 psi, you need to pump the tank up to ~3600-3800 psi. It's not as good for the life of the tank as a nice gentle fill, but it doesn't worry me much when it gets done to my tanks. There's no way they're pumping them above 4000 psi, much less 5000 psi.

$6 for air is OK. Check other local shops to compare.

Do not paint the tank. There are two good reasons. First, the paint will scratch and chip in short order and look like crap. Second, the protective galvanization *requires* a large exposed galvanized surface in order to effectively repair scratches in the zinc coating. The way galvanization works is that when there's an imperfection in the zinc surface, exposing steel, a chemical reaction occurs that results in zinc being redeposited to cover the imperfection. Painting a galvanized tank means scratches will rust.

 

[Nemrod]

Dive-aholic, I mean in a production environment---ie---filling many tanks as fast as possible for customers who need to get going. A circulating cool water bath helps to get a solid fill. It still takes some effort and common sense. Yes, filling as you are doing with your compressor should result in a good fill for you--absolutely. During college I worked in a shop and filled thousans of tanks, dry and wet. It is not rocket science. N

 

[jonnythan]

Nemrod, I remember someone commenting on a quick informal study they did regarding a water bath.

IIRC, the conclusion was that the final tank psi was the same whether filling in a water bath or not when filling to the same psi.

Sure, the tank *feels* cooler when it comes out of the water bath, but apparently it doesn't actually change the internal *air* temperature any significant amount.

Did you ever do any direct comparisons such as this? What were your findings?

 

[Darnold9999]

...and here I always thought the water bath was for safety. If the tank ever exploded the water absorbs most of the kenetic energy. Only know of one tank that exploded, don't remember why but do remember that the damage was significant. Anyone else remember - I seem to recall it was at Frank White's shop quite a few years back.

 

[Dive-aholic]

A 12 inch cinder block wall won't do much to buffer a tank explosion. So water definitely won't do anything but get you wet.

 

[Dive-aholic]

Dive-aholic, I mean in a production environment---ie---filling many tanks as fast as possible for customers who need to get going. A circulating cool water bath helps to get a solid fill. It still takes some effort and common sense. Yes, filling as you are doing with your compressor should result in a good fill for you--absolutely. During college I worked in a shop and filled thousans of tanks, dry and wet. It is not rocket science. N

I understand what you're saying about a production environment. I've only seen this type of demand in N. Florida, though. Most have water baths. I prefer to just back my van in and fill the tanks in the back of the van. I've also tried the water baths and get the same results. Tanks are same temp. Pressure is same. One way the tanks get wet, the other they don't.

 

[captain]

I fill tanks with my 3.5 cfm compressor. It takes about 20 minutes to fill a steel 72 from 500 to 2500. If I fill it dry in the summer it will drop 200 psi when it cools. I have a length of 8" diameter plastic pipe capped on one end. It is the same length as the tank not including the valve and there is a fitting on the botton to connect a garden hose. When filling I put the tank in the pipe and crack the water on until I get a slight overflow over the top of the pipe. The tank valve and filler never gets wet. The slight overflow of water keeps the tank cool and I don't have to over fill or top off. I have been doing it this way for years and have never had a problem with water getting in a tank.

 

[ScubaKris]

I have been certified for 32 years, instructor ratings include VIP.

The scuba tank will absorb condensation if submerged in water. I have heard about a lot of people saying things about doing this, I think that they are all wrong and don't have a clue. I think it is a good idea to submerge the tank in water and I do think it helps. Hey,..It's a SCUBA TANK, it is designed to go into the water, people often dive in very polluted water ,.. not only with the tank, but, their bodies.

The fill rate should be very, very slow. idealy take about 10 minutes to fill a tank. You should never over fill/stress the tank. If they are filling it this high, why is the burst disc not blowing. This stress on the burst disc will make it go in shoujrt order.

Painting the tank, go for it. I have done many a tank. The only thing you do not want is to bake on the finish, do not expose it to heat. If you do paint it a shop should ask a question about it. Some shops have the policy not to fill tanks that have been painted.

When choosing a paint ,just make sure it will not desolve the existing finish. When I paint tanks, I scrape or sand off the flaking areas. I hang the tank from the valve with a chain, good idea to tape up the valve. then you can use primer if you want. I like to use Rustoleum spray paint from Walmart, often Yellow for vis.

I have seen many of an instructor go from non diver to instructor in less then 6 months without the min dives. To fill and VIP tanks in a 5 star shop, one that has been in business for a year. Anyone can do it with no training. I remember the 18 year old instructor girl that told me I needed to have my tank VIP'd before she would fill it, . when I asked why, it looks bad, was the response. I mumbled something about she should wish she looks this good when she is as old as the tank.

 

[MikeFerrara]

Nemrod, I remember someone commenting on a quick informal study they did regarding a water bath.

IIRC, the conclusion was that the final tank psi was the same whether filling in a water bath or not when filling to the same psi.

Sure, the tank *feels* cooler when it comes out of the water bath, but apparently it doesn't actually change the internal *air* temperature any significant amount.

Did you ever do any direct comparisons such as this? What were your findings?

The inside of the tank can, of course, still get hot when it's in a water bath. However, water conducts heat much quicker than air does so it cools faster. How hot it can get inside the tank depends on the temp of the bath and the fill rate.

Any study that claims different is trying to change the laws of physics and I'd call BS.

I've not only filled a bunch of tanks but I've done quit a bit of PP bleding and using a water bath I can get more accurate mixes, with less guessing (with practice you can get good dispite temp changes as long as you're consistant) and with less cooling time between blending stages.

In the shop we stopped using the water bath because of the mess on the floor and the pain in the neck of keeping the water tank clean. Besides lifting lots of tanks in and out of the tank is a pain especially if you're filling/lifting doubles. At home, the time associated with allowing some cooling time and retopping was never an issue so we never went back to using a water tank.

We never had any problem with getting water in the tanks and any one who can't figure out how to use a water tank without getting water in the tank should be filling tanks in the first place.