Steel tank ?..again

[The Chairman]

Hey all,

Yesterday at the pool saw me teach BSA/PADI Snorkeling to a bunch of Boy Scouts. I took along my son's and my gear to test out my PS HP Steel 120's against an aluminum tank as well as have my son get used to his BC. Well before the class we hit the pool... I did not have any weights... still didn't need any weights after I got it down to 300 psi either... (lots of practice on free flow breathing -grin-) In fact I was still rather negative! I am fairly hefty... you might call it fat! And in all honesty, I float like a cork. For me to not need any weight in my BC was outraegeous. That has only happened with dual steels. I do plan on putting in some weight out in the salty blue, but I don't think I will need much. I like that! I gots extra pockets!!!

 

[devjr]

Warhammer, you've got it together. Your post concerning the weight and buoyancy of steel tanks vs aluminum is on the mark. These are exactly the tradeoffs you need to consider when providing your wife with the proper tank. Much of the rest of the stuff on this board is off topic but interesting and sometimes very knowledgeable.

You really need to keep an eye on her on the first day with the new equipment. However, there should me a minimum of problems as you already know how to calculate the approximate initial buoyancy.

Again, the point is, she needs a relatively small and lightweight steel tank with only slight negative buoyancy. You already know that.

As to your question concerning 3500 psi yoke valves; all MODERN yoke valves are capable of handling up to 4000 psi. In fact, USD used to stamp their yokes "4000 psi" until the industry started arbitrarily fiddling with standards due to liability concerns. The main problems, which were solved in the 70's, were with yoke strength and the valve O ring. The valves were modified by reducing the O ring annulus to insert a thinner O ring. This small change allowed the regulator to make metal to metal contact with the valve face effectively trapping the O ring, similar to the DIN setup. These WILL NOT blow out at 3500 psi, not even close. Since it would be inconvenient to have double system standards, my personal HP steel tanks are fitted with yoke valves. The standard straight pipe thread has been machined to 7/8-16 to fit the high pressure tank. The burst disks have been upgraded. No problems whatsoever with this type of setup. Also, no problems with high pressure on the regulator first stage and I really don't expect any.

 

[Jonathan Starr]

I recently switched from an Alum 80 to a Pressed Steel HP 100, and I am very happy. My local shop, Maui Dive, is able to fill it nicely, and I got to drop off a couple of lead pounds. It gives me more bottom or reserve time in a slightly smaller, lighter package. I find no down side.

I can't imagine a single HP 80 creating enough negative bouyancy to cause a problem to a small diver; however I am fairly new to this and never even thought of the considerations that must be taken into account with negatively bouyant big twins till I read this thread.

Interesting stuff.

Aloha,

JOnathan

 

[Iguana Don]

jobowkr,

Your problem with short fills is/is not the fill
station's fault. They fill your tank (hot) to 3000 psi and as it cools down to ambient temp the pressure inside decreases. If your station filled cold as in setting tanks in tank with water you would get a true 3000 psi fill. Or if they filled your tank to 3300 psi hot and when it came down to ambient temp it would then be at 3000 psi.

3" shorter Al tank. Try Luxfer's "Super 80", I have 2 of them, they are 23' tall as compared to the 26" of a normal AL tank and are 8" in diameter. Wouldn't trade them for the world. Hope this helps.

ID

 

[The Chairman]

When it's short filled...

Hey ID & all,

As for short fills (or over-fills), I fault the fill station. Whether they did not understand physics and use a "cooling bath" (which is safest) or just let the tank's temp stablize to room temp, it is STILL the fill station's competence which comes into play here.

One of the great things about NITROX is that it forces the end user to verify WHAT the fill station has done. When I find that my favorite fill station has only filled my HP 120s to 3300 and not the full 3500, then I have a problem with them. Not near as big a problem as if I ordered 32% EAN and got %44... but still I would see it as a quality control issue... the lack of attention to detail. Where else might they be short changing me on???? Just how big a filter stack have they used, and how old is it??? Is my air really as clean as they say it is???

Nope, if a fill station is going to blend EAN or simply squirt my tank full of air, I want that they should understand fully, the physics they are dealing with. I also don't want them to be lazy or make WAAs (Wild Ass Assumptions!!!). My butt is on the line, Chester!

 

[Warhammer]

She's going to try the HP80s next time she dives and we get there by car. My shop has agreed to let her try them before buying. If she likes them, and I'm sure she will, then I'll probably end up buying them for her. But the thing is, she's not much of a local diver, so it would be a big expense with not much use. She only dives if the water is above 75 degrees and with at least 60' vis. Exception being that ever now and then she'll dive with me in the local quarry. But that may change this fall. She's expressed interest in getting a drysuit, so we'll see. Damn this sport just keeps breaking me. Does it ever stop? Do you ever have enough gear?

 

[Iguana Don]

NetDoc,

I don't think any reputable fill station makes any "WAA" as you stated, the laws of physics are pretty simple and cannot be altered. Under ideal conditions a 3k fill is ia 3k fill all things being equal.
But you throw temp into it and it changes all things.

If you get a 3k fill and take your tanks out into the sun, or your trunk or the back of your truck and it becomes a 3.5k fill do you go back to the station and thank them, I doubt it. Or if you get a fill in a station that is 72 deg and take them outside where it is 40 deg and the tank pressure drops, you go in and complain. Ideally you would get tanks filled in the same water temp as you are diving.
But that is not real world.

All stations I know of have a tank pressure gauge, it would be very easy to verify your fill. Even without it being a nitrox fill. I carry one with me in my spare parts box. I don't think it's a problem of quality control or incompetence at all, just a problem with the physics involved.

ID

 

[Iguana Don]

Getting a bad Nitrox fill is another question! Then it becomes an incompetence issue.

ID

 

[The Chairman]

Hey ID, & All,

I agree more with your previous post. Not using heat sinks or common sense is the most common cause and the ultimate price with any compressed air scenario is catastrophe. Lets look closely at the physics…

A tank going from 3000psi to 3500psi: convert to absolute pressure (add 14.7psi) 3014.7psi/3514.7psi= 1.16%
Convert 80 F to Absolute temp (add 459.7)= 539.7 F
539.7 F * 1.16 = 626.0 F
626.0 F – 459.7 F = 166.352 F
THAT’S HOT!!! This will cause first degree burns easily! It might also cause distortion in the metal/O-Ring interface, which could lead to the ultimate failure of the O-Ring. “Cold Fills” can be accomplished in one of many ways… the most common being the use of a heat sink (water bath, but check the temp of that water from time to time- running is actually preferred) and filling from pre-compressed air (big tanks filled from another compressor off site usually). Compressing air “concentrates” the heat from that air. You either have to “bleed” it off, or it will get in the way. Either way a mindset of “Paying Attention to Detail” is what I want…

Use a temp to fill my air? I will find another more professional shop first. Leave my tanks in the direct sun??? I don’t think I have ever got them to 166.3 F… Prudent minds like to cover things from ol’ Sol…

 

[The Chairman]

Oh yeah,

"Throwing a temp into it" is in itself a journey into the land of WAA. They should go through a standard sign off procedure, or they shouldn't touch a tank.