Small Steel Doubles vs. Single Tank?

[DA Aquamaster]

Point taken about being a bit off topic.

Another example that is more on topic: My X7-100's hold about 12 liters each.

I can find this by converting the volume of the full tanks to liters then dividing by the fill pressur ein bar to get the volume of the tank at 1 bar.

The rated fill pressure is 3442 psi which is 237 bar (1 bar = 14.5 psi)

1 cu ft = 28.3 liters, 100 cu ft = 2830 liters

2830/237=11.94 liters

So once you know your tanks volume, you can figure out what you have at greater bars. At 50 bar you have 12 liters times 50 bar which equals 600 liters, at 200 bar you'd have 2400 liters.

The "advantage" with this system is that it makes the math fairly easy when looking at your SPG and trying to figure out how much gas you actually have in the tank. Of course that assumes you have an SPG marked in bars since converting from psi to bar, but the complication right back into the problem.

Since most divers in the US do not have spg's that are marked in bar, there is no real advantage to knowing the capacity of the tank in liters.

All is not lost however. To do the same thing in psi, you create a "tank factor" for the tank. For the X7-100 that is 2.9. You find that by taking the 100 cu ft it holds when full divided by the pressure in psi (3442) times 100. (100/3442)*100= 2.9052 2.9 then is the number of cu ft per 100 psi. In other words, if your tank has 100 psi in it, it currently has 2.9 cu ft of gas in it.

So if you have 500 psi you have 5 times 2.9 cu ft, which is roughly 15 cu ft. 14.5 cu ft if you are good with math in your head and 15, if you are not (3x5=15). If you have 2000 psi you have 20 times 2.9 cu ft or 58 cu ft (20X3=60 if you are again not good with math in your head.)

So either way with the metric system and an SPG in bar and a tank in liters, or in the imperial system with an SPG in psi and a tank factor it is failry easy to figure the capacity remaining in your tank with fairly simple math.

Now obviously if you are math challenged, you will probably round the tank factor to "3". If you then still stuggle with things like multiplying 3X5, 3X10 or 3X15 in your head, you may have sufferred brain damage from breathing air out of those new fangled aluminum tanks and may want to consider another sport.

Personally, I think the tank factor approach is even easier than the metric system unless you are diving with a 10 liter tank. That said, an AL 80 is 10.6 liters, so it is reasonably close to being a 10 liter tank.

 

[4sak3n]

I was going to get all nitpicky and remind you that 1 bar = 14.7 psi but stopped myself before I embarrassed myself. Its 1 atm that equals 14.7 psi, not 1 bar.

I was actually pondering the differences between European and American cylinder capacity systems earlier today and realised that your tank factors do exactly what the European system does.

I was so chuffed with myself and then log on and, lo and behold, DA Aquamaster has posted already! Still ... twas to be expected.

 

[DA Aquamaster]

12 liters per bar or 2.9 cu ft per cu ft works the same - a fixed volume per unit of pressure is a fixed volume per unt of pressure.Unit analysis is unit analysis.

Miles per hour or furlongs per fortnight, it does not matter much once you under stand how the two units involved interact.

Besides its kinda fun to first agree with the "metric is vastly superior crowd" then deflate them slightly by doing the same thing with the imperial system. Metric is great, but much of the advantage is lost anytime you get away from the ability to work with a 10 and just move the decimal.

Where it gets really complicated is where people try to mix the two systems and that as been the biggest barrier to countries just adopting the metric system - they refuse to do it outright and feel the need to convert.

What I have never understood is why a bar is 14.5 psi while an ATM is 14.7 and why as a consequence 1 bar does not exactly equal 1 ATM.

 

[pescador775]

I gather that a "dumpy" is just a short, fat tank. The Brits don't like them because they don't "sit" well. This is important to divers who place a tank or tanks on a bench and back into it.

Here are a couple of the links I dug up in my history. It's not the initial website I saw the idea, that one seems to be hiding from me, but both of these were found in the process of researching the idea. I don't understand what a "dumpy" tank is, either..

 

[Luis H]

12 liters per bar or 2.9 cu ft per cu ft works the same - a fixed volume per unit of pressure is a fixed volume per unt of pressure.Unit analysis is unit analysis.



What I have never understood is why a bar is 14.5 psi while an ATM is 14.7 and why as a consequence 1 bar does not exactly equal 1 ATM.

Talking about units, you may want to correct that typo.


IIRC the bar is based on Pascal. A Pascal is one Newton per square meter.

I forget how the meter was defined, but it is a defined unit of length.

The Newton is the unit of force based on one kilogram times the acceleration of gravity, as in force equals mass times acceleration (F = m * a ).

The point is that all those units are derived from just a few basic units.

The SI (or Systema Internacinale, sp?) is just a particular metric system that is supposed to be consistent. In contrast with our system where we have pounds mass and pounds force, etc.

To make things somewhat confusing, there is also a metric system that uses kilograms mass and kilograms force. I am sure you have seen some pressure gauges that read in kg/ cm square.

The fact that the pressure of the atmosphere is close to one bar, I believe is mostly a coincidence. It is kind of the same with the density of water in the metric system. It is close to a round number, but not exactly.


I hope this helps.

 

[Luis H]

Here are pictures of my two sets of European tanks. This are 7 liter tanks and are rated for 200 bars. If you do the math you will see that they work out to be basically double 50 cu ft at 2900 psi in our system.

The yellow ones are Poseidon and the white ones are Drager. The first hydro date on the Drager is from 1957.

They are great tanks. They balance just perfectly and with a basic harness (no back pack) they are the most comfortable rig in or out of the water.

When I started using this tanks the DIN fitting was not even called a DIN fitting and no US Scuba manufacturer had regulators that would fit them.

The manifold has two male DIN fittings that screwed into the tank valves and it has one DIN outlet. I can separated the tanks and dive them as singles at any time.

 

[jamiep3]

Is that some kind of reserve lever behind the DIN fitting on the manifold?

 

[Luis H]

Is that some kind of reserve lever behind the DIN fitting on the manifold?

Yes, that is the reserve mechanism.
It came with a rod that was routed between the two tanks so you could reach it easily. I never used the rod.

 

[SparticleBrane]

Of couse anyone stupid enough to use the new AL80's not only would be grossly underfilled, but brain damaged from breathing air from an AL tank (according to my local Scuba Pro dealer, times have changed).

 

[DA Aquamaster]

My grandmother was very active in civil defense and other war related civilian activities during WWII and in the 70's she still had lots of WWII era literature left over showing that aluminum pots and pans were dangerous to your health.

I have always suspected a big motivator for that view at the time and for the wide dissemination of that view by the government had much more to do with a desire to have the public turn in aluminum pots,pans, utensils, etc for scrap due to shortages of aluminum for war production purposes.

The only thing I have ever seen with any real scientific basis is the research showing a correlation between aluminum in the brain and alzheimers disease, which when i first saw it in college gave grandma's anti-aluminum biases some potential legs and made it worth researching a bit more. However the big offender there is usually believed to be aluminum compounds found in some antiperspirants and entering the body through skin absorption. To a much lesser and more controversial extent, dietary sources of aluminum (supporting to some degree the aluminum cookware theory) in baking powder, processed foods, Al contaminated drinking water, etc. are cited, but aluminum compounds in antacids are not correlated with increased alzheimers risk suggesting that Al compounds are not effectively absorbed through the digestive track or mucous membranes which suggests the aluminum cookware theory is still bogus.

I have never seen anything regarding the potential for aluminum or aluminum oxides to be aspirated via aluminum scuba or O2 tanks, but I can see how some divers and shop owners in the 60's who were well indoctrinated in the "aluminum cookware and utensils are bad for you" school of thought might think so. I guess to be safe don't eat your AL 80 or use it as a deodorant.

Personally I like both steel cookware and steel tanks, so I don't worry about it much.