Scuba Tank Sizes

[acelockco]

I'm not an expert on this. My wife & I just switched over to Steel Tanks 2 months ago. I wish I had switched sooner. ......... I now dive with only 4 lbs instead of 12 lbs and I do not go buoyant and the end of my dive. Pamela has shed all her weights. .................
and the higher capacity in the smaller package has made our diving experience much more satisfying.

Steel cylinders are great, but aluminum do have their place as well. After using some steel cylinders on vacation, I have to agree with you 100% on how much of a difference they actually do make. The additional air you get really increases your margin of safety as well, giving you enough air to get to your no-deco limit with a nice amount to spare.

The one thing I would warn is about your wife loosing all of her weight. Remember you should have enough ditchable weight that in an emergency situation she could drop it if necessary.

The only reason I still dive aluminum cylinders is because I own 9 of them right now and don't really want/have to spend the money it would take to replace them all.

[amtrosie]

I have to add my 2 cents! The size as mentioned is not as important as capacity at a given air pressure. HP (High Pressure) tanks have a problem, that is filling them full! When you fill a tank it heats up and the air expands and then when it cools or hits the water it will drop about 10% to 20% of the volume of gas and the proportional pressure. A slow fill works better not heating as much. Now over 3000psi is hard to pump! It is hard on the equipment and uses the most energy and creates the greatest wear on the equipment. Most dive boats can pump 3000psi which then being a hot fill will be 2600 to 2800psi. So if you have a 80qft tank filled to 90% you have 90% of 80qft of air or 72qft of air. The steel tank can take the place of lead on your hip, but the aluminum tank will float when near empty. So if you have a 119qft steel tank rated at 3500psi and you get a fill that cools to 3000psa you have apx 100qft of usable air. The best is a steel 95 to 105qft Low pressure tank. Rated to hold 95qft at 2400psi. Now most boats and shops will fill it hot to 3000psi plus or minus and you have 20% over fill or 95qft will have close to 118qft of air compared to the HP 119qft that only has Apx. 100qft! So to me a LP steel is the way to go! So as you should have covered in your basic class the letters and numbers on the tank do matter. After you have been diving awhile as a woman you will blow the doors off most male divers as far as consumption!

Several things need clarification here.

1. A L.P. tank volume rating is predicated on the volume of gas with 2640 psi. in the cylinder. NOT 2400 psi.!!!!

2. The correct symbol for measured volume is cu.ft. (cubic feet). The "q" symbol is a symbol denoting a heat quotient. The "Q" symbol is utilized in the stock trades group for stock trading.

3. Nitrox denotes a mixture found with in the cylinder and stands for Enriched Air (Oxygen). It can be any mixture that is not 21% (20.9% to be exact-er) and 79% ish Nitrogen. So 32%, 36%, 40%, 23% are ALL Nitrox mixtures. It is the content and make-up of the gas found within the cylinder, it is not a different cylinder. Just greater care is taken in the cleanliness of the cylinder.

4. L.P. stands for Low Pressure, and is any "working pressure" below 3000 psi. That includes tanks rated for a working pressure of 2640 psi., that are stamped 2400 + (the plus is the 10% over fill component). This also includes the "old" steel 72's that are rated for 2150 psi. (if memory serves me).

5. H.P. stands for High Pressure and denotes a tank with a working pressure of more that 3000 psi. Steel 120's have a working pressure of 3500 psi. Steel 119's have a working pressure of 3442 psi.

NOTE: It must be noted here that it is fairly common practice to "over fill" tanks, that is pressurize past the rated tank pressure, I will not comment on that practice, nor advise on the wisdom of that practice

6. Most Aluminium tanks have a rated pressure of 3000 psi. (some 3300 psi). These are neither denoted as high or low pressure cylinders.

ALL SCUBA CYLINDERS ARE HIGH PRESSURE CYLINDERS!! It is just that some are high er pressure than others.

7. Volume of gas in a cylinder is noted as cu. ft. and is the quantity of gas that the cylinder is capable of handling. All other nations rate this in "BAR"s (derived from the Greek "baros", which is a weight measurement)

8. P.S.I. (pounds per square inch) is the "power", or better yet, the number of gas molecules that are crammed into a specific space. So the rush of gas that escapes the valve as it is turned on can be a good indicator of the psi. in a cylinder.

Now as to the comments of why one type of cylinder is better than another (steel vs. aluminum) The basic argument can be boiled down in to the buoyancy characteristics of a cylinder when it reaches it's lowest practical volume. ( this is why weight checks are done with 500 psi in the tank) An Aluminum tank becomes positively buoyant when it reaches critical pressure (500 psi. for the scuba diver). Most Al. 80 tanks will be 4.5 to 5.0 lbs. positive when empty, that is it "pulls" you up when empty, as opposed to "pushing" (weighing) you down when full. (about 8 lbs. negative) So the total "swing" of weight is 12 lbs. which must be accounted for on the weight belt. The benefit if the steel tank is the greatly diminished "weight swing" from full to empty. Most steel tanks will still be slightly negatively buoyant when empty. So the diver does not require as much weight to dive due to the better buoyancy characteristics of the steel tank. The added bonus of this tank is that, in not carrying as much weight on the belt, the diver does not work as hard on the dive carrying the lead on the belt, so has better gas consumption. Which equals longer bottom times.

This may be more information than originally sought, but hopefully it will also answer the questions after the question.

[acelockco]

amtrosie,

Thanks for posting that, hopefully that gets everyone on track!

Just a quick note, not all aluminum cylinders are positive when empty. A good example of that is the aluminum pony bottles that are always negative (even if it is very little and rarely gets empty anyway...hopefully.)

[Papa Bear]

First I know it is Cubic just like BTW and other abbreviations I use for the qft. Just shorter and easier. Next the "old 72's" were rated at 2250 not 2150! Next I dive with 105s and 120s that ARE 2400psi! So I not sure where you info comes from and I don't care weather it is right or wrong, but don't correct me about tanks, I use quite a number of them in a lot of different circumstances. If you want to state you opinion then fine! But don't speak for me please or change what I said in anyway. I live and dive in the real world all over it and see a lot of combos and know that some places in the world you are lucky to get 2800psi! And some of those places are right here in the states! I have been on many dive boats in California that advertise 3500psi fills. What they don't tell you is you will be waiting a long time for hot fill. My 2400psi 120 is filled to 2800 with ease and that my friend equals almost 20% over and then I have over 140 CUBIC Feet of air. So my point was not to be too technical, but to suggest the differences in tanks and amount of air each will hold. Some people think that a smaller 119 HP 3500psi tank is going to hold as much as my 120 LP 2400psi steel and my point is now way no how! Not in the real world all theory aside! So please don't quote me in the future when opinions are being given! C U later!

[hbh2oguard]

And let there be peace
Papa I have to say that you have convinced me to try to trade the hp100 DIN I have that I can't use, for a lp steel.

[rubber chicken]

Several things need clarification here....<snip>

7. Volume of gas in a cylinder is noted as cu. ft. and is the quantity of gas that the cylinder is capable of handling. All other nations rate this in "BAR"s (derived from the Greek "baros", which is a weight measurement)

8. P.S.I. (pounds per square inch) is the "power", or better yet, the number of gas molecules that are crammed into a specific space. So the rush of gas that escapes the valve as it is turned on can be a good indicator of the psi. in a cylinder. .....<snip>

Just a small correction on these points.

Most of the rest of the world measure cylinder volume in LITRES, 3, 5.7, 7, 10.5, 12, 15, 18, 20.... whatever.
"BAR" is the SI unit of measurement for pressure, being slightly less than 1 atmosphere. 1 bar = 14.5 psi,
Hence a 12L steel cylinder at 232bar contains 2784 litres of gas, a 10.5L cylinder at 300bar contains 3150 litres etc etc.

HTH.



PS: Come on America, get with the program, Join the Metric system!

[Tigerbeach]

Thanks for that.
let me get this straight - even if you have a small and a larger tank - both filled with 3000psi. Am i under the assumption that you will have longer (time) underwater with the larger tank??

Lottie, there are too many variables at play to really answer your question.

The simple answer is; I think you are asking if all tanks that have a full working pressure of 3000 psi hold the same amount of air, the answer is "no".

In this case the tank with the larger capacity should last the longest time underwater. However, there are many other variables at play.

Such as: Are the two tanks going on the same dive with the same diver?
If a man has a bigger tank than his female buddy, he might suck up more air and finish sooner. What depth are you diving? How hard will you be working? What is the water temperature?
All these can impact the bottom time with a tank of any size.

[Papa Bear]

Lottie, there are too many variables at play to really answer your question.

The simple answer is; I think you are asking if all tanks that have a full working pressure of 3000 psi hold the same amount of air, the answer is "no".

In this case the tank with the larger capacity should last the longest time underwater. However, there are many other variables at play.

Such as: Are the two tanks going on the same dive with the same diver?
If a man has a bigger tank than his female buddy, he might suck up more air and finish sooner. What depth are you diving? How hard will you be working? What is the water temperature?
All these can impact the bottom time with a tank of any size.

Well said! A single 72 has as much air as a closet. If you close the door and just sit there you will have a long time, but if you jump up and down and run in place, it won't last so long. The best thing I can tell you about air consumption is just dive a lot and become comfortable and you will be bringing extra air back!

[hbh2oguard]

The best thing I can tell you about air consumption is just dive a lot and become comfortable and you will be bringing extra air back!

Almost true, you'll be lasting longer, or atleast on my part because I'll still suck my tank dry

[Papa Bear]

Almost true, you'll be lasting longer, or atleast on my part because I'll still suck my tank dry

Your not a girl! Most I dive with even in warm water bring some back! Most run out of bottom time before air!