Scuba Tank Sizes

[lottie]

Okay, I'm showing my dive naivety here.

What is the difference between the different sized tanks out there? - I'm just talking about the recreational tanks (not nitrox). Also is there a difference between Aluminium and Steel? (apart from the buoyancy aspect)

So for example: if a diver (okay, me) was at a steady 60fsw, not hoovering up my air with the tank filled to 3000psi. Would the different tank sizes make any difference to the length of time that i was under the water?

I just got back from doing a dive and took a smaller tank (normally used by the DMs/Instructors) rather than the normal (think they are AL80s - whatever that means) - so i was only under for 31 minutes instead of my average of 45 mins

Thanks

[hbh2oguard]

The size of the tank is how many cubic feet of air. So an 80cf tank has 80 1ft cubes of air(If that makes sense) The bigger the tank the more air you have, so the longer you can last under water. Steel tanks last A LOT longer than AL, I have one that has a hydro stamp of 1963. Also my AL tanks are at a working pressure of 3000psi, while there are numerous working pressures for steel like (2200,3000,3442,3500) and I'm sure there are more. Probably one of the nicest tanks is a high pressure steel tank.

[lottie]

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 underwater with the larger tank??

[yohanson]

First off, air and nitrox scuba tanks are the same thing. All air tanks can be filled up to 40% O2 as long as it isn't a partial pressure fill. Anything over that requires that the tank be O2 cleaned. But, they are the same tanks.

The larger the tank, the longer one can dive on it assuming it is filled to the same pressure as the smaller tank. Also, the higher the pressure, the more gas it contains. For example, I have two HP 119 steel tanks and I also have a LP 95. All three tanks are the exact same size but the HPs carry about 20 cubic feet of gas. The fill pressure for the HPs is 3442 PSI and the LP is 2400 (well, it's stamped + so I can fill it 10 percent over...but, have had it filled to 3000). And as far as capacity goes, the material a tank is made out of makes no difference.

HTH

[lottie]

okay, i understand now...
thanks guys

[Papa Bear]

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!

[yohanson]

. 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!

I've never had my HP119 cool to 3000 psi. I had 3400 in them when I dove Wazee on New Years Day and they dropped to 3200 when I hit the 37 degree water. The AL80s drop about the same. But, how is a LP95 better than a HP119 when they are the same size and have the same buoyancy characteristics when they are at the same pressure? Also, if you have an LP95 filled to 2650 at a shop and you hit the same water temp, doesn't it also drop about the same amount of psi? One last thing, I've never had a problem with getting 3500 psi fills. If I travel and do a liveaboard, I'm not bringing my tanks anyway.

[Papa Bear]

On the west coast we have 1/2, 3/4. and day boats where you bring your own tank. And you get hot fills. But boats pump to 3000psi without a problem and they cool to around 2700 to 2800. still 15% over fill! If you own your own compressor you would know how hard it is on them. That is why it takes so long for that last 500psi! You must be getting a slow fill or it is in a cold climate! Live aboards usually have 80qft @ 3000 and they consider a good fill to be 2800psi!

[Sarah]

So psi's is the force behind the air, cubic feet is the amount of air available to breathe.

I would rather have 100 cubic feet available at 2500 psi's than 80 cubic feet available at 3500 psi's, all other things being equal.

[Papa Bear]

In cave diving you use 105qft or 120qft steel LP tanks filled to 3800plus psi! with 105qft tanks that gives you about 290qft of air for your dive! The steel allows you to take off hip wight and in fresh water with doubles you don't use a wight belt! So yes all things equal LP is the way to go and even if both tanks were 80qft one HP and one LP at 3000psi one is 20% over and one is 15% under! One is a 95qft tank at the start and the HP is a 72qft tank at the start. You can see that in wreck and cave, buddies have to work out tank dissimilarities because you have to have enough air to get the biggest air hog and yourself out just in case it hit the fan!