High Capacity Tanks - which one?

[roakey]

Originally posted by Seahorse
...because they appear to all hold the same amount of air which doesn't seem right.

Ok, I'm confused that you're confused.

The 80 cubic foot cylinder holds 80 cubic feet of air when full, the 100 cubic foot cylinder holds 100 cubic feet of air when full and the 120 holds 120 cubic feet of air when full.

So no, they don't all hold the same amount of air; each cylinder you listed holds more air than the previous cylinder in the list.

Roak

 

[Mike M]

When Andrea(girlfriend) and I bought our first set of tanks, she went with the HP steel 80. She did this because it was about the same size as an alu 63 and a little lighter than the HP steel 100. After a few dives we decided that we needed more tanks She bought a HP steel 100, alittle heavier, and longer but... in the water she claims she feels little deference.
You should consider what your buddies are using and buy what will be compatible with what they use. Her HP steel 100 is very compatible with my HP steel 120. Her next tank will be another HP steel 100, since she claims "the 80 just goes to fast"

Mike M

 

[The Chairman]

Hey Seahorse,

MANY people are confused by all of the ways we measure tanks. One of the most common mistakes is to confuse the working pressure (3500 psi) with volume (80cf, 100cf, 120cf). All of the tanks have the same working pressure, but they have huge volume differences.

The cubic foot referred to is the gas at @ 70 degrees and at one atmosphere of pressure-NOT pressurized.

The 80cf tank has stuffed into it a quantity of room pressure gas that would measure 2'x4'x10'=80cf. Your tank is obviously NOT that big, so as they stuff this gas into it, the pressure rises... in fact once they get ALL of the 80cf of room pressure gas stuffed into this tank, it should read 3500 psi. With me so far?

The 100cf tank has 20 cubic feet (or 25%) MORE gas stuffed into it than the 80cf tank. The measurements of the room pressure gas used is 2'x5'x10'=100cf!

The 120cf tank has 40 cubic feet (or 50%) MORE gas than the 80cf tank stuffed into it. The measurements of the room pressure gas used could be visualized as 3'x4'x10'=100cf.

A good study for you would be to get a Chemistry book that deals with the ideal gas laws. It should also be in the advanced books of diving. In that you would learn that to stuff 120cf of room pressure gas into the 80cf tank the pressure would half to be 1.5 of what the 80cf was rated for. 1.5x3500psi=5250psi. In other words to get the SAME amount of air held by the 120cf tank into that 80cf tank you would have to fill it to 5250psi... THIS IS NOT RECCOMENDED!!! That is why the 80 cf tank is smaller than the 100cf tank which is also smaller than the the 120cf tank. I hope this sheds some light on the issues involved...

 

[budgy]

Rick could you improve your air consumption? I have reduced my air consumption quite considerably over the past few years. If you are a heavy breather you may find that it is better and cheaper to concentrate more on your breathing technique. I dive with air and find that the small lower pressure tanks have more than enough air for non deco diving limits. It wasn't always the case, I have used up a 12 litre 232bar in twenty min on a 20 foot dive. I was taught to swim fast and breath quickly with fast expultions, this is good for surface swimming but uses breathing patterns like a steam locomotive and gets through an air tank quickly. Fast finning and over weighting leaves me puffing, as do long surface swims. There are some good tips on air conservation on Rodale. Over in the U.K. steel tanks are 8-9% cheaper than aluminum. I never see anyone using the aluminum tank. I find the steel 15 litre tank heavier but more buoyant than the smaller diameter 12 litre. On my average 30 min 60 ft dive I am not using 2/3 of the 12 litre and have enough for a second dive, or I switch tanks. I am not sure that 300 bar tanks are reallly worth while, for sport non deco diving. In fact I am thinking of using an eight or a ten litre tank as they are much more managable for shore diving. If you are a nitrox diver with higher non deco limits or a tech diver needing large quantities this of course will not apply.