Tank Sizing Questions

[clayton]

But why use lp tanks,my 3000psi 80's weigh 44 lbs filled,my new 100cf hi press are 4 lbs lighter filled and the same physical size,as long as they are filled properly i get 20 extra cf and slight less weight,but i go offset the weight with lead so not really aconcern.

[navyhmc]

I have a LP because that are some areas that you flat out can't get a full HP fill...

In "Cave Country" a lot of cave divers will overfill the LP's to get more cu ft of gas. And when you're 1,000' back in a cave, any extra gas you have means life! Or deeper penetration, your choice.

[in_cavediver]

Originally Posted by clayton

But why use lp tanks,my 3000psi 80's weigh 44 lbs filled,my new 100cf hi press are 4 lbs lighter filled and the same physical size,as long as they are filled properly i get 20 extra cf and slight less weight,but i go offset the weight with lead so not really aconcern.

I have LP tanks because I got really good deals on some and because the HP tanks didn't exist when I got others.

Case in point - I got an LP85 for just under $200 (OK 4 of them). The HP equivalent is over $300 for the HP100. The characteristics of the 85 and 100 are darn near the same - its just less gas (when filled to rated pressure). If you get short HP fills, you are even closer.

There are all kinds of academic reasons to say to go for one or the other the reality is for most people it doesn't matter in practice.

[Splitlip]

I sold my LP's because it was becoming difficult to get the over fills. My main shop at the time changed their policy, and limited the fills to the rated pressures.

A cave diving buddy was happy to take them off my hands. LOL. Up in cave country he is still regularly getting fills to 4000 on the tanks "rated" for 2400.

Here is an interesting link. Mind you, you cannot even get these aluminum tanks filled today.

How much energy is there in a filled scuba tank

[navyhmc]

I've been looking for that link! Thanks Split!

[sanjose80]

I think the steel tanks are thin walled too so for the same outside dimensions have more capacity inside.

[in_cavediver]

Originally Posted by sanjose80

I think the steel tanks are thin walled too so for the same outside dimensions have more capacity inside.

Yes but the bottoms of steel tanks are rounded where Al tanks are flat. Any minor increase in volume from wall thickness is grossly overshadowed by the loss of volume with the rounded bottoms.

[navyhmc]

Originally Posted by sanjose80

I think the steel tanks are thin walled too so for the same outside dimensions have more capacity inside.

Wall thickness has very little impact on total gas. The pressures do.

When you calculate psi vs. physical tank volume, the 1.5" of height difference between a HP119 and HP130 is pretty close to the compressible area needed at 3442psi to make up 11 cu ft of gas. The same goes for the height of a HP100 vs HP120 vs HP80

If you compare the LP vs. the HP tanks, a HP119 is 5.2" shorter than a
LP121 (120 cu ft actual) which is about right for the cubic inch difference in volume for the lower psi.

[WD8CDH]

Originally Posted by in_cavediver

Yes but the bottoms of steel tanks are rounded where Al tanks are flat. Any minor increase in volume from wall thickness is grossly overshadowed by the loss of volume with the rounded bottoms.

AL tank bottoms are rounded on the inside so you loose about the same. The volume differance (in cubic inch water capacity) is primarily from wall thickness.

[CompuDude]

Originally Posted by WD8CDH

AL tank bottoms are rounded on the inside so you loose about the same. The volume differance (in cubic inch water capacity) is primarily from wall thickness.

Well, the corners are rounded off a bit, but I seem to recall seeing a cut away aluminum tank, and I could swear I remember the inside bottom being pretty flat.

In fact, here's a random image from the net that shows this: