Pros and cons - steel tank vs aluminum?

[RJP]

Next question: when I did the bouyancy check a month ago, 16# seemed to be right. I got in the water, let my breath out, let the air out of the BC, then sank down to eye level. It was as close as you could get it.

But I learned from that dive that it was hard staying neutral at 15' with approx. 500 lbs.

Is there an easy, quick way to guess at what a better weight should have been? It would have to be more - 1 or 2#?

Well, first things first. Do your bouyancy check correctly!

You want to float at eye level with BC empty while HOLDING a full breath, not with breath fully exhaled. When you exhale you should sink.

If you are floating at eye level with BC empty and lungs empty you are UNDER weighted.

 

[RJP]

As to tanks - just about the only AL tanks you'll see at Dutch or in Lake Atlantic are rentals/students.

Just about everyone who dives here in NJ dives steel. Why?

- Less additional weight needed; whether for a given tank size or whatever
- More gas; bigger HP tanks hold more air

I dive steel 119's. They are 11lbs negative full, 3lbs negative empty. Compare that to an AL100 which are only 4lbs negative full, and 4lbs POSITIVE when empty. That's seven pounds you can take off your belt right there. I also have enough gas for 60min dives at 70ft or so. I wave bye-bye to my buddy with a smaller LP tank at about 40min.

Mount it on a BP/W setup and that's another 6-12lbs of lead you can lose, depending on whether you use an STA or not.

With a drysuit and three layers of undies, I use a DiveRite BP/W, 6lb STA, and a steel 119. With that setup I require ZERO lead in salt water. For fresh water I simply take off the STA weight and I'm fine.

Ray

 

[jeckyll]

....

You are taking weight from your belt and putting it elsewhere, in the tank. Yes it's better to take weight from the belt and put it in the tank, but it's also better to take it away from the tank by switching to AL and putting it on your BP.

......

Umm you use less weight over all. You should really crunch some numbers on tanks given full and empty buoyancy characteristics.

 

[NJMike]

There's a lot of stuff going on here. First, I think that sometimes our messages are a little hard to understand, because of the communication medium and our own deficiencies in communicating.

Case in point: I said earlier that I checked my bouyancy without air in the BC and after exhaling. My mistake - I had the full breath of air. That was on the first set of dives and I was able to descend without much difficulty (I probably could have used 2 lbs more).

On the next dive, without the benefit of my instructor friend, I didn't use enough weight and had the problem descending.

So the miscommunication was my fault.

On the aluminum vs steel tanks issue, I downloaded all the data from the site that someone linked above, and have been reading it for 2 days. I am beginning to get '4' when I add 2+2...

Some tanks have a higher pressure (PST E7-80 at 3442 psi) than others (Luxfer 80 at 3000 psi). So, I thought that was good, with the SS tank I have 442 more lbs of air to use. But then I saw that the volume was 80 cu.ft., less than 3 cu. ft. more than the Luxfer 80 (AL). But then the light bulb goes off, I see the length and the SS tank is 6 inches shorter than the AL, hence the higher pressure...

Am I right?

So if you get the SS, it ends up weighing 3.4 lbs less, and is 6 inches shorter, PLUS you get 3 cu. ft. more air...so far sounds like a no-brainer.

Now I look at the bouyancy numbers...the AL has a +5 lb change from full to empty; the SS has a +6 lb change. Okay, so...everything else being equal, that means I would have to carry 1 additional lb with the SS? Good so far?

That means the tradeoff for the lower weight and shorter length of the SStank would be to have to carry 1 additional pound of weight. That might be a good tradeoff, especially if you are physically smaller, right?

Now here's where it gets a little foggy for me...and yes, I've read the thread (only about a thousand times hee hee). What is the effect of the actual bouyancy figures in my logic? For example, for the 2 tanks mentioned above, the AL tank has -1.4 lbs bouyancy when FULL, +4.4 lbs when EMPTY...a +5 lb change. The SS tank has a -8.5 lbs bouyancy when FULL, and -2.5 lbs when EMPTY...a +6 lb change. The SS tank is more negative bouyant when EMPTY than the AL tank is when full!

So what does that mean?

If we assume that EVERYTHING ELSE IS THE SAME...SAME DIVER, SUIT, BC, ETC.

Then at first glance it looks like the SS tank would require 7 lbs less weight than the AL tank to start the dive, and 6 lbs less to end the dive.

Is this right?

Now, to add confusion to it (at least for me), there's the "trim", "lateral position", and other issues that people brought up that I haven't been able to figure out. It sounded like...there might be differences in the way SS and AL tanks change bouyancy characteristics...forward and aft...from the beginning of the dive to the end...?

Is that right?

 

[jeckyll]

NJMike:

I'm going to use my own figures as I don't know yours. I'm 6'1 - 195 and I dive in a Bare nexgen drysuit with a fair bit of underneath (Water is typically 46 - 49 degrees).

With an AL80 I need 34 lbs when diving around here. That way I can hold a 5 ft stop with 500 psi left in my tank.

If I dove the steel tank you used in your example, I could go and take off 2.5+4.4= 6.9 lbs off my weight belt and I would have the same buoyancy characteristics and still be able to hold my 5 ft stop.

What does that mean for overall weight I'm carrying to the water?

Well the AL80 is 31.4 lbs
The PST E7-80 is 28 lbs.
Tank savings: 3.4 lbs.

Lead savings (from above) 7 lbs.

So over all I'm carrying 10 lbs less to the water.

I still have the same amount of air over all so this is a pretty fair comparison. With the steel tank, you need less weight hanging off your lower back on the belt _and_ you are carrying less weight on your back over all. But, the steel tank costs more.

You may find that with a longer steel tank (such as an LP) it's easier to trim out. This would be due to where the weight sits on your back. Longer tank = weight spread out over a larger aread of your back.

Here is a link to an interesting link on balance

I hope that answers some of your questions

Bjorn

 

[NJMike]

I did make one mistake that I caught after sending my post. When I said that with the SS tank I needed 7 lbs less at the start and 6 lbs less at the end, it's actually 7 lbs less for both.

So my reasoning sounds correct, then. I have to eat dinner now but afterwards I'll check the link on balance.

 

[RJP]

But then the light bulb goes off, I see the length and the SS tank is 6 inches shorter than the AL, hence the higher pressure...

Am I right?

Right, but backwards. The shorter tank (steel shorter than AL) does not CAUSE the higher pressure, per se.

It is the fact that steel is stronger than aluminum that you can put more gas into the same - or in this case smaller - size container.

The steel tank is smaller because it CAN BE smaller while still holding a greater amount of air at a given (or smaller) absolute volume of the tank. This is due to the differences in strength between steel and aluminum. Has effectively nothing to do with size.

Don't overthink the tank thing.

Just buy the largest HP steel tanks you can comfortably dive, and go diving!