Aluminum vs Steel tanks

[headhunter]

I have not taken the DIR-F class yet, but hope to do so in Jan. or Feb.

It is my understanding that (according to DIR methodology) when diving wet you always dive an aluminum tank and that to dive with a steel tank you must dive dry for the redundant buoyancy in case of wing failure.

I know I'm missing something when I think this thing through, but here is what I keep tripping over.

Let me use the example of diving wet.

I'm assuming that ditching weight is an undesireable thing to do and that it should be avoided. With that in mind, if I am weighted to be neutral at 10 ft with 500 psi the weight of the gas is my only difference from full to empty. This should hold true with an AL 80 or a steel 80. Given equal sized tanks, the difference in buoyancy at depth will simply be due to the compression of my wetsuit.

I understand that with an aluminum tank I put more weight on my weight belt and that with a steel tank I take some weight off of my belt. The buoyancy swing on an AL 80 goes approximately from - 3.5 to + 2.5 and a steel 80 goes from - 8.5 to - 2.5. So the difference on your weight belt is simply 5 pounds.

I also understand that this difference increases with a larger steel tank. For instance, if one opts for an E-8 130 the swing goes from - 1 to - 10.5. That's a difference on your weight belt of 7 pounds when full and 3.5 pounds when empty compared to an AL 80.

If properly weighted, the difference seems minimal even when taking the full 7 pound difference between an AL 80 and steel 130.

Where is it that I am not thinking this through properly? I'm not looking to argue the point. I'm willing to accept that this is true, but I just want to understand why. Hypothetical examples may be helpful.

Thanks.

Christian

 

[Boogie711]

People get hung up on al vs steel, wet vs. dry. It's not about that. It's about diving a balanced rig - one that you can swim up in the event of wing failure.

More often than not, the simplest way to accomplish this, especially in warmer water where you're wearing minimal exposure protection, is via a single aluminum tank. But, for example, if you're diving colder water with a 7mm wetsuit, there's nothing wrong with a steel single tank.

What you DON'T want to be in is a situation where you're diving an LP 120 in a dive skin. That's just all bad.

 

[headhunter]

People get hung up on al vs steel, wet vs. dry. It's not about that. It's about diving a balanced rig - one that you can swim up in the event of wing failure.

More often than not, the simplest way to accomplish this, especially in warmer water where you're wearing minimal exposure protection, is via a single aluminum tank. But, for example, if you're diving colder water with a 7mm wetsuit, there's nothing wrong with a steel single tank.

What you DON'T want to be in is a situation where you're diving an LP 120 in a dive skin. That's just all bad.

Thanks Boogie. That makes sense.

Someone else was also just explaining to me that many of the DIR principals are laid out with doubles in mind. In that case, my example of a 7 pound difference turns into a 14 pound difference, which is now significant. With a pair of E-8 130s I would most likely be grossly overweighted.

So, for my own clarity on this point, I'll make a statement and someone correct me if I'm wrong or let me know that I've got it now.

Since I dive in a 7mm wetsuit, it would be OK for me to dive using a single steel 130 as long as I still had a "balanced rig" that was not overweighted. For the sake of arguement, let's say that I still needed 5 pounds on my weight belt to be neutral at 10 feet with 500 psi. Assuming that my trim was in order, this would be considered a balanced rig and according to DIR principles, I would not be doing something that would be considered "unsafe".

Did I get it now?

Christian

 

[ShakaZulu]

What you DON'T want to be in is a situation where you're diving an LP 120 in a dive skin. That's just all bad.

Nice, like to see that one.

 

[JimC]

Let me add one thing. You need to be able to swim it up with a broken wing.

Can you swim up that 130, when your down at 90 feet and your wetsuit is compressed?

A balanced rig means neutral with almot no air inthe jug, none in the wing in the 5-0 foot range. Your wetsuit is going to loose 8-12# of boyancy at depth. The weight of the gas in a 130 is like 12#. Thats a heck of a lot of weight to swim up - worst case. In an AL80 your only looking at 6# of air. Still a lot - but within the realm of doable.

Thats without diching ofcourse, something youd want to avoid needing to do prior to getting into the water.

 

[jagfish]

Does the consideration of an alternate lift device enter into this mix? A lift bag or SMB for example...

 

[headhunter]

Let me add one thing. You need to be able to swim it up with a broken wing.

Can you swim up that 130, when your down at 90 feet and your wetsuit is compressed?

A balanced rig means neutral with almot no air inthe jug, none in the wing in the 5-0 foot range. Your wetsuit is going to loose 8-12# of boyancy at depth. The weight of the gas in a 130 is like 12#. Thats a heck of a lot of weight to swim up - worst case. In an AL80 your only looking at 6# of air. Still a lot - but within the realm of doable.

Thats without diching ofcourse, something youd want to avoid needing to do prior to getting into the water.

Thanks Jim.

So, with the decreased buoyancy of the wetsuit and the weight of the gas, it would in essence be like swimming up with an extra 20 pound weight with the steel vs a 16 pound weight with the aluminum (assuming that the bottles are full).

I'm getting a clearer picture with each real world example that I'm given.

Christian

 

[Boogie711]

If you're using 20 as a baseline for a full E8-130 and a crushed wetsuit, I'd argue it's closer to 13 lbs for an AL-80 - not 16 lbs.

 

[headhunter]

Does the consideration of an alternate lift device enter into this mix? A lift bag or SMB for example...

I was actually wondering about that myself, but didn't want to muddy the waters with that question.

Since you've brought it up though, I'd like to know. Can someone shed some light on this?

I don't know if a lift bag or SMB would be an acceptable method of compensating in an emergency from a DIR perspective. I imagine that if the situation can be avoided through proper procedures, then the answer would be to not get yourself into that position in the first place.

This also begs the question of whether or not your buddy should be considered a redundant system for your wing in an emergency. For this last issue, I'm assuming that you can swim your own rig up, but that a little help from your buddy could save some effort.

I'm specifically interested in the DIR answer on this, since I've made a concious decision to learn the system in its entirety. I must say that every answer I've been provided from a DIR perspective has been logical and made a lot of sense to me, which is why I've decided to pursue the formal training soon.

By the way, I'm not avoiding using common sense here. I'm just trying to get an understanding from a purely academic standpoint. In any actual situation, I would simply do whatever I thought I needed to do to stay alive. I just find that if you take the time to think these things through, that if and when the need arises, you make better decisions.

Thanks to all of you who are helping me increase my level of understanding.

Christian

 

[headhunter]

If you're using 20 as a baseline for a full E8-130 and a crushed wetsuit, I'd argue it's closer to 13 lbs for an AL-80 - not 16 lbs.

That's fair and logical. In my original scenario, I had a 7 pound difference with full tanks. I'm not entirely sure whether the number should be 16 and 23 or 13 and 20.

I'm using a 7mm Henderson Hyperstretch. I'm not sure exactly how much lift I'm actually losing at 100 ft. I wonder how I could figure this out without doing a science experiment. Maybe an email to Henderson can give me a ball park for their suits.

Christian