Choice of Cylinders

[Henryville]

I want to quit renting tanks, and am researching the options. Having concluded I should go with LP steel, I came across this "DIR" advice on the website of an "authorized Halcyon dealer" (I added emphasis):

"1. Aluminium In general terms aluminium cylinders change from being negatively buoyant to positively buoyant as the volume of gas they contain reduces. This characteristic, when combined with correct weighting can be capitalized upon to provide redundant buoyancy. This makes the use of aluminium cylinders when wet suit diving an absolute necessity.

2. Steel The use of steel cylinders when wearing a wet suit should be avoided at all costs. Steel cylinders represent total negative buoyancy. Whilst they too reduce in overall weight as the gas volume reduces they never achieve positive buoyancy and remain negative through out the dive no matter what the circumstances.

3. So what? Imagine diving in a wet suit and experiencing a BC failure, the only option would be to fin yourself back to the surface. If you're using a steel cylinder no matter what equipment you ditch you'll still have to swim up carrying on average a 5kg lump of steel, twice as much if you're using twins.

However if you use an aluminium cylinder and have balanced your equipment correctly, by removing your ditchable weight you will be able to take advantage of the the cylinders integral buoyancy during your swim back to safety. Now, in an emergency what would you prefer a 5kg millstone or a gentle helping hand?"

Do you agree with these statements? I assumed that unless you were diving with no weight, you'd just carry less to begin with for the negatively bouyant steel so this makes no sense to me. Further, a Faber LP 95 and a Luxfer Al 80 seem to very comparable on weight an bouyancy so I can't see a difference under the secribed BC failure scenario. From looking at specs it appears to me that for a comparable volume all tanks change in bouyancy by the same degree so I figured the delta wasn't a relevant factor arguing for or against steel vs al.

This site also indicates that the relationship between pressure and volume changes at higher pressures which, unless they mean to say that tanks heat up as a fill goes on and therefore the temperature change comes into play, is incorrect.

Also, what do you think of PST eliminating their LP line? Does anyone have comments on experience with their E series, the OMS Fabers and on the White Fabers?

Thanks

 

[Diver0001]

<snip>

Do you agree with these statements? I assumed that unless you were diving with no weight, you'd just carry less to begin with for the negatively bouyant steel so this makes no sense to me. Further, a Faber LP 95 and a Luxfer Al 80 seem to very comparable on weight an bouyancy so I can't see a difference under the secribed BC failure scenario. From looking at specs it appears to me that for a comparable volume all tanks change in bouyancy by the same degree so I figured the delta wasn't a relevant factor arguing for or against steel vs al.

This site also indicates that the relationship between pressure and volume changes at higher pressures which, unless they mean to say that tanks heat up as a fill goes on and therefore the temperature change comes into play, is incorrect.

Also, what do you think of PST eliminating their LP line? Does anyone have comments on experience with their E series, the OMS Fabers and on the White Fabers?

Thanks

I agree with the general principle of thinking about which cylinders are best to use with the exposure suit you have.

However there are a couple of things that I believe are inaccurate or "assumed" in the piece you posted:

1) I've never seen any cylinder (steel or otherwise) that was 5kg negative unless full. So what they're doing is comparing the buoyancy of full steel cylinder to an empty aluminium cylinder.....I would guess to make their argument sound stronger. I would reject this comparison as apples and oranges. An empty large steel cylinder is about 1.5-2kg negative max. Smaller steel cylinders (comparable to an AL80) are can be neutral when empty but typically offer 1/2-1kg negative buoyancy when empty. And apples and apples discussion would compare empty cylinders of the same size.

2) breathing your tanks down to create extra buoyancy is possible but so it ditching weight. I think you need to look at the gear you have and see how much buoyancy (pos or neg) you have when you have ditched your ditchable weight at the deepst depth you will be diving. That's your baseline.
Ditching weight is also the first thing you would do on the surface to create maximum positive buoyancy in the case of a bcd failure. Funny that they don't mention that.

3) I believe that one of the assumptions they're making is that the wetsuit is thin and doesn't have much intrinsic buoyancy. The thicker the wetsuit the less accurate their argument becomes. The thinner the wetsuit, the more concerned you need to be about the bouyancy of your tanks.

One thing that they didn't mention is that in a real emergency you could ditch the steel tank/bcd for the swim back. You could, for example, attach a reel and dsmb to your kit to mark it's location and ditch it and come back for it later.....

They also didn't mention alternative types of redundant buoyancy that could be applied to wetsuit diving. For example, the dsmb can be used to create and upline and/or extra positive buoyancy on the surface. In fact, there are much more effective and sensible ways of creating positive buoyancy than to breathe your tanks empty.....

to name a few. I'm sure others will add to this.

R..

 

[SDAnderson]

In the event that your BC/wing fails, can you swim your rig to the surface? That's the only question that matters. If you're diving wet, don't forget to compensate for suit compression at depth - a 7mm farmer john adds a lot of bouyancy at the surface and very little at 100 feet. You don't want to end up in deep water with a nearly empty tank, wondering how you're going to get to the surface.

 

[jhelmuth]

I want to quit renting tanks, and am researching the options....

Further, a Faber LP 95 and a Luxfer Al 80 seem to very comparable on weight an bouyancy so I can't see a difference under the secribed BC failure scenario.

From looking at specs it appears to me that for a comparable volume all tanks change in bouyancy by the same degree so I figured the delta wasn't a relevant factor arguing for or against steel vs al.

Also, what do you think of PST eliminating their LP line? Does anyone have comments on experience with their E series, the OMS Fabers and on the White Fabers?

Thanks

These tanks do seem to be comparable. Volume can imply the comparable gas available, or it can be a literal interpretation of the amount of water displaced, or it can also be a literal interpretation of the internal volume of the cylinder (each describe a different characteristic in diving). But if you think about it, they are not as much the same as you might expect.
Consider that the Faber can hold 15 cu/ft more gas at 80% of the pressure. Using the gas laws, you'll figure out that the internal volume of the Faber is considerably larger (if the Faber were filled to 3000 psi, the tank would hold ~ 118 cu/ft of gas - an increase of 48% more gas than the Luxfer). From an external volume (displacement) perspective, they are less than 5% different (~50 cu/in) - so they are very close to identical (seawater weights 64lbs per cubic ft, so 50 cu/in of seawater weights ~ 1.85 lbs).

Cylinder..........psi...ft3..dia...len....wt...Empty/full
Luxfer 80 Alum 3000 80 8.0 22.93 35.2 +2.3/-3.6
Faber 95 Steel 2400 95 8.0 23.80 37.3 +2.4/-4.4

Still, the real question remains on the buoyancy characteristics and how that may affect the ditchable weight a diver would carry. Consider a diver who achieves perfect buoyancy with any given configuration and can ditch exactly 4lbs (regardless of their exposure protection choice) of lead. Not impossible at all if the overall weight can be divided into ditchable and non-ditchable portions (such as ditchable weight integration). Also consider that the diver will find him/her in the same circumstances at the moment they need to get to the surface (keeping all variables constant for comparisons sake - time especially), then the greatest delta in buoyancy the diver will experience is ~ 1.2 / .842 = 1.43 lbs (largest change in buoyancy divided by the ratio of the gas supply available).So with all variables held to the same circumstances, there could never be a difference of more than 1.42 lbs of lift (-1.42 lbs. buoyant) between the two. Therefore the diver will probably want to add/shift 1-2 lbs of weight to be ditchable in their configuration when diving the steel tank (or remove/shift in the case of the aluminum).

Conclusion: For these 2 tanks, the difference is negligible. A practical shift in ditchable weight solves the problem. But that does not always hold true for other comparable tank selections. In general, all divers should carefully examine their gear configurations. The above example does not take into account that a diver could find themselves over-weighted at depth. Tanks are not a "one size fits all". A diver should use their training and experience to more carefully consider all variables when choosing tanks. Having said that, it's neither rocket science nor extreme in the sense that every dive requires a different tank. People just need to use their head for something more than a place to put their mask...

 

[DA Aquamaster]

Some steel tanks can be quite negative compared to an AL 80 but to say that all steel tanks are unsuited for wet suit diving is to badly over generalize the issue.

As indicated above a Faber 95 is comparable to a AL 80 in terms of bouayncy. The generic run of the mill steel 72 is also suitable very well suited wet suit diving. And I should note that divers used steel 72's with wet suits for at least a decade before BC's were even used at all.

In the end what matters is that you can swim your tank and exposure suit up from in the event of a BC failure and that involves many more factors than just the metallurgy of your tank.

 

[jhelmuth]

...by the way, here are a couple of nice links that relate to this thread.

http://library.thinkquest.org/28170/i.html

http://scubatanks.us/scuba_tanks.htm

Oops! - that last link is a commercial website. I did not put it there for any reason other than as a reference to different cylinders (I am not affiliate with nor do I have any interest in this company or its website - sorry if that was not appropriate)

Enjoy!

 

[mgdive]

Also, what do you think of PST eliminating their LP line? Does anyone have comments on experience with their E series, the OMS Fabers and on the White Fabers?

Thanks

With regards to PST "eliminating" their LP line, I'd be more inclined to call it "consolidating".

The E8-119 *is* an LP-95 that you can fill with 119 cf @ 3442 psi. (95 cf @ 2640).
E8-130 -> LP 104.
Same dimensions as their LP counterparts.

 

[Don Burke]

"1. Aluminium In general terms aluminium cylinders change from being negatively buoyant to positively buoyant as the volume of gas they contain reduces. This characteristic, when combined with correct weighting can be capitalized upon to provide redundant buoyancy. This makes the use of aluminium cylinders when wet suit diving an absolute necessity.

"Absolute necessity" is overstating the case.

2. Steel The use of steel cylinders when wearing a wet suit should be avoided at all costs. Steel cylinders represent total negative buoyancy. Whilst they too reduce in overall weight as the gas volume reduces they never achieve positive buoyancy and remain negative through out the dive no matter what the circumstances.

"At all costs" is also overstating the case.

3. So what? Imagine diving in a wet suit and experiencing a BC failure, the only option would be to fin yourself back to the surface. If you're using a steel cylinder no matter what equipment you ditch you'll still have to swim up carrying on average a 5kg lump of steel, twice as much if you're using twins.

You're not swimming up 5kg of metal, you're swimming up whatever the buoyancy of the tank is.

However if you use an aluminium cylinder and have balanced your equipment correctly, by removing your ditchable weight you will be able to take advantage of the the cylinders integral buoyancy during your swim back to safety. Now, in an emergency what would you prefer a 5kg millstone or a gentle helping hand?"

Now all they need to show us is the aluminum to steel tank comparison that shows 5kg of buoyancy difference.

Do you agree with these statements?

No

I assumed that unless you were diving with no weight, you'd just carry less to begin with for the negatively bouyant steel so this makes no sense to me. Further, a Faber LP 95 and a Luxfer Al 80 seem to very comparable on weight an bouyancy so I can't see a difference under the secribed BC failure scenario. From looking at specs it appears to me that for a comparable volume all tanks change in bouyancy by the same degree so I figured the delta wasn't a relevant factor arguing for or against steel vs al.

The buoyancy change of a tank is due to the change in weight of the gas in it as gas is used. The material the tank is made of isn't an issue.

This site also indicates that the relationship between pressure and volume changes at higher pressures which, unless they mean to say that tanks heat up as a fill goes on and therefore the temperature change comes into play, is incorrect.

Around 3100-3300psig, the pressure increase is no longer linear with the amount of gas added to a tank. Users of high pressure tanks need to be aware of this.

Also, what do you think of PST eliminating their LP line?

They appear to have recertified and reclassified the tanks that were LP tanks.

Back to the steel vs aluminum debate, I prefer to have a viable option for getting to the surface if my wing fails.

The scenarios are:
1. I jump in the water and shred my inflator hose by catching it on something on the boat. With the drysuit, I've already got enough gas in the drysuit to keep me on the surface, so I get back on the boat. With the wetsuit, I drop some weight so I can easily stay on the surface untill I get back on the boat.

I get that fixed and:
2. I jump back in the water, descend to the bottom and shred my inflator hose on some wreckage. With the drysuit, I just use the suit for buoyancy control and abort the dive. With the wetsuit, I will probably be able to swim up the rig and ditch weight on the surface, although I might have to ditch weight to ascend.

In order to have enough ditchable weight in a wetsuit, I'll normally be using aluminum tanks. Some have shortened this to "aluminum wet, steel dry" which is an oversimplification. This has been adopted as gospel by some people despite statements to the contrary by GI3.

Dive what's safe.

 

[Diver0001]

Around 3100-3300psig, the pressure increase is no longer linear with the amount of gas added to a tank.

<snip>

Don, you gotta reference for that? I want to read up on it a bit.

R..

 

[Don Burke]

Around 3100-3300psig, the pressure increase is no longer linear with the amount of gas added to a tank.

Don, you gotta reference for that? I want to read up on it a bit.

I'll have to dig it out. I'll get back to you.