Choice of Cylinders

[Don Burke]

(addressing jhelmuth) Now we get into displacement vs bouyancy. While I understand your point, it is meaningless to me without the tank weight etc. Again it boils (no pun intended) down to + or - bouyancy empty at 10ft. and weight of air, which is one of the changes during the dive and the other being suit compression or change. Ditchable weight vs fixed weight and can you exit with reasonable safety.
I also think this is a discussion maybe slightly derailed from the original topic, but I dont believe anyone is trying to or has been offended.

This is probably the most enjoyable discussion I have had on Scubaboard.

Addressing other points:

The volume of a given tank doesn't change appreciably with a change in internal pressure, so it displaces pretty much the same amount of water empty or full. The only way to change the buoyancy of the tank is to change the weight by adding or removing gas.

If we are talking about changing the tank, we've changed the displacement and we get to start over.

Making 104 standard cubic feet of air weigh 3.17 pounds, when the air weighs .075 pounds per standard cubic foot would indeed require an addendum to the laws of physics, specifically conservation of mass. All three of those numbers can't be correct. I can personally verify the .075, so one or both of the other numbers has to be off. My money is on the 3.17 being in error.

You'll find that the weight of a given amount of air will depend on the number of molecules you've managed to put in the alloted space. The temperature will affect how easy it is to get the air in that space, but not how much that number of molecules weigh. If you hot fill a tank to 3000psig, you can expect the pressure to drop as the tank cools. The weight won't change. If, after it cools, you top the tank up to 3000psig, it will indeed be heavier than it was because you've stuffed more molecules in the space. If you then heat the tank to the temperature of the hot fill, the pressure will go up, but the weight won't change from the top off weight.

Humidity will indeed affect the weight of air. Standard conditions will fix that, but what dive compressor operates in a standard temperature and pressure? In any case, the air we are talking about is pretty dry stuff and I'm not so sure we could even detect the difference between one shop's air and another shop's air by weight.

What started all of this was the boilerplate statement that steel tanks are never safe for wetsuits and aluminum tanks are the only safe backgas tanks for wetsuits. While we've beaten the area around this bush pretty severely, I think we are all in agreement that the statement is only true part (perhaps most in the case of doubles) of the time.

I've looked over the posts I've made in this thread and haven't seen what my error is supposed to be. I suspect something I posted has been misread.

As I said above, I'm enjoying the heck out of this thread and certainly have no objection to continuing. I consider it time well spent.

Don Burke

 

[jhelmuth]

Don,

The weight of the molecules is not in dispute. I totally agree that a given amount of air weighs what it weighs - no more no less. Buyancy is the result of that gas being compressed into a cylinder and considering the entire bottle with gas to the weight of the volume of water it displaces (all other factor constant). I thought this was what you were refering to when commenting that the bouyancy charicteristics were wrong (did not meet the laws of physics). I think I now better understand your comment. My appologies for that mistake.
As for tank tables, I don't have any data or experience on their accuracy. If widely known as unfaithful, I am unaware of such claims. Clearly it was posted here that someone held a different table with different #'s. I don't know which came from where - but it is safe to say that the difference in a tank empty vs full should be the weight of the gass consumed.
I tried to do some research on the "standard weight (mass) of air (which I would have assumed is avail and would be quoted at sea-level at a given temperature and very likely at 29.92 Hg). Unfortunately, this was not to be found. So I then turrned to the different tank charts I could find. I noticed that if I tried to derive it from that, the values were indeed diferent (Luxfer seemed to agree with the 0.074 you cited). I would like to know where you got your information. I do not have a background in compressed gas cylinder design and I am unaware of how they get their numbers. I also know that the "air" (gass) filled into a scuba cylinder is virtualy free of moisture. To some degree, this would make that air lighter, but I don't think it would be considerable.

Don - can you please cite your source for the weight of air? I'm going to look more into the manufacturers claims and try to understand how this is arived at. I'm aware of the variations in defined "empty", etc. But I now want to know how a 104 cubic foot cylinder can have a delta of something other than between 6.1 - 7.6 lbs.
I can understand how lp steel at 2400 psi working would have a difference from a PST E series rated at 3442 (the % of gas remaining vs full is quite different with a baseline of 500 psi defined as empty).

I'll be back as I find out more. Maybe someone else can chime in who has more knowledge on the tank ratings.

 

[Don Burke]

jhelmuth,

The figure I have for the weight of air comes from "Pocket Ref", compiled by Thomas J. Glover and published by Sequoia Publishing, Inc., Littleton Colorado.

http://www.sequoiapublishing.com/

I bought my current copy, the third edition, at Harbor Freight.

The figure of .075 pounds per cubic feet matches what I see when diving. .074 is certainly close enough.

Every time I have tried to use the tank tables, I've come across some anomaly like the empty/full numbers for the PST 104. Back when the techdiver email list was up and running, we had a discussion about the tables and it seemed there was more wrong than right. At the time, I got the impression that I was the only one who didn't know it. Welcome to the inner circle.

As for how the tank ratings are derived, my guess is the marketing people have too much input. There is no standard for whether the valve is part of total weight, whether salt or fresh water is used for buoyancy computations, or even the relationship between the number assigned to a tank and the actual capacity. Note I used the term "buoyancy computations"; it appears some don't even dunk the tanks to get numbers to publish.

For instance, an aluminum 80 holds a bit over 77 cubic feet. If you're in a situation where you can only get air at about 2200psig (some fire departments have SCBA compressors like that), a steel 72 is actually larger than an aluminum 80.

It is all pretty weird. In other parts of the world the capacity of the tank is expressed as the amount of water it would hold, which sounds like a better system, but I wouldn't bet against that being corrupted.

Perhaps we could start a thread where people would post what their tanks weigh along with the buoyancy.

 

[jhelmuth]

I'm still working on this. Thank you for the ref. I am making an advance "guess" that the number for SCUBA air is 0.075lb/ft3 minus the moisture (whatever that may be factored into the 0.075). I don't think the valve weight will change the properties for the swing weight (sorry if that is not the correct term) - at least I don't see how that it would. I can understand how the different manufacturers can have variation due to measuments made in salt vs fresh water, their definition of "empty" (0-500 psi left in the tank), etc. While we think of the gas laws as ideal, they are not precisely so (by my recollection). It doesn't seem plausable that manufacturers would try to cheat on their #'s if this is as straight-forward as it seems - it would be too easy to invalidate their #'s. I hope there is more to it than that.

 

[jhelmuth]

OK, I got this data from www.engineeringtoolbox.com

Density and Specific Weight of Air at Standard Atmospheric Pressure in Imperial (BG) Units

Temperature - t
(oF) Density - ρ
10-3 (slugs/ft3) Specific Weight - γ
10-2 (lb/ft3)

40 2,469 7,942
50 2,420 7,786
60 2,373 7,636
70 2,329 7,492
80 2,286 7,353
90 2,244 7,219
100 2,204 7,090
120 2,128 6,846

So it is easy to calculate that air should weigh in at 0.074 @ 76 degrees F. I beleive that his is a dry (IE - no moisture in the air) number, but I do not know that for sure.

 

[Don Burke]

BOC has a pretty good publication about gasses in general, including air.

http://www.boc.com/capability/Gases.pdf

 

[jhelmuth]

Well, I may have discovered how/why some of the tank buoyancy charts seem to be off. It seems that these measurements can be the result of a hot fill with a bleed down to 500 psi at much cooler temps. A hot fill would generate a total molecular weight which is much lower than a standard fill, thereby actually starting out with less gas at the "full" test.
If this is the case, it seems stupid on the part of the manufacturers that participate in such practices. Thery're not fooling anyone, so why do it? Hopefully I'll eventually find out something different, but it seems that this may be the answer.

 

[MartiniTime]

LOL, I spent the weekend diving, so I missed all of this. Great discussion.

 

[Don Burke]

Well, I may have discovered how/why some of the tank buoyancy charts seem to be off. It seems that these measurements can be the result of a hot fill with a bleed down to 500 psi at much cooler temps. A hot fill would generate a total molecular weight which is much lower than a standard fill, thereby actually starting out with less gas at the "full" test.
If this is the case, it seems stupid on the part of the manufacturers that participate in such practices. Thery're not fooling anyone, so why do it? Hopefully I'll eventually find out something different, but it seems that this may be the answer.

That would account for some of the slightly off numbers.

However, many aren't even within two ZIP codes of being correct. While some dive shop salespeople will claim some pretty strange things about buoyancy swing, the manufacturers should know better. My guess is that those specifications aren't considered very important by the maufacturers and are assigned to very new people who believe whatever the first computation or measurement tells them.

 

[jhelmuth]

I'm going to see if I can contact someone at PST or Luxfer. I can't imagine that those spec's are considered so unimportant that they are disregarded for all practicle purposes.