Tank selection -- help!

[SeaJay]

...Sorry 'bout that... I should have clarified a little more...

I hadn't heard of a 109 cuft tank, so I went to http://www.diverlink.com/gear/tankspecifications.htm and searched for the numbers, "109." There was only one tank listed there that had any sort of rating that included the numbers, "109," and that was in the Pressed Steel section... Their steel 120 is actually 109 cuft at it's rated pressure of 2400psi. Sure you can overfill it... I was simply using the tank's rated working pressure on all of the tanks in order to make a fair comparison.

Anyway, I assumed that you were talking about this particular tank, and did some comparisons to the Luxfer AL100 and the Catalina AL100... The two most common AL100's.

...So that's where I came up with those numbers.

The bottom line is that it's possible that you're going to have to adjust your weight and weight placement to be able to go from your AL100 to the steel 109's... And it's possible that the new tank will leave you in a position where you're overweighted, badly trimmed, or diving without enough ditchable weight. You're going to have to try these out to know for sure.

 

[perpet1]

I don't mean to "belittle" anyone, as I do realize you don't know what you weren't taught and are in fact smart enough to ask those questions you don't know the answer to, if I came across that way, I apologize. Keep it up.
However, your not the first diver I've run across that does NOT know about cylinders, what the tank markings mean, etc. It WAS covered in our class and since the cylinder is a very essential part of your SCUBA gear, something you should know and understand. I've run across 2 people who left an LDS with rental cylinders with out of date visuals and were subsequently unable to get fills. They never learned to check.

I too have to appologize but this is a little more then not knowing the tank markings.

If your instructor did not relate tank volume to you in a way that made sense then how did he or she present gas management. This goes to the heart of your question because you are interested in gas management or you would not have asked.

One of the more important things associated with planning a dive is to develop a plan for gas management. If your instructor did not clearly relate tank volume to tank pressure then IMO they did not prepare you for open water.

There are other aspects of a tank ratings that may or may not be critical for an open water student but a basic understanding is.

BTW I agree it is a good idea to look for the right answers when you find something you do not understand and for that I commend you.

 

[Hal]

I too have to appologize but this is a little more then not knowing the tank markings.

If your instructor did not relate tank volume to you in a way that made sense then how did he or she present gas management. This goes to the heart of your question because you are interested in gas management or you would not have asked.

One of the more important things associated with planning a dive is to develop a plan for gas management. If your instructor did not clearly relate tank volume to tank pressure then IMO they did not prepare you for open water.

There are other aspects of a tank ratings that may or may not be critical for an open water student but a basic understanding is.

BTW I agree it is a good idea to look for the right answers when you find something you do not understand and for that I commend you.

There are a couple of things that I find pretty ridiculous about your replies and those of a couple others as well. First, my question essentially boiled down to whether the pressure rating of a full tank affects available air, i.e., whether two full tanks of the same size (cf) with different ratings would provide the same amount of air. Stupid question? You and several others certainly appear to think so. However, despite witty answers like "109>80," it appears from the various (substantive) replies, that there is more to it than just the cf rating.

Second, instead of attempting to address the substantive question and help someone who is seeking to understand this, you and others chose to poke fun. You then add statements like:

"If your instructor did not relate tank volume to you in a way that made sense then how did he or she present gas management. This goes to the heart of your question because you are interested in gas management or you would not have asked."

My question reflects the fact that I was not clear on how to compare two tanks when the cf ratings were different. Perhaps you might clarify what you mean by “gas management.” I suppose I would have thought it means calculating approximate amount of air for planned bottom time, ensuring diving buddies agree on the reserve and on a turn-around amount, confirming the proper functioning of equipment, etc. To suggest that a failure to understand how the pressure does, or does not, affect a comparison of available air in full two tanks of different size seems to be no more a legitimate basis for questioning how gas management was taught than would, for example, your spelling mistakes (e.g., appologize) necessarily suggest the inadequacy of your education in English.

 

[perpet1]

There are a couple of things that I find pretty ridiculous about your replies and those of a couple others as well. First, my question essentially boiled down to whether the pressure rating of a full tank affects available air, i.e., whether two full tanks of the same size (cf) with different ratings would provide the same amount of air. Stupid question? You and several others certainly appear to think so. However, despite witty answers like "109>80," it appears from the various (substantive) replies, that there is more to it than just the cf rating.

Second, instead of attempting to address the substantive question and help someone who is seeking to understand this, you and others chose to poke fun. You then add statements like:

"If your instructor did not relate tank volume to you in a way that made sense then how did he or she present gas management. This goes to the heart of your question because you are interested in gas management or you would not have asked."

My question reflects the fact that I was not clear on how to compare two tanks when the cf ratings were different. Perhaps you might clarify what you mean by “gas management.” I suppose I would have thought it means calculating approximate amount of air for planned bottom time, ensuring diving buddies agree on the reserve and on a turn-around amount, confirming the proper functioning of equipment, etc. To suggest that a failure to understand how the pressure does, or does not, affect a comparison of available air in full two tanks of different size seems to be no more a legitimate basis for questioning how gas management was taught than would, for example, your spelling mistakes (e.g., appologize) necessarily suggest the inadequacy of your education in English.

It is obvious that you had (I assume you now understand) absolutely no concept of what the rating of a tank is or that it directly related to volume.

Did find a place that rents Steel 109 cf tanks. Problem is that the AL 80s are rated to 3000 psi (or perhaps a little more) and the Steel 109s are rated to 2450 psi. I'm not sure how to compare, i.e., will the amount of air I get in these steel tanks be more than what I would get in AL 80s at 3000 psi? More than what I would get with the AL 100s at 3000? I assume that there is some mathematical formula that would help me compare but also assume it would be too easy just to multiply size by psi.

Now Hal, As far as scuba equipment goes I have to say this is one of the most ridiculous comments I have heard from a certified diver!!! Now I have to ask what else don't you know that you should as an open water diver? Although I can't spell to save my life I would be ashamed if I had a student that asked this question.

Gas management has everything to do with the volume of gas in your tank. Since you did not understand the relationship between tank pressure and volume how could you have any clue on how to manage your gas during a dive? A lot of instructors just relate everything to PSI but this is not clear as you change tanks. In other words, gas management is actually looking at the volume of gas in your tank and managing it accordingly.

I also would question if you knew about the 10% over fill on a steel tank and the fact the an AL80 is not actually 80CF. So generally speaking your replay is flawed in that you did not ask your initial question looking for these answers. You were clear that you wanted to know if a 109CF@2450# had more air then an 80CF@3000# or a 100CF@3000# (by the way an AL100 is rated at 3300#). I did answer the question you asked, 80<109 I probably should have added 100<109 just in case. If you wanted to know more then you should have asked (assuming you had the knowledge to ask). Instead of comenting on my English you should be thanking the others who not only answered the question you asked but the question you did not know to ask!

Now if, in general, that is not what you were asking then you should use your superior grasp of the english language and express yourself more clearly.

 

[Groundhog246]

(by the way an AL100 is rated at 3300#).

I don't own and have never dove an AL100, so this IS news to me. I thought the limit on AL tanks, even with 6061 alloy was 3000PSI. Of course to properly use to 3300PSI, you'd have to use DIN valves, as the rated limit for a yoke (and stamped on the side of the yoke on my reg) is 3000PSI.

On the gas management issue, I have to agree that if you weren't taught some of this, then how DO you plan? If we're diving, we want max bottom time and we've got an LP ST72 @ 2250PSI (I just went and looked and it IS stamped 2250PSI) and an AL80 @ 3000PSI which has the most air? Assuming you've got the higher SAC rate, which tank should you dive? Using the rule of thirds, what is your turning pressure and what is my turning pressure? I DON'T think you're questions are "stupid", but if you can't easily solve the above, you're instructor has done you a grave dis-service.

 

[LUBOLD8431]

I don't own and have never dove an AL100, so this IS news to me. I thought the limit on AL tanks, even with 6061 alloy was 3000PSI. Of course to properly use to 3300PSI, you'd have to use DIN valves, as the rated limit for a yoke (and stamped on the side of the yoke on my reg) is 3000PSI.

The AL-100 does have a service pressure of 3300 psi, and it does have a yoke on it. SOme yoke valves are rated to a higher service pressure than 3000 psi... And some yoke regulators are rated to 3500 psi...

For proof, look at the new PST cylinders that have a DIN/K valve that is service rated to 3442 psi... (Using either the 232 bar DIN, or as a K).

 

[perpet1]

Thanks to those of you who provided meaningful comments -- useful information that has helped me understand this better.

Was also starting to worry that these boards are not intended for exchange of information to increase knowledge but a place for those, ironically including the moderator, to ridicule those who are still trying to understand how things work.
Again, thanks.
Hal

Hal,
Here is the post from Roakey, the only moderator in this thread previous to this (your) post. I have to say that this does not sound like ridicule to me. In fact, it sounded to me like he was trying to help and answer your question. Interesting how you treat those that help you.

The question was clear, we're amazed that your instructor didn't cover this.

A tank that holds 109 cf tank at 2450psi has 9cf more gas than a tank that holds 100cf at 3000psi.

You don't care about the pressure, only the capacity (at least on the West side of the pond).

So a steel HP 80 that holds 80cf at 3500 psi has exactly the same amount of air as an AL80 that holds 80cf at 3000 psi. 80cf=80cf.

Roak

 

[Krisscuba]

Which weighs more,.. a pound of feathers or a pound of lead?

 

[WarmWaterDiver]

Perpet's statement that the challenge on temperature is not necessary underscores the lack of understanding of "actual" versus "standard" cubic feet. One can determine the ratio without even having to know what the "standard" volume rating of the tank is.

From high school physics, the ideal gas law can be employed. Set one condition at 100 degrees F and the other at 50 degrees F, and then ratio. For the same gas composition, the molecular weight is the same, the ideal gas constant (by definition) is the same, the actual cylinder internal volume is (essentially) the same, leaving temperature as the only change. The temperatures have to be converted to absolute temperatures, so since we're starting with Farenheit lets use the Rankine scale, where degrees R = degrees F + 460 (with roundoff)

100+460 is 560. 50+460 is 490. Divide 560 by 490, and it is evident the tank at 50 degrees F contains 1.14 times more gas than the tank at 100 degrees F. This could be verified by weighing both tanks, as the mass of the cooler tank will be greater. The intermediate algebra steps are left as an exercise for the more curious.

No one, especially me, knows it all, but we each have something to contribute. It's the whole being more than the sum of its parts that occurs in discussion boards like these where I find value in participating.
I'm encouraged by the apologies extended to the original poster here though.

 

[onfloat]

I'm a new diver (less than 15 dives.) I received my OW about 9 months ago in a 3 day class. (This is going to give some ammo to PADI bashing). The only thing that I can remember being taught was there was a yoke and din valve and to be aware of the difference. I don't recall anyone mentioning that there were different sizes, pressure ratings or that they could be made of steel or aluminum. I sure didn't know that I had an AL80 strapped to my back until the last day of class. Heck, I just learned in the last couple of months ago that there were LP steel tanks and what that meant. I have spent the last 6 months or so just lurking on this board trying to pick up tips and what mistakes not to make. I spent most of my time trying to figure out what type of BC, fins, regs, I wanted. I didn't even become aware of VIPs and hyros until I actually started to look at buying my own tank. That being said.....I think there is something to be said about the information being provided. It's important not to take an answer personally and look at the information that is being provided and use what you can learn from it. Most of the answers seem be on the right track even if there is some sarcasm attached, you just have to read between the lines. Maybe some of my thick skin comes from my profession, I don’t know. I just know that something can be learned even from the wise a**es.