Tank selection -- help!

[LUBOLD8431]

Thats just sad... Thank god for Scubaboard (and other scuba message boards)... Keep asking your questions, no worries. The people that dont want to add anything constructive should not type anything...

 

[perpet1]

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.

OK you proved you can use the ideal gas law but it has nothing to do with this thread really. I will agree that it is a concern when trying to figure out why you did not get a full tank when you got a hot fill or why your tank pressure decreased when you jumped into the cold water. I would also go as far as saying that it is a concern for planning your gas management for a dive if you are diving in very cold water. Although this is all a valid discussion the general idea in this thread is that a tank at its rated pressure will have its rated volume regardless of telperature.

I agree with you that if you then change the temperature then the volume will change but so will the tank pressure (in fact the change will be proportional). A good thumb rule to know (although not exact) is that tank pressure will change 5psi per every degree F. In other words is you have your tanks at 80F and you jump into 50F water you can expect your tank pressure to ruoghly decrease by 150psi (yes, actual volume will decrease proportional to that). This is a rough thumbrule and it is not 100% consistant as you use different tanks but it is easier then whipping out the ideal gas law at the dive site.

 

[WarmWaterDiver]

Perpet,

Well, if you look at the question which began this thread, and STILL think my contribution is not "what this thread is about", then I give up on trying to help you understand.

By the way though it's because of science like this that definitions of "standard' gas volumes (and for things like crude oil, "standard" liquid volumes like the US Barrel at 60 degrees F) came into being. Otherwise, for example, if you used natural gas for home heating and cooking, and I had a place I purchsed the gas supply from, I could merely heat the gas, reduce the pressure, etc. and make a tidy profit without delivering the "real goods", which in the end is the mass of gas or number of gas molecules purchased.

Does your state have natural gas deregulation??? Have you shopped around?? Need a consultant to help you???

The tank at 50 degrees F, ALL else being equal, actually has MORE air molecules and will last that much LONGER before reaching the designated pressure to begin ascent - but you didn't take this into account evidently.

Please note the original poster did ask about "the math" so my comments were directed for that person's benefit all along - NOT Perpet1's personal opinion of what the original poster's question was about.

 

[perpet1]

Perpet,

Well, if you look at the question which began this thread, and STILL think my contribution is not "what this thread is about", then I give up on trying to help you understand.

By the way though it's because of science like this that definitions of "standard' gas volumes (and for things like crude oil, "standard" liquid volumes like the US Barrel at 60 degrees F) came into being. Otherwise, for example, if you used natural gas for home heating and cooking, and I had a place I purchsed the gas supply from, I could merely heat the gas, reduce the pressure, etc. and make a tidy profit without delivering the "real goods", which in the end is the mass of gas or number of gas molecules purchased.

Does your state have natural gas deregulation??? Have you shopped around?? Need a consultant to help you???

The tank at 50 degrees F, ALL else being equal, actually has MORE air molecules and will last that much LONGER before reaching the designated pressure to begin ascent - but you didn't take this into account evidently.

Please note the original poster did ask about "the math" so my comments were directed for that person's benefit all along - NOT Perpet1's personal opinion of what the original poster's question was about.

So your absolutpoly correct that temperature has an affect on the density of the air in a scuba tank BUT you are absolulty WRONG that is will increase the number of air molecules in a tank. The density will change as temperature changes but the absolute number of air molecules will be constant (something to do with conservation of matter yet another high school physics concept).

Now the requlations in other industries as to how to measure a things like natural gas are for consistancy and consumer protection. Think about it, if there were no regulation defining how to measure the amount of gas in a tank the vendor your just raise the temperature of the tank fill and fill it to the rated pressure at a higher temperature. Then once the consumer actually recieved the product it will have cooled (the density will have changed) and they would actually recieve less gas (yes the same exact number of gas molecules just at a very different density). This is not because there are fewer gas molecules as you imply but because the density has changed.

Now back to the scuba problem. If I have a tank at 3000psi that has exactly 80cf of air at 80F. Now I jump into 50f water. If I read your post correctly you are implying that I will have more air to breath. Is this really what you were trying to say?

 

[jonnythan]

So your absolutpoly correct that temperature has an affect on the density of the air in a scuba tank BUT you are absolulty WRONG that is will increase the number of air molecules in a tank. The density will change as temperature changes but the absolute number of air molecules will be constant (something to do with conservation of matter yet another high school physics concept).

What he said was absolutely correct. You can't change the number of molecules of air in the tank by changing its temperature, obviously, but having two tanks of air at the same pressure but different temperatures, one will have *more* air than the other.

Now back to the scuba problem. If I have a tank at 3000psi that has exactly 80cf of air at 80F. Now I jump into 50f water. If I read your post correctly you are implying that I will have more air to breath. Is this really what you were trying to say?

No, he's not saying that. As the air in the tank cools, the pressure will drop. Bringing a tank with 3000 psi of air down 30 degrees will make the pressure drop. The actual mass of air remains constant. What he's saying is that between two tanks at 3000 psi, one at 80 and the other at 50 degrees, the one at 50 degrees holds *more* air.

 

[jonnythan]

So your absolutpoly correct that temperature has an affect on the density of the air in a scuba tank [/I]

Also, changing the temperature of a tank has no effect at all on its density.

 

[WarmWaterDiver]

Thank you jonnythan- this should maybe be moved to "The Physics of Diving" for Perpet1 if it goes much further, but here's the final illustration that in the USA, a STANDARD cubic foot of gas is the number of gas molecules that occupy one cubic foot of volume AT 60 degrees F AND 1 ATA pressure.

The PADI OW manual contains references to the effect that a standard scuba tank (let's use a 3000 PSIG aluminum 80ft3 tank as referenced) contains "about the same amount of air as a small walk-in closet". Using basic geometry, 80 cubic feet would be contained, for example, in a space 10 feet by 8 feet by one foot (already, this is larger than what commercially available scuba BC straps would accomodate in terms of size of a container, but let's ensure this is absolutely clarified).

Using the ideal gas law Perpet1, calculate the ACTUAL volume occupied by a volume of air (80 cubic feet at 1 ATA and 60 degrees F) when it is compressed to 80 PSIG and equilibrated to 60 degrees F.

Here's the extra credit assignment. A 3000 PSIG 80 cubic foot aluminum tank is referenced in terms of its liquid water volume capacity in Europe, so in Europe, the exact same tank is known as a "10 liter" (or carrying an extra decimal place, 10.2 liter).

Demonstrate that 80 cubic feet = 10 liters, given these basic conversion factors (look them up yourself if you doubt their accuracy).

One cubic foot equals 7.4805 gallons.

One gallon equals 128 fluid ounces.

One liter equals 33.8 fluid ounces.

You must show all your work to obtain credit.

BEGIN!


Changing the number of molecules present by changing temperature is akin to attempting to be an alchemist trying to transmute lead into gold.

Jonnythan, I'm unsure how to read your last post here The pressure exerted by a compressible fluid is a function of the number of molecules and their activity level (temperature). Using the ideal gas law, the density of a gas (Mass divided by volume is the basic definition of density) is defined as [Pressure(absolute)* molecular weight] / [ideal gas constant * temperature(absolute)]. Unless you are referring to the density of the material the tank itself is made from (I'm not sure how to read your last post). Or are you illustrating that after equilibrating, you then cool the tank further, that pressure must also drop?

Think of the directional change in barometric pressure associated with weather phenomena like cold fronts and warm fronts if you like.

So, all the scuba tank "cubic foot" capacity ratings are well demonstarted to be in terms of US units definiton of STANDARD volume at 60 degrees F and 1 ATA, which is why different tanks rated for the same "cubic foot" capacity can be different size (physically, like a LP 100 cubic foot tank being 8" diameter and 24" long with 2640 PSIG pressure, vs. aluminum 100 cubic foot being 8" diameter and 27.3" long vs. a HP steel 100 being 7.25" diameter and 23.9" long) at different tank pressure ratings.

Which is in reference to the original poster's question . . .

So how many cubic feet are REALLY in your 80 at 80 degrees F Perpet1? You allege 80, but that's not the answer unless its pressure is 3116 PSIG when it's at 80 degrees F, and there is the slight differences in actual internal volume (Luxfer vs. Catalina etc.)

Perpet1, please see what I originally wrote about EVERYTHING except the two tank temperatures being equal - meaning both tanks have already equilibrated . . . The intermediate algebra steps were left for the more curious (which you seem to fit into the category of based on your subsequent postings) . . . and if performed, should demonstrate better clarity.


Adios!

 

[roakey]

I told myself that I had my say, and that was enough, but against my better judgement, here goes...

First off, before I start stepping on toes, let me point out something. As contributors to this file (or even if you're just a read-only person out there), you're some small fraction of 1% of the diving community that actually cares about the sport and wants to improve; you're not content to fly to Cozumel every other year and take a stroll UPON the reef like the vast majority of divers, and that's to be commended. Everyone can bring something to this file and take away so much more; I both have a lot to offer and a lot to learn.

One more point: Being a moderator doesn't mean squat in a discussion, both in terms of knowledge OR power. We're all peers in here, both in my philosophy as well as formal guidelines for the moderators. As a contributor I can't use any of my moderating powers on this thread. If the thread becomes nasty, I report it like any other board member and some other moderator that's not involved with the thread "does the right thing" up to and possibly including deletion of my notes. So forget that my name is bold -- there's both a formal and informal division of moderator and poster -- I'm just another poster in this thread.

Ok, here goes:

Hal asked a very basic question that deserved a very basic answer. Most of us come out of OW, or at least learn soon after that if we leave our cylinders out in the sun, the pressure goes up, then drops back down when we get in the water. We're also unhappy if we pick up a very warm cylinder at the shop that's filled to the "correct" pressure because we know that it'll be a lot less than that when we get in the water (assuming the water is colder than the air). The effects of temperature on pressure are not to be ignored, but neither are they pertinent Hal's question.

Hal's question was basically, does a "full" Steel 109 have as much air (or more) as a "full" AL100, despite their different service pressures? Simple answer, "yes" -- that's all he needed...

So, WarmWaterDiver, though your points are correct as well as interesting and make a good aside to the discussion at hand, I would not call them pertinent, if for only one small fact: Whatever your cylinder contains when you get in the water, is after all, all your cylinder will contain for the dive. If you got a hot fill or if you're jumping into cold water on a hot day or warm water on a cold day, whatever your SPG reads after a couple minutes in the water is what you get, no more, no less.

And, after all, your gas planning is based on your pressure towards the end of the dive (when the gas has reached ambient with the surrounding water), so whatever pressure your cylinder may contain at any other time in the history of THIS particular fill is moot.

You also need to be a bit more careful; because of your expertise in this area you can make assumptions without realizing it, and, as my wife who's a teacher is fond of saying "It matters not what you say, it only matters is what the student heard..."

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.

This is *great* news! So I take my full cylinder from lying in the sun at 100 degrees, climb into a 50 degree Colorado lake and poof! just like that I get 14% more air! WooHoo! That's pretty cool. Of course, this isn't what you're trying to say, but that's what you DID say since you left out the critical "at the same pressure" part of the explanation.

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.

Well, actually the cf rating IS all there is for your application. Your AL100 gets filled to various pressures (some "good" fills and some "bad" fills), and ends up at various pressures as you're diving and the same thing will happen with a 109, so all other things being equal, 109>100. If someone asks me which will go further, a car with one gallon of fuel or a car with one liter of fuel, I automatically assume that the car is a constant. I don't go off in the weeds and start discussing a hybrid electric/gas car versus a NASCAR race car.

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

Which weighs more, a pound of feathers or a pound of gold? (Anyone? A trick question within a trick question!)

I didn't even become aware of VIPs and hyros until I actually started to look at buying my own tank.

Has that information been removed from the PADI Open Water Dive Manual? I've got a couple laying around, none recent, but they cover visuals and hydros in Chapter 1 in the equipment section, though, strangely, it's under valves, not cylinders.

...and during my composition of this reply now we're talking about barometric pressure. All very interesting, none pertinent to the original question...

Thank you jonnythan- this should maybe be moved to "The Physics of Diving"

I'll take this as an admission that you've realized that it in fact didn't belong here as an answer to Hal's question. Hal's question was simple, it didn't deserve to be hijacked into physics trivia.

Roak

 

[onfloat]

I didn't mean to hijack (If that's what I did.) I was just trying to add a little perspective from a new diver to some that have been doing this for such a long time that all of this knowledge seems to be second nature. Just becuase someone is a certified diver dosen't mean that they know the manuals backwards and forwards. Maybe they should, but you have to start somewhere.

(Roakey, I know you were not the one that implied this. You seem to be the voice of reason.)

Has that information been removed from the PADI Open Water Dive Manual? I've got a couple laying around, none recent, but they cover visuals and hydros in Chapter 1 in the equipment section, though, strangely, it's under valves, not cylinders.

Roak

It's still in the manual, I just don't recall it being covered in my 3 day course. I know I wasn't tested on it. In fact I thought the manual was full of fluff along with information that needed to be presented and I skipped this area. Now that you have had me go back and look, I see a whole section about regulators that I didn't see before, Thank you!!

I will stop now before I take this further down the road about diver education or the lack there of.

 

[Groundhog246]

I didn't mean to hijack (If that's what I did.) I was just trying to add a little perspective from a new diver to some that have been doing this for such a long time that all of this knowledge seems to be second nature. Just becuase someone is a certified diver dosen't mean that they know the manuals backwards and forwards. Maybe they should, but you have to start somewhere.
..
It's still in the manual, I just don't recall it being covered in my 3 day course.

Was that 3 days including the OW dives? The LDS we trained at have a minimum of five classroom sessions, each followed by a pool session. I've seen some do 8 or more pool session and/or resit a couple of classromm sessions, with lots of assigned reading in between, After that and your written tests, then a weekend of diving (minimum 5 dives, starting at 15 feet, working your way down to 55 feet) and demonstrating skills. Some students also end up with extra OW dives required.

If you search you'll find lots of threads on the quality of divers (or lack thereof) being turned out and it's not knowing items such as how to read tank markings, the need for hydro/VIP, or even gas laws, that are of concern. Not all of us responding have been diving all that long. And many that don't learn it in class, never learn it at all. The "lucky" ones find there way here or end up mentored by a more experienced diver who does know.

As the program director stated when he passed out our OW certificates, it's not a license to dive and it's not the end of your diving education. It's a license to learn and completion of the first step