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...
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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.
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: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 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?
perpet1:So your absolutpoly correct that temperature has an affect on the density of the air in a scuba tank [/I]
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.WarmWaterDiver: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.
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.Hal: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.
Which weighs more, a pound of feathers or a pound of gold? (Anyone? A trick question within a trick question!)Krisscuba:Which weighs more,.. a pound of feathers or a pound of lead?
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.onfloat:I didn't even become aware of VIPs and hyros until I actually started to look at buying my own tank.
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.WarmWaterDiver:Thank you jonnythan- this should maybe be moved to "The Physics of Diving"
roakey: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
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.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.
..
It's still in the manual, I just don't recall it being covered in my 3 day course.