Steel tank ?..again

[turnerjd]

Originally posted by NetDoc

A tank going from 3000psi to 3500psi: convert to absolute pressure (add 14.7psi) 3014.7psi/3514.7psi= 1.16%
Convert 80 F to Absolute temp (add 459.7)= 539.7 F
539.7 F * 1.16 = 626.0 F
626.0 F – 459.7 F = 166.352 F
I don’t think I have ever got them to 166.3 F… Prudent minds like to cover things from ol’ Sol…

Please keep CRAP like this away.

Firstly use the general gas law

P1V1/T1 = P2V2/T2

OK, the tank volume restraining the air is constant so we can get rid of the two volume terms

P1/T1 = P2/T2

Now, as this is the general gas las we can work in whatever units we like (can mix imperial / metric - provided you use the same unit for both pressure, volume, or temperature measurements)

Lets use 20 Deg C (a warm room) for our fill temperature
lets start with 3000PSI
Now, what temp do we need for 3500PSI?

P1=3000, T1=20
P2=3500, T2 is what we want to find

using P1/T1 = P2/T2

T2 = 45° C (not 166F.. = approx 75°C)

Now, in some hot places, it IS possible for it to be 45 Deg. in direct sunlight. Now lets just leave it in a car..... it will get above 45° in car left in direct sunlight....

Remember, 3000 - 3500 is a 16% (not 1.16% as netdoc said!) increase in pressure. If you have a US tank with a + after it (AFAIK - I'm not american!) you can overfill by 10%...... if it doesn't have a plus....

Jon T

 

[The Chairman]

Gosh Jon T,

I am sorry you don't have a grasp on this physics "crap"... but here goes...

Multiply 3000psi by 16% and you get what??? 480psi Hmnnnn, not the number we were looking for... however multiply it by 1.16 and walla... we get 3480psi, far closer to the 3500 we were looking for, n'est pas??? Percentages to determine total volume are additive, you are indicating only the increment of change. My number simply had an erroneous % sign for which I will be eternally penitent. It was in fact a decimal ratio.

Secondly, any fool claiming to know physics (and even quote the law) should realize that ABSOLUTE temperatures and ABSOLUTE pressures govern gas laws... NOT mere Celsius or Fahrenheit but Kelvin!!! Now add 273.16C (the increment in C from absolute zero to 0 C) to your erroneous Centigrade numbers and you will find out that the real temp you are referring to is actually 68.66 Celsius... Still too damn hot(you were off by a mere 23.66C)!!! P S, don't forget to add 14.7psi to make your pressures absolute as well (just can't forget that first atmosphere)

(3000 +14.7)psi/(20 + 273.16) K= (3500 +14.7)psi/(68.66+273.16)K

However, if P1/T1=P2/T2 (where volume is a constant), THEN… P1/P2*T1=1/T2

OR (easier to compute)

T1*(P2/P1)=T2 (no physics here, just plain High School Algebra)

So if you convert 80F (which is still not THAT warm to me compared to your refreshing 20 C)to Celsius…
(80-30)*0.56= 28.0 C

(28.0 +273.2)K * (3514.7/3014.7)= T2K
301.2*1.17=352.4K (does that 1.17 look familiar to you yet???)(OK, OK, I rounded it differently, sue me!!!)

352.2K-273.2C=79.2C Hot enough to burn you!!!

BTW (20C *1.8) +32=68F… that is NOT warm in my book… At least not down here in Sunny Florida where its warm enough for us to understand our physics! (no, we can't count, but we can certaintly calculate!!!)

As for overfilling a tank by 10%… ONLY if it is plus rated!

There you have it… more physics “crap” for you to read, and hopefully finally to understand!!! As a side note, PADI gave us some vague finagle factor to "easily" calculate pressures based on temperature variations for my Dive Master Class... I think even they called it a rule of thumb, and not too precise.

Last side note... before you call something somebody wrote "crap", please think it through. It is highly inflamatory, and not something we need here. If you disagree with me, do so politely, and maybe I won't have to deal with as many hard feelings as I have right now. Being opinionated is fine, until you go and stick your foot in your mouth... It is then we learn the full value of humility and speaking gently. I have the same problem with a group of divers who think they have it all figured out and that no one else matters for squat.

 

[turnerjd]

Originally posted by NetDoc
Gosh Jon T,
Secondly, any fool claiming to know physics (and even quote the law) should realize that ABSOLUTE temperatures and ABSOLUTE pressures govern gas laws... NOT mere Celsius or Fahrenheit but Kelvin!!!

Unfortunately, the general gas law is the only case in gas calculations where you CAN mix units, and DON'T have to use absolute temperatures. - High school physics

Now, if you are getting into a more thermodynamic consideration and starting to use constants like R, Boltzman etc.. then yes absolutes apply, but not in the case of the general gas law.

You went on to calculate by % increase. The reason that you CAN calculate by % increase is that the general gas law isn't covered by absolute conditions.

Originally posted by NetDoc

Last side note... before you call something somebody wrote "crap", please think it through. It is highly inflamatory, and not something we need here. If you disagree with me, do so politely, and maybe I won't have to deal with as many hard feelings as I have right now. Being opinionated is fine, until you go and stick your foot in your mouth... It is then we learn the full value of humility and speaking gently. I have the same problem with a group of divers who think they have it all figured out and that no one else matters for squat.

OK, I called it crap. But, unfortunately this is what your calculation, and your follow-up calculation ARE. Uptill now I had your posts down as reasoned, and usually to the point. However, for some-one that has done the DM class to make silly mistakes like this (OK the 1.16 was the decimal ration, and there was an errant % sign is a trivial error) with something as simple as the general gas law needs pointing out.

I think that if you look in the PADI encyclopedia under the physics section it not only gives the gas law, but also at least one example of it. In this example °K ARE NOT used, and IF I remember correctly (I don't have the encyclopedia here at work) it even said it works will all units provided both T's, or P's, or V's are in the same unit.

Appologies for being inflamatory, but a spade is a spade, and unfortunately, your response was rather inacurate!

If you want further information on the general gas law, look it up in the equation section of a book like Engineerings Handbook - it will give you all the units and everything you have to use, and will say wether or not you have to use absolute temperatures.

Any of the other engineers or scientists on the board like to comment?

Jon T

 

[The Chairman]

Hey Jon T.

I can reccomend many chemistry or physics books, but just look on the web for the answer here. The "General Gas Law" (or STP -Standard Temperature Pressure Equations or Ideal Gas Laws) MUST use absolute temperatures. If you use an "arbitrary" standard (such as C or F), than you can NOT have repetitive calcs using different scales...

Using YOUR reasoning,,,

20C 3000 going to 3500 will yield a C increase of

3000/20=3500/x x=23.33C ???
Convert this to F... (23.33*1.8) +32= 74F

20C= (20*1.8)+32= 68F

Same Calc for F

3000/68=3500/x x=79.3F

Your Xs don't equal... 74F does not equal 79.3F. IF you convert them to absolutes (add 273.2 to the C or add 459.7 to the F you get the following...

First using Kelvin...
3000/(273.2+20)= 3500/(273+X) X= 68.87C
Convert it to F (68.87 x 1.8) +32= 155.95F

Now for the Imperial Absolute 0 (which I can not remember the name for)...
3000/(459.7 +68)= 3500/ (459.7 +X) X=155.95F

Here the Xs EQUAL, because that's the right way to do it.

PADI may have given you the equation, but obviously they did not tell you how to plug in the proper values. If you call yourself a Scientist, then dig into the facts (yes, it might take some work, and reading) before you go and try to contradict science again. Now, since I don't expect you to believe me, I went over to Encarta and copied this from their web page on the topic...(merely copied NOT edited) Notice their use of the verbage "absolute temperature".

Empirical laws have been developed that correlate macroscopic variables. For common gases, the macroscopic variables include pressure (P), volume (V), and temperature (T). Boyle's law states that in a gas held at a constant temperature the volume is inversely proportional to the pressure. Charles's law, or Gay-Lussac's law, states that if a gas is held at a constant pressure the volume is directly proportional to the absolute temperature. Combining these laws gives the ideal gas law: PV/T = R (per mole), also known as the equation of state of an ideal gas. The constant R on the right-hand side of the equation is a universal constant, the discovery of which is a cornerstone of modern science.

The URL is
http://encarta.msn.com/find/Concise.asp?ti=0638E000
if you want to read yet further...

Jon, I hope after this, you can still see my posts "as reasoned and to the point" (I do take taht as a compliment). I do not want to make an enemy here, as I usually agree with everything you have to say as well. I say enough with this "crap"... lets go diving!

 

[Atomox]

NetDoc’s calculations for ideal gas relationships and the assertion that the temperatures need to be converted to absolute scales are correct. (BTW imperial T is Rankine).

What is being overlooked is the criteria for an ideal gas calculation (in this case Charles law which states that the product of the volume V and pressure P is proportional to the absolute temperature T is that no heat is transferred to or from the gas during the compression process (Also known as adiabatic compression) AND that the number of moles (or quantity) is not changed as well.

When we look at the final temperature of the compression (with no addition of gas) from 3000 psig & 80 degF to 3500 psig the resultant temperature of 160 degF, while correct according to ideal gas calculations, is rather high. It has been my experience from filling my own tanks, that the tank does get warm but it does not get to 160 degF. What this suggests is that a significant amount of heat is lost to the surroundings – Therefore, the process is probably not adiabatic!

It has also been my experience that when filling a tank from 3000 psig to 3500 psig that I am actually adding gas to the tank! (It is obvious when you think about it) I believe that this point is being over looked. I know that Jon said the volume is not changing which is correct but the amount of gas is! Therefore the relationship of (P1*V1)/ T1 = (P2*V2)/ T2 is not valid.

I would suggest that the ideal gas law(s) are only a good approximation of ideal conditions and do not reflect actual conditions.

It is also my experience that keeping the tanks in a water bath while filling does not really result in a large amount of heat transfer. For a water bath to be of any significant use you need to keep the tanks in the water for an hour or so.

brad

 

[The Chairman]

Atomox,

Thank-you, I agree with you whole heartedly... and you really agree with my whole point. You would have to heat that tank AN AWFUL LOT to get the same gas (no change in moles) to increase from 3000psi to 3500psi. It was earlier stated that such a high tank pressure might NOT be from incompetence on the filler's part. I felt that a 16% increase in pressure was unreasonable given how much higher the tank temperature would have to be. You would NOT be able to handle such a hot tank. I would fault the filler for negligence.

Entropy was not considered as we were adding heat to a closed system. This was an exercise in Enthalpy. Of course, in real life, as tanks are filled their steel/aluminum wall radiate the collected heat and cool the inner gasses to the temperature of the ambient atmosphere. There will a direct correlation of a pressure drop as well. This is why the first pressure measurement when your tank is filled seems to go down appreciably within the first hour or so. I also agree on your rule of thumb to let it sit in a water tank for an hour or more to cool off. (maybe I need that too... (grin))

BTW, add some condensation/evaporation theory and you have the basis of modern refrigeration...

Also, I did leave out the adjustment for the tank's absolute pressure in the last post. Sorry in advance if that confused anyone. The discourse seemed to be concentrating on the absolute temperature side of the argument.

 

[neil]

So....I still can't hold my breath, right?
Neil

 

[devjr]

Netdoc and atamox have it right. Actually, I found netdoc's first explanation to be the easiest to follow. Absolutes must be used, of course.

My first hand experience with a portable compressor might be illustrative. I've made numerous fills while diving in the Bahamas. The compressor has a 4 cfm fill rate and the terminal pressure is set at 3200-3300 psi. The fills are usually made at approx 80degF air temp with no water bath. The tanks become quite warm, almost "hot". After filling, the tanks are plunged into sea water at about 75 degF for a minute or two. Immediately following retrieval, the tank pressure usually reads 2900 psi. The tanks are topped off again to terminal pressure or nearly so.

I started doing this after comments about "low pressure fills". In fact, given time to air cool, a tank which has originally been hot filled may register 2800 psi or so as the diver submerges.

I have no particular thoughts about the physics of the situation. As a practical matter, the hot fill should be avoided or the tank cooled and later topped off.

 

[Iguana Don]

NetDoc,

The figure that I stated in my post refering to fill at 3k taken out to the sun and it becomes 3.5k was just a figure I pulled of the top of my head. And you all went and blew it out of proportion. Regardless of the final pressure whether it be 3.5k or 3.2k you missed the point all together.

The point is what the tank pressure is at the shop or fill station is not going to be the pressure you dive with. Put all the figures you want to. This is real world.

ID

 

[The Chairman]

Hey ID,

I did not mean to blow anything out of proportion here. I just feel that there are too many fill stations playing fast and loose with the rules. Filling a tank over it's rated capacity (unless its marked +) is NOT a good practice. It might be "real world", in that many shops do it, but its not right. And as you put it, put a temp into the mix, and you really have no idea how your tank was treated/abused. As was pointed out in a previous post, let the tank cool and then top it off, but don't use a WAA and pump it higher hoping you guessed it right.

Heck, I know one guy in the Orlando area that is bent on breaking every gas handling rule he can, just to prove it to everybody. Over filling it by 15% to 20% is not uncommon, and he thinks he is doing you a favor. He carries my doubles by the manifold, and I could just go on. Needless to say, I don't visit him any more.

My favorite fill is my Nitrox fill station. He fills my tanks from already compressed air & oxygen. There is no compression heat to deal with and the fill is alway within a few psi of the max. Heck I usually will pay the few bucks more to have my regular tanks filled there too. Although it has never been proved, I have had friends get sick with "odd air". But they don't complain about it until AFTER the dive. I want mine as pure as possible!