Charles' Law and exploding hairspray cans, a question

[The Iceni]

Originally posted by Ber Rabbit
Unfortunately my assigned lecture is Charles' Law. The way I was taught its only use in diving is when you are dealing with hot fills or leaving your scuba tank in the sun. The basic relationship I was taught was that as temperature increases, pressure increases (the molecules move faster).

Thanks,
Ber

Hi Ber and Jamie,

Add in Van der Waals', Graham's and Poisseuille's Laws and Avogadro's number and you can really confuse!

This is Van der Waals' version of the gas equation;

(P + n^2 *a/V^2)(V-nb) = RT


(Wher R the universal gas constant, and about 22.4 litres per mol and "a" and "b" are constant for each gas)

On a more serious note though, Ber, perhaps your instructor is after an explanation for "short fills".

By the way, did you know that it matters not a jot what the tempertaure of the cylinder is when it is put to use? A cold cylinder (under the artctic ice) may show a drop in pressure due to the cold, or it may show an increase in pressure due to heat from the sun (in the tropics) but provided it has been filled to working pressure at the rated temperature (usually 15 degrees C) it will contain exactly the same amount of gas.

It will therefore last just as long.

This is because every molecule of each breath that you take is heated to body temperature (and water vapour is added) by the time it reaches - and fills - the lungs. So for all practical purposes you will be breathing your air or gas mix at 37 degrees C.

Does this make any sense? :mean:

 

[Walter]

Jamie,

The Idea Gas Law is more useful when actually working with numbers in real situations as it takes more factors into consideration. Which law has what name is also not important.

OTOH, we aren't actually teaching people how to work with the gas laws (except for Boyle's) what we are teaching them are a number of concepts. Amonton's Law is an important concept and is easier for most students to learn if it is presented alone. When the tank gets hot, the pressure goes up. The same is true for Boyle's Law - when pressure increases the balloon get's smaller. Each concept is easier to learn by itself. After they've learned the concepts (who cares about the names except for a few odd balls like me?) you can combine them into the Ideal Gas Law if needed. I never explain the Ideal Gas Law because, frankly, they don't need it for diving.

 

[Ber Rabbit]

jrtonkin
That's why I married an engineer, they're usually full of facts that come in handy when you have a project to tackle! Thanks for the explanation of the exploding hairspray can! We're trying to keep the physics to a minimum for the OW class so we don't get to the ideal gas law. It's easier for technical applications (physical chemistry, quantum mechanics, etc.) but let's not blow away the liberal arts students (I will be teaching at a college).

Dr. Thomas
Yes, that does make sense after some contemplation about volume and such, the volume would increase or decrease while going through the breathing cycle. I think she's just looking for a way to encourage students to check for short fills as you said and to keep them from leaving their tanks in the sun at the dive site or in their trunks all day in the parking lot after they pick them up from the dive locker at school. I've sat through her class 3 times and have exactly 10 LINES of notes from her Charles' Law coverage! I have to turn those 10 lines into a 10 minute lecture! Maybe she's just trying to see how I solve difficult problems!

Walter
She's gotten a little nicer since the first class she taught solo (mine). We had to work math problems with Charles' Law--YUCK! We had to be able to calculate what the psig on an spg would be if the tank was heated from one known temperature to another! THANK GOD we don't have to do THAT anymore!

I've really appreciated all the help, d-day is tonight so we'll see how it goes. I'm using Zept's bottle idea for a training aid since it's easier to transport and cheaper than a pony bottle with digital gauge!
Ber

 

[Ber Rabbit]

Just thought you'd like to know the lecture went well! I used Zept's demonstration with the bottle of cold air and it worked like a charm. I had even left the bottle out for several hours after putting the lid on tight. It still worked so I know the bottle doesn't have to be kept cool for transport. I did the demo first with a bottle that had been at room temperature. My instructor was VERY impressed!
Thanks for everyone's input! I've actually gotten better at gas laws because of you! (Ber's comprehending physics, be afraid, be very afraid!)
Ber

 

[jbd]

and all this time you thought physics was hard. Look what you accomplished with an empty plactic bottle and a freezer.

Zept that was a great suggestion!

 

[The Chairman]

Glad to hear it!!!

BTW... concerning the statement by Dr. Thomas.

It will therefore last just as long.

Uh... not quite true. Matter is conserved, but if a gas is denser, then you will consume it faster. This is why they put intercoolers on turbochargers, so that you can get more air into the cylinder. An ideal gas' density is inversely related to it's absolute temperature. Everything else is spot on.

For a regular SCUBA class I go straight by standards. For my SCUBA Crew 7, I teach STP, or the ideal gas laws. I have the luxury of seeing my kids every week for 1 1/2 hours.

 

[Hobbs]

The plastic and cooling was a nice touch, what about adding visual confirmation on that test (and you can do it both ways from cold to hot and hot to cold) by adding a baloon ?
if you take the bottle and have it at room temp then add the ballon over the bottleneck then heat it with somethin (or cool it) wach the ballon expand / retract into the bottle.
(bigger bottle + more difference in heat = more result)

(this is fun doing at the beach also, bring a bottle down to a couple of meters with a baloon with a little whole in it then return to the surface and watch it squirt out water like a fountaine while holding it upside down this is best done freediving )


just my 2 cents