boyles law

[Bomber]

Tutt, tutt, mixing metres with atmospheres You'll be doing bars and feet next Firstly get on the correct page and identify that a 10m depth increment is not an Atmosphere (14.7 PSI) but a Bar (14.5 PSI) there's the difference! If you think that's being pedantic - it aint - it's being correct.

Berko

isnt that also meters of sea water??

the old grey matter is flagging a difference in preasure between sea water and fresh water.... but not quiet recalled it all ATM

[John63]

I thought the discussion was about Boyle's Law and not gas planning.

I think it is important to understand exactly what is going on, and my point is that you breathe at the same rate, but that the same volume uses a greater proportion of the gas in the cylinder.

I've found that trying to explain Surface Equivalent Volume is harder to do than to explain that the volume is the same, but that more gas molecules are involved and the cylinder empties quicker.

I saw someone confuse the 60 l/min rate (SAC=20, depth = 20m) with the 60 l/min rate for a diver under severe stress and who then missed a factor of 3 out of the equation.

Cheers, Chris

It is about Bolyes law which, very simply, is that pressue is inversely proportional to volume for a fixed mass of gas at constant temperature.


However, it had also developed (as threads do) and has become about the application in diving (as this is a dive forum).

The consequences / application in diving are many as we all know (mask squeeze, equalisation etc) but one of the most important is the effect on gas rates. Introduction of discusion of molecules is (in my humble opinion) more confusing than using the depth/pressure x SAC.

Telling someone that they will breathe the same volume of gas at any depth but that it's more dense so will empty the cylinder quicker is, whilst correct, just waiting for someone to say

"okay, I have a 10 litre cylinder of 200 bar, so 2000 litres. Now I have a SAC of 20 l / min, so 100 minutes from the tank. I remember that Chris told me that I breathe the same at any depth so 30m 100 min here we go. Yeah, he mentioned molecules, but thats all science stuff...."


Whereas if they get fixed in their minds that their gas consumption increases with depth they will (hopefully) at least be aware that the cylinder will not last anywhere near as long and can directly relate consumption to depth.

Cheers, John

[adamr]

The Feynman Lectures on Physics

everything you wanted to know about everything.

Agreed, Richard Feynman was a legend. I would have loved to have had him for lectures......

[James The Badger]

Tutt, tutt, mixing metres with atmospheres You'll be doing bars and feet next Firstly get on the correct page and identify that a 10m depth increment is not an Atmosphere (14.7 PSI) but a Bar (14.5 PSI) there's the difference! If you think that's being pedantic - it aint - it's being correct.

Berko

Could you just clarify why you are expressing pressure in PSI rather than metric? Are you also saying that 10m of seawater does not weigh the same as 1 ATA?

[Morag]

I can't beliveve this isn't covered in GCSE level physics.

I don't even remember it being in GCE 'O' level physics. But then, I guess it was a long time ago

[James The Badger]

I don't even remember it being in GCE 'O' level physics. But then, I guess it was a long time ago

This is the way they taught physics when you were at school Morag

[nickb]

I don't even remember it being in GCE 'O' level physics. But then, I guess it was a long time ago

It was when I did it (1974). Boyle's Law, Charles' Law and the combination of the 2: PV/T = Constant (the ideal gas law)

[Morag]

It was when I did it (1974). Boyle's Law, Charles' Law and the combination of the 2: PV/T = Constant (the ideal gas law)

The syyllabus obviously changed between the year you did yours and several years later when i did mine. Either that or my brain has had far more important things to remember over the years, and i have subconsciously evicted the physics

[PBrown]

Could you just clarify why you are expressing pressure in PSI rather than metric? Are you also saying that 10m of seawater does not weigh the same as 1 ATA?

I suspect that Berko was trying to point out that atmospheres are an imperial measurement, bar is a metric measurement. For diving purposes we tend to interchange them without a second thought, making the assumption that they are the same, this drives scientific types up the wall.

1 atm = 1.01325 bar = 101.3 kPa = 14.696 psi (lbf/in2)= 760 mmHg =10.33 mH2O = 760 torr = 29.92 inHg = 1013 mbar = 1.0332 kgf/cm2 = 33.90 ftH2O

cheers,
Paul

[James The Badger]

I suspect that Berko was trying to point out that atmospheres are an imperial measurement, bar is a metric measurement. For diving purposes we tend to interchange them without a second thought, making the assumption that they are the same, this drives scientific types up the wall.

1 atm = 1.01325 bar = 101.3 kPa = 14.696 psi (lbf/in2)= 760 mmHg =10.33 mH2O = 760 torr = 29.92 inHg = 1013 mbar = 1.0332 kgf/cm2 = 33.90 ftH2O

cheers,
Paul

Thanks Paul, I didn't know ATA/ATM was imperial BTW. I was just hoping that after 8 years of teaching this I wasn't missing something...