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What does PO2 mean?

  • Partial Pressure of O2

    Votes: 59 67.8%
  • Percentage of O2

    Votes: 21 24.1%
  • Never heard of it

    Votes: 0 0.0%
  • Don't care

    Votes: 5 5.7%
  • No idea what you are talking about

    Votes: 2 2.3%
21 - 38 of 38 Posts

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Nigel Hewitt
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In Physics, just like English, usage does not imply correct usage.

But Physics is less forgiving.
 

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Perpetual Learner.....
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No doubta a contentious first post from me, but....
I was taught a long time ago that p=Pressure. And that P=partial so, more correctly PpO2 is partial pressure of O2.

In addition to this NOAA refer to partial pressure a PP02.

pO2 is the oxygen pressure, if it is mixed with other gasses in a container then you need to take into consideration the fact that the pressure of each gas is equal to the pressure that each gas would produce if it occupied the container alone, thus each gas' individual pressure becomes known as it's Partial Pressure.

How this relates to diving with enriched mixes is something that I don't yet fully understand. Although I do know about O2 toxicity!
 

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I think, if I remember my schoolboy physics (hell of a long time ago) , that this is Dalton's Law of Partial Pressures and should be expressed as PPO2, but the 2 should be subscript. Anyway, my very modest mosquito shows it as PO2. It is the same thing, and definitely not to be confused with FO2.
 

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On reflection, i seem to remember that all gases, regardless of atomic weight, exert the same pressure. So if the cylinder is full of air then the partial pressure of oxygen as 0.21 x the cylinder pressure in bar. Not bthe same as the % volume, which will always be the same. B*****ks, I think somebody already explained that. Must do my Nitrox course again.
 

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Making God laugh...
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But to refer to a % of O2 as a fraction is wrong.

32%, written down as a fraction would be 32/100ths, the laws of maths then say that you should round down, so in effect the FO2 of a 32% mix should be written as 8/25ths - never ever seen it written like that......
 

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Er.. to be pedantic, and I don't care if you shout at me because I am doing a course on 'HTFU', that is not rounding down. That is finding the lowest common denominator. In fact if you shout I might consider it part of my course. Anyway mathematically fractions and decimals and percentages are exactly the same things expressed different ways. using percentages or decimals just makes it easier to be more accurate.
 

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Making God laugh...
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Er.. to be pedantic, and I don't care if you shout at me because I am doing a course on 'HTFU', that is not rounding down. That is finding the lowest common denominator. In fact if you shout I might consider it part of my course. Anyway mathematically fractions and decimals and percentages are exactly the same things expressed different ways. using percentages or decimals just makes it easier to be more accurate.
Correct, and thanks for reminding me of the correct terminoligy - my old and lunchtime drinking session addled brain couldn't remember "expressed using the lowest common denominator"

But the fact remains - a fraction is written as 36/100 (which would then be expressed as 9/25), not 36% therefore the use of the term FO2 is wrong.

Regards

Stevie H
 

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Fcuk, you could have shouted at me and helped with my HTFU ttaining. Now you have reminded me it's Sunday afternoon I'm off to have a look in the fridge.
 

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Nigel Hewitt
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And there was me thinking that a percentage was a way of expressing a dimensionless quantity, normally between 0 and 1.
Which seems quite appropriative in this case.

You guys are confusing fractions with vulgar fractions.
 
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Don't think so, vulgar fractions are (unsurprisingly) fractions that are capable of being refined. By applying the LCD. Again, sorry for being pedantic but feel free to shout. It will count towards the course content of my HTFU degree.
 

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Nigel Hewitt
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Don't think so, vulgar fractions are (unsurprisingly) fractions that are capable of being refined.
Sorry. A vulgar fraction is just the N/M style of representation. Vulgar as in common.
It is about style of writing not whether it can be reduced by removing common factors
Again, sorry for being pedantic but feel free to shout. It will count towards the course content of my HTFU degree.
Never argue mathematics with the physics police.
 

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I think I may have just realised there's another great way to confuse the crap out of a gas monkey.

"I'd like 9ths in this one please"
"Sorry?"
"9/25ths O2"

The chances of ending up with 9/25 trimix are almost worth it for the laugh at their tiny little brains :D

Digs.
 

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Still young enough to know everything
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But to refer to a % of O2 as a fraction is wrong.

32%, written down as a fraction would be 32/100ths, the laws of maths then say that you should round down, so in effect the FO2 of a 32% mix should be written as 8/25ths - never ever seen it written like that......
It's not wrong really though - as you say, 32% is exactly the same as 32/100, it's just another way of writing it. 32% means 32 out of 100, and 32/100 means 32 out of 100... Even though as you say that the fraction could be simplified, thirty-two one hundredths isn't wrong.
 

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Nor is it wrong to mix percentages and fractions. It is quite reasonable to say, for instance, 36 1/4%.

However if you were using fractions it is more sensible to say 8/25 than 32/100.
 

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Coming to this discussion a bit late, however I wonder if the discrepancy between PO2 / PPO2 comes from the two equations we use:-

PO2 = P x FO2
and
P total = P gas1 + P gas2 + P gas3 ...

The 2nd equation is Dalton's Law ; a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.

PO2 calculated in the first equation could be substituted as P gas1 (or PPO2 ) in the 2nd equation.
 
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