Altitude Diving And Nitrox

[rsingler]

mod is deeper for altitude diving

Nitrox and Diving at Altitude – Utah Scuba Divers

Read this article before @boulderjohn replied. Didn't this article forget to account for fresh water too? Or is Flaming Gorge salt water at 6000 ft?

 

[rsingler]

This isn’t about salt vs fresh.

So which units are the appropriate units for ppo2? Bar or atm? If bar then surely the calculation of mod needs to be in bar. And the pressure of 10m of fresh water is 1 bar.

Not that Wikipedia is definitive, but it would appear that atm is the appropriate unit. Additionally, fresh water is indeed greater than 10m/bar (by about 2%).

From Wikipedia:
The International Bureau of Weights and Measures (BIPM) lists the bar as one of the "non-SI units [that authors] should have the freedom to use", but has declined to include it among the "Non-SI units accepted for use with the SI".[1] The bar has been legally recognised in countries of the European Unionsince 2004.[2] The US National Institute of Standards and Technology (NIST) deprecatesits use except for "limited use in meteorology" and lists it as one of several units that "must not be introduced in fields where they are not presently used".

The notation bar(g), though deprecated by various bodies,[2] represents gauge pressure, i.e., pressure in bars above ambient or atmospheric pressure.

Definition and conversion
The bar is defined using the SI derived unit, pascal: 1 bar ≡ 100,000 Pa ≡ 100,000 N/m2.

Thus, 1 bar is equal to:

• 1,000,000 Ba (barye) (in cgs units);

and 1 bar is approximately equal to:

• 0.987 atm
• 14.5038 psi absolute
• 1019.72 centimetres of water (cmH2O).

Therefore, to continue the discussion with the Wikipedia data above, 1 bar is ~1.020meters.
1.02m=33.46ft
33.46ft/0.987 atm per bar = 33.9 ft.

Tomayto/tomahto with regard to bar vs atm, but fresh water is more than 10m/33ft per atm OR bar. But if 2-3% is not significant to you, then yeah: bar, atm, sea water, fresh water...close enough.

 

[Exigent]

The dive tables I have refer to PO2 as bar/atm. One would assume it to be bar per atm but it actually means they are interchangeable in this case. They are approximately equal, like mentioned above.

 

[KenGordon]

Not that Wikipedia is definitive, but it would appear that atm is the appropriate unit. Additionally, fresh water is indeed greater than 10m/bar (by about 2%).

From Wikipedia:
The International Bureau of Weights and Measures (BIPM) lists the bar as one of the "non-SI units [that authors] should have the freedom to use", but has declined to include it among the "Non-SI units accepted for use with the SI".[1] The bar has been legally recognised in countries of the European Unionsince 2004.[2] The US National Institute of Standards and Technology (NIST) deprecatesits use except for "limited use in meteorology" and lists it as one of several units that "must not be introduced in fields where they are not presently used".

The notation bar(g), though deprecated by various bodies,[2] represents gauge pressure, i.e., pressure in bars above ambient or atmospheric pressure.

Definition and conversion
The bar is defined using the SI derived unit, pascal: 1 bar ≡ 100,000 Pa ≡ 100,000 N/m2.

Thus, 1 bar is equal to:

• 1,000,000 Ba (barye) (in cgs units);

and 1 bar is approximately equal to:

• 0.987 atm
• 14.5038 psi absolute
• 1019.72 centimetres of water (cmH2O).

Therefore, to continue the discussion with the Wikipedia data above, 1 bar is ~1.020meters.
1.02m=33.46ft
33.46ft/0.987 atm per bar = 33.9 ft.

Tomayto/tomahto with regard to bar vs atm, but fresh water is more than 10m/33ft per atm OR bar. But if 2-3% is not significant to you, then yeah: bar, atm, sea water, fresh water...close enough.

Fair point, I guess g is only 9.81 m/s/s so a 10m column 1cm^2 weighs only 9.81N not 10N. My mistake.

 

[boulderjohn]

Basic Partial Pressure Lesson
Each gas that is a part of the mixture of air contributes to the total pressure of the air in relation to its fraction of the total. If you are at sea level breathing air at 1 atmosphere (ATA) with 21% oxygen, then the partial pressure of oxygen is 0.21. As you descend, the added pressure of the water increases the total pressure of the gas being breathed. As Pascal discovered hundreds of years ago, 33 feet of sea water provides the same pressure as the atmosphere at sea level. Because of decreased density, fresh water needs 34 feet to equal one atmosphere of pressure. That means that at 34 feet of sea water (FSW) or 34 feet of fresh water (FFW) at sea level, the total pressure of gas will be 2.0 ATA. Oxygen will still contribute 21% of that total, so its partial pressure is now 2 * 0.21 = 0.42.

That gives us the basic equation we use for all calculations:

Partial Pressure = Gas Fraction times the pressure (in atmospheres). PP = FG * P.​

Using that basic formula, we can find pretty much anything we want to know.

To find the pressure (P) at which you will hit a specific Partial Pressure, divide PP/FG. (This will give MOD.)
To find the fraction of gas (nitrox percentage in this case) that will give us a certain PP, divide PP/P. (This will give best mix for a certain depth.)
​

To convert pressure (P) to depth, we need to subtract the weight of the atmosphere (1.0 at sea level; a fraction of that at altitude) from the total ATA and then multiply by the weight of the water. In imperial units, we would use 33 for salt water and 34 for fresh water. In metric units, we would use 10 for salt water and 10.3 for fresh water. Those are inexact approximations, but they are close enough.

 

[Charles2]

To find the fraction of gas (nitrox percentage in this case) that will give us a certain PP, divide PP/FG. (This will give best mix for a certain depth.)

I'm not sure I understand this one. I would have thought FG=PP/P.

 

[boulderjohn]

I'm not sure I understand this one. I would have thought FG=PP/P.

Yep--I mistyped. I'll fix it.

 

[yle]

So which units are the appropriate units for ppo2? Bar or atm? If bar then surely the calculation of mod needs to be in bar. And the pressure of 10m of fresh water is 1 bar.

1 bar and 1 atm are almost identical. There is only a 1% difference... which is typically lost in the rounding anyway.

1 atm = 1.01 bar