Monitor oxygen as nitrogen

[EFX]

Actually both gasses are measured in atm pp (partial pressure). Exceeding the O2 limit is, relative to N2 loading, more or less instantaneous. Hence, the limit is expressed as a pp without time constraints. The N2 pp in the tissues loads and unloads in a time dependent fashion also based on depth (the ambient pressure). Your computer is always calculating the pp of N2 in each tissue compartment. Since the onset of bends is time delayed the instantaneous display of ppN2 is not as important as the NDL time in keeping you out of DCS. More importantly, offgassing of N2 to prevent DCS is rate limited which implicitly calls for a time measurement.

 

[rjack321]

I would much rather find myself in NDL violation with some minor deco than to be at 1.8 and find the dive taking longer than I had planned for whatever reason. An O2 hit is serious trouble.

[-]Let me give an example.[/-]
Wrong example

No its a perfect example, bent is fixable. Toxing is almost never fixable.

We don't actually measure N2 because its pointless. On nitrox the partial pressure of N2 is the total pp minus the O2 fraction. You only need to measure one fraction to know them both. Rebreather divers monitor the O2 fraction because its dynamic. Open circuit divers don't need to because you measure the fraction of O2 ahead of time, then at any given depth you know the pp of O2 and N2. The fraction of O2 and N2 are fixed and the only thing changing the pp of O2 and N2 is depth.

 

[Polari]

No its a perfect example, bent is fixable. Toxing is almost never fixable.

We don't actually measure N2 because its pointless. On nitrox the partial pressure of N2 is the total pp minus the O2 fraction. You only need to measure one fraction to know them both. Rebreather divers monitor the O2 fraction because its dynamic. Open circuit divers don't need to because you measure the fraction of O2 ahead of time, then at any given depth you know the pp of O2 and N2. The fraction of O2 and N2 are fixed and the only thing changing the pp of O2 and N2 is depth.

The wrong example is for the example i wrote and deleted. I agree and is a common sense what Steve said about O2 toxicity and i agree with you too

 

[scagrotto]

We use a constant partial pressure for oxygen.Usually 1.4 (a limit which i do not find so practical in some cases).
Why not use a constant pp for nitrogen too?
Or the opposite.Why not use oxygen same way as nitrogen?Not have a constant but calculate it based on accumulation.

Dive a rebreather.

Without a 3rd gas and the ability to vary the fraction of that gas it's an either/or choice. Do any rebreathers actually use 2 independent diluents?

If not, then you can maintain a constant ppO2 by keeping fO2 inversely proportional to ambient pressure or you can maintain a constant ppN2 by keeping fN2 inversely proportional to ambient pressure (of course you'd maintain that constant ppN2 by adjusting the fO2). Either way, the partial pressures of all the gases will have a total pressure of 1.0 * ATA. Change the ambient pressure and at least one of the partials has to change. Keeping a constant ambient pressure would severely limit your dive profile, no?

Of course it would be possible to maintain a constant ppN2 instead of a constant ppO2, but that would mean you're deliberately keeping the ppN2 higher than necessary at least some of the time. What's the advantage to that?

 

[Doppler]

I believe the OP is asking about choosing the flavor of gas used based on vacant partial pressure equations.

For example: If we take 1.3 bar of O2 as our top limit + 3.16 bar of nitrogen as the top-end of our inert gas narcosis threshold we have a COMBINED OXYGEN AND NITROGEN DEPTH OF 4.46 bar (1.3 + 3.16).

Therefore, any dive below 34.6 metres will require a third gas to fill the vacant partial pressure... for example, a dive to 50 metres (6 bar) will have a vacant partial pressure of 1.54 bar... which must be filled with He or something...

This method is common with trimix divers and covered in several books... including mine.

 

[boulderjohn]

In addition, we actually DO set a limit on PPN2, it is about 3-4ATA. This is the limit many people use to avoid narcosis (for air that is about 100 or 130 ft). The analogy to O2 toxicity limits of PPO2=1.4 is pretty good; in both cases it is the partial pressure causing the problem, not the depth or the time.

I believe the OP is asking about choosing the flavor of gas used based on vacant partial pressure equations.

For example: If we take 1.3 bar of O2 as our top limit + 3.16 bar of nitrogen as the top-end of our inert gas narcosis threshold we have a COMBINED OXYGEN AND NITROGEN DEPTH OF 4.46 bar (1.3 + 3.16).

Therefore, any dive below 34.6 metres will require a third gas to fill the vacant partial pressure... for example, a dive to 50 metres (6 bar) will have a vacant partial pressure of 1.54 bar... which must be filled with He or something...

I was guessing this was what he had in mind as well.

In normal recreational diving, we have only two gases to think about, so whatever limit we put on oxygen limits directly impacts the amount of nitrogen.

In technical diving with trimix, we normally use helium to limit both the amount of oxygen and the amount of nitrogen in the gas we breathe. By using a little math and mixing the right amounts of helium, oxygen, and air, we will plan for limits on both the problem gases.

 

[aquaregia]

A lot of dive computers actually do integrate CNS% along with the decompression obligation. However, it's important to remember that the USN CNS toxicity models top out at 1.6 (although I think there are British tables up to 2.0), so extrapolation above that is going to be pretty risky.