A question about the Partial Pressure of Oxygen

[FPDocMatt]

The partial pressure of oxygen at sea level is 0.21 atmospheres. Oxygen can cause seizures at pressures above 1.4 atmospheres. To keep the partial pressure of oxygen at or below 1.4 atmospheres, it is mixed with proportions of other gases, such as helium, when diving at great depth.

Let's say you had a device which automatically adjusted the partial pressure of oxygen in the mixture to keep it at the desired level. (Actually, I think such a device does exist.) The question I have is, why wouldn't you just keep the pp of oxygen at 0.21 atmospheres? Is there a reason to ever have it higher than that?

I understand that you want the total pressure of the gas you're breathing to be the ambient pressure of the depth you're at. Otherwise, its pressure would not overcome the external pressure on your chest wall, making it impossible to breathe the air into your lungs.

But is there any reason for the partial pressure of oxygen to be higher than 0.21 atmospheres at depth?

 

[DevonDiver]

Let's say you had a device which automatically adjusted the partial pressure of oxygen in the mixture to keep it at the desired level. (Actually, I think such a device does exist.)

Closed Circuit Rebreathers (electronic) control the input of oxygen and other dilutent gasses - which can be programmed to maintain set PPO2.

The question I have is, why wouldn't you just keep the pp of oxygen at 0.21 atmospheres? Is there a reason to ever have it higher than that?

Higher PPO2 is beneficial for off-gassing.

You would have to decrease the % O2 to enable proportionally decreased PPO2 upon descent.

If decreasing the %O2, you would have to replace it with another gas.

Helium is very expensive.

 

[CuracaoJ]

But is there any reason for the partial pressure of oxygen to be higher than 0.21 atmospheres at depth?

Sure there is, as Devon alluded too, Helium is expensive, so usually the gas that is used to make up the difference is Nitrogen. As you probably remember Nitrogen is what's responsible for Narcosis at depth, and its build up is what gives us DCI or DCS. So by adding more O2 into the mix, we can reduce the negative effects of Nitrogen. Which also has the side benefit of extending no decompression limits at recreational depths. You get approx an extra 5 minutes of no deco time at 30MSW on 32% Oxygen as opposed to air (depending on your chart etc).

Jeff

 

[FPDocMatt]

Sure there is, as Devon alluded too, Helium is expensive, so usually the gas that is used to make up the difference is Nitrogen. As you probably remember Nitrogen is what's responsible for Narcosis at depth, and its build up is what gives us DCI or DCS. So by adding more O2 into the mix, we can reduce the negative effects of Nitrogen. Which also has the side benefit of extending no decompression limits at recreational depths. You get approx an extra 5 minutes of no deco time at 30MSW on 32% Oxygen as opposed to air (depending on your chart etc).

Jeff

Thank-you for the information.

The extended NDL's with a higher percentage of oxygen are due to what, the lower percentage of nitrogen?

If that's true, then the reason to have more than 0.21 atmospheres of oxygen is not related to the oxygen itself. So if you could have 0.21 atmospheres of oxygen at all times, but use some other gas which has no ill effects for the remainder, that would be good, it seems.

I understand that helium is expensive.

 

[Doppler]

Yep, the increase in available bottom time is due to decreased nitrogen rather than the increased PPO2... And Yep, a CCR is the way to manage one's PO2 at a "constant" level (or oxygen depth... which is another way to express partial pressures), but it would be extremely unwise to try to maintain one's oxygen partial pressure that low... at least on scuba of any sort, OC or CCR or SCR.

At issue are two factors: the decay of PPO2 because of metabolic use, the potential to drop PPO2 to hypoxic or anoxic levels when ascending.

There is also the secondary factor of much higher than optimal PPN2... as mentioned.

Think back to what Dalton's Law tells us. Let's say we dive to 30 metres. That's an ambient pressure of 4 bar/ata. If we were able to maintain a PPO2 of 0.21 at that depth, our PPN2 would be 3.79 bar... on air the PPN2 would be around 3.1 bar. Way more narcosis. Do the calc for a dive to 40 metres!?

When we think about adding He to our mix, we can play around with maintaining our oxygen and our nitrogen depths at constant values... my personal levels in cold water conditions are 1.3 and 3.1 bar respectively. Add these two values and we have 4.4 bar... that's 34 metres. Deeper than that, I start to think about adding He (this is on OC).

Simple?

Of course there is the added compliation that keeping one's PPO2 at 0.21 bar at depth on anything but a CCR (with warnings and overrides shut off) would be impossible.

Oh, and there is a flaw -- commonly made -- in one of your basic assumptions. PPO2 of 1.4 bar is NOT a measure of oxygen dose. CNS toxicity measurements are a combination of OXYGEN DEPTH and TIME. An elevated partial pressure of 1.4 is no safer than one of 1.6 bar without reference to time. I would -- and do every time I execute a staged decompression dive -- breathe a gas that delivers a PPO2 of 1.6 for ten or fifteen minutes; however, I would NOT breathe a gas delivering 1.4 bar for three hours.

 

[DevonDiver]

The extended NDL's with a higher percentage of oxygen are due to what, the lower percentage of nitrogen?

Correct.

If that's true, then the reason to have more than 0.21 atmospheres of oxygen is not related to the oxygen itself. So if you could have 0.21 atmospheres of oxygen at all times, but use some other gas which has no ill effects for the remainder, that would be good, it seems.

At depth certainly - because lowering the PPO2 decreases the risk of oxygen toxicity.

Helium is used to replace O2 in mixtures used below the depth where the PPO2 would otherwise exceed (approx - depending on agency) 1.4.

It's often used shallower to also replace the nitrogen, thus decreasing the impact of narcosis.

I understand that helium is expensive.

Very - which makes it prohibitively expensive for 'merely' reducing the PPO2 - when that PPO2 has no other detrimental effects below the stated threshold for oxygen toxicity.

 

[Bob DBF]

If that's true, then the reason to have more than 0.21 atmospheres of oxygen is not related to the oxygen itself. So if you could have 0.21 atmospheres of oxygen at all times, but use some other gas which has no ill effects for the remainder, that would be good, it seems.

I understand that helium is expensive.

Although not in the scope of your origional question, replacing nitrogen with helium is not a perfect "fix". There are positive and negatives on any gas type and mix, the choice depends on the objective.

As you probably remember Nitrogen is what's responsible for Narcosis at depth, and its build up is what gives us DCI or DCS.

To clarify for the OP. Yes, Nitrogen is what's responsible for Narcosis at depth, but what usually gives us DCI or DCS is not properly ascending and not staying within NDL's or performing proper deco. This can happen on any mix of gas.



Bob
--------------------------
I may be old, but I’m not dead yet.

 

[FPDocMatt]

At issue are two factors: the decay of PPO2 because of metabolic use, the potential to drop PPO2 to hypoxic or anoxic levels when ascending.

1. How is the metabolic use of the oxygen a factor? I metabolize the oxygen I breathe all day long, but the 0.21 atmospheres of oxygen available is quite adequate.

2. In the hypothetical apparatus in my OP, the 0.21 atmospheres of oxygen would be maintained during ascent.

Is there something I'm missing?

Oh, and there is a flaw -- commonly made -- in one of your basic assumptions. PPO2 of 1.4 bar is NOT a measure of oxygen dose. CNS toxicity measurements are a combination of OXYGEN DEPTH and TIME. An elevated partial pressure of 1.4 is no safer than one of 1.6 bar without reference to time. I would -- and do every time I execute a staged decompression dive -- breathe a gas that delivers a PPO2 of 1.6 for ten or fifteen minutes; however, I would NOT breathe a gas delivering 1.4 bar for three hours.

Thanks for the correction. Makes sense.

 

[TSandM]

Steve's talking about CCRs, which are the only diving equipment at present which can maintain a constant ppO2. On a CCR, you are rebreathing the gas in the loop until or unless you inject some more gas, so the ppO2 will slowly fall as you metabolize the oxygen. In addition, if you are at .21 at 33 feet, the ppO2 will fall as you ascend, and you'd be at .1 by the time you got close to the surface -- UNLESS you add O2.

There are three considerations in choosing diving gases: Oxygen toxicity, narcosis, and decompression. The more oxygen you can have in your breathing mixture, the less decompression you have to do. The choice of other gases to use in a breathing mixture is not very broad -- there aren't that many gases from which to choose, and many of them are toxic. Of the ones which aren't poisonous or anesthetic, many are narcotic. It has come down to nitrogen and helium because they are available, nontoxic, relatively affordable, and helium is virtually non-narcotic. Although there are controversies about the comparative behavior of nitrogen and helium in decompression, I think it's legitimate to say that both require attention to ascents rates and times. Minimizing inert gas makes for shorter deco, but raises oxygen toxicity risk. GUE, the agency through which I have some of my training, chooses to keep a relatively low ppO2 (1.2 or lower) for long exposures, but most of us will tolerate 1.4 for shorter dives, and 1.6 on deco, where there is very little exertion and the exposures are generally fairly short.

So there's really the answer. Although you COULD, I suppose, run a CCR at a .21 setpoint, you'd rack up a ton of deco, because the deeper you went, the higher the ppInert would be, and the more time you'd have to spend getting rid of that gas at the end.

 

[Jim Lapenta]

And just one other point that was mentioned about oxygen being used to reduce narcosis. There is some thought than O2 itself is narcotic and as an MD you know the effect it has on nervous or upset patients. It calms them so simply reducing the N2 with O2 may not have the desired effect of reducing narcosis but may have none or possibly add to it. Hence He is the preferred method at this time. He though does require much greater attention to ascent rates and it also has a greater heat transfer efficiency. Ie you get colder faster breathing helium. I also never use back gas with He for drysuit inflation. Being so light there is the possibility of isobaric counterdiffusion if you use it as a suit gas. The theory is that it can actually be absorbed through the skin while you are trying to off gas it, if used as a suit gas.