Question Why would Peregrine TX give me PPO2 alarm?

[PeterRabbit]

I just completed my open water training with my local SSI training facility and used my Peregrine TX on the open water dive at Homestead Crater which is at elevation 6,000 feet. It only goes down about 65 ft max, and I'd say my average depth was about 30 ft. This was my very first "real" dive with my dive computer, that wasn't in a 15 ft swimming pool. We did 4 dives total over the course of two days. When I surfaced, I noticed my dive computer had an alarm I had to acknowledge, it was a PPO2 alarm. I remembered reading about partial pressures, so I kinda understood what it meant, but at the same time, kinda didn't. I asked my dive instructor about it, and he said it was an alarm of partial pressure (which I already understood from the reading material). I will admit I don't fully understand partial pressures, other than they increase with depth. Since our dives were less than an hour each, and never went deeper than 60 ft., why would my Peregrine TX have given me a PPO2 alarm? I looked back at the data on the Shearwater app when I got home, and don't really see anything glaringly obvious of why I would get a PPO2 alarm after syncing everything. Also, since we were so shallow and the dives were brief, the computer counted my dive at two dives instead of one. I went into the settings to fix this, so that the end of dive delay was set to the MAX at 600 seconds to prevent this in the future.

Why would I have gotten a PPO2 alarm? Sorry for the seemingly stupid, and ignorant question from a new SSI student. I don't feel I was in any real danger at any time during the multiple dives, and can only assume it was a "false alarm." Can someone please dumb it down for me, and "ELI5", why the dive computer would give me a PPO2 alarm on such shallow dives, short in length of time? I acknowledged the alarm and it went away, but it still leaves me wondering why it gave me that alarm? It's also frustrating because even after syncing the dive computer to the app, I don't show any data that indicates that it gave me a PPO2 alarm, which leaves me wondering even further, WTF am I missing? Sorry in advance, I am new and still trying to understand everything. ALSO, my dive instructor is not very patient!!! If I don't get something, he moves on to the next thing. And just in case he is a member of this online board and reads this in the future, I mean that as constructive criticism, I am absolutely not bashing you and I am very thankful that you have gotten me this far!!!

 

[J-Vo]

We use the term partial pressure to define amount of a specific gas in a mixture referenced to on bar of pressure.

So pure oxygen at seal level has a partial pressure oxygen of 1.013 or 101.3% of oxygen density @ 1 bar.

Pure oxygen at 6k ft has a partial pressure of 0.80 or 80% of the density of oxygen @1 bar pressure.

This matters because our physiology responds to the pure number of molecules of a gas we are exposed to rather than the % mixture of gasses. So even though your breathing air (20.9% O2) at 6k ft, your body responds like you breathing 16.7% O2.

Edit: As @inquis said, those of us diving Shearwaters in the Rock Mountain area have reduced the Low PP02 warning so we don't have to deal with it constantly.

 

[rongoodman]

If there’s only one gas in the mixture its partial pressure is the same as the total pressure. Breathing air at sea level gives you a pO2 of .21 ATM. Pure O2 at 20’ gives a pO2 of 1.6 ATM. It’s the elevated pO2 which can cause a diver to seize and drown, not the percentage. Conversely, if the pO2 is too low, whether from too little pressure or too little O2 in the mix(a hypoxic mix), he can become unconscious, also not a good thing.

 

[macado]

Yeah and I'm still trying to wrap my head around why the term "partial" is used. The thing my brain isn't understanding is how a gas can just have a partial pressure. I had originally thought before even getting into scuba, that the pressure of gas contained within a space, IS the pressure. So the pressure is what it is, and can't be anything else, or any other value. For example if I blow up my car tire to 40 psi, the pressure is the pressure. It's 40 because 40 = 40 no matter what. So I'm to understand on a physical level, how a gas doesn't just have a simple whole pressure value, and how it can have a partial value in the first place. Does partial pressure only come into play when it is a mixture of more than 1 gas? For example oxygen and nitrogen? What if the container only had 100% pure oxygen in it, no other gasses, is it still possible for a partial pressure value to exist or is that thrown out the window because it's a single, pure gas?

Partial pressure is a scientific term. Someone can also probably explain this far better them me but partial pressure is the pressure exerted by an individual gas in a mixture. You are correct that the pressure contained within a container is the total pressure however each specific gas in the mixture also exerts a partial pressure.

In your car tire example the total pressure may be 40psi as you stated but the each gas in the mixture has a partial pressure that makes up the total pressure.

Let's think about this in terms of your tire pressure example. 40psi is approximately 2.7atm.
(40 psi) / (14.7 psi/atm) = 2.7atm

Where did I get 14.7psi? The weight of the atmosphere at sea level exerts a pressure of 14.7 pounds per square inch of force or 14.7psi. We call this 1 atm (atmosphere of pressure).

1 ATA equals 14.69psi. This is the approximate pressure we’re experiencing at sea level. Every 10m/33ft the pressure increases an additional 14.7psi. The one thing that is rarely mentioned is that most gauges are calibrated against gauge pressure as opposed to absolute pressure.

40psi on my pressure gauge is actually ~54.69psi absolute pressure (taking into account atmospheric pressure) but let's just stick with 40psi for now as an an example.

partial pressure = (total absolute pressure) × (volume fraction of gas component)

At 2.7atm or 40psi here are the partial pressures of air in your tire.
.209 x 2.7 = .5643 partial pressure of oxygen
.78 x 2.7 = 2.106 partial pressure of nitrogen
.01 x 2.7 = 0.027 partial pressure of trace gases
-------------------
This adds up to 2.697atm (2.7) or 40psi. The total pressure in your 40psi tire.

Think of Dalton's Law (actually called Dalton's law of partial pressures) which states that: 'The total pressure of a gas mixture equals the sum of the partial pressures that make up the mixture."

Some "light" reading for you..

Dalton's law - Wikipedia

en.wikipedia.org

I'm over simplifying this here but partial pressure refers to the pressure (maybe it would help you to think of this as ratio?) of a specific gas in a total mixture. For example, air is a mixture of 20.9% oxygen, 78% nitrogen and ~1% trace gases. Each of these gases exerts a partial pressure. That would add up to 1.0

So if you use regular air as an example at sea level these are the partial pressures of the gases in air.

.209 oxygen
.78 nitrogen
.01 trace gases
---------
1.0

This is important in diving terms because the partial pressure of gases changes the shallower or deeper you go.

Regarding your question about oxygen. Yes partial pressure also comes into play it's simply easier to a calculate as it's a single gas. Oxygen is 100% oxygen (I could be pedantic and say that it's technically 99.999% but let's skip that for now).

At sea level the partial pressure of oxygen is 1.0. That's it. There are no other gases in the mixture.

EDIT: Updated to add an explanation of gauge pressure vs. absolute pressure.

 

[PeterRabbit]

Ok thanks everyone for the explanations, and I will keep doing a lot of reading on it so I understand it better. I just paid for 4 more speciality courses, Nitrox is one of them. Trying to pass off my Nitrox quickly because I might go to Cayman Brac in February with my local dive shop, and the resort has free Nitrox upgrade and it would be a shame if I couldn't take advantage of it. So I'm studying through the Nitrox course now, and my instructor is going to try and get me certified on it before I go. Even if I pass it, I will only use the Nitrox if I feel 100% comfortable using it, and I fully understand what's involved in it.

 

[macado]

100% Take a nitrox course. Nitrox is an awesome and very helpful course. If you have a good instructor they will explain partial pressures a bit more and why they are important in diving. I assume the eLearning will also do a good job in explaining this.

In essence, nitrox is a math course. There are no special skills required to breath it. You just need to understand there is effectively a depth limit depending on the mixture you are using. If you are doing a lot of diving in Cayman Brac then it will totally be worth it to have nitrox training.

Asking questions is good. Far too many people are afraid to ask questions and just accept what their instructor tells them or worse, they assume their instructor is right when they are actually wrong. Believe me, there are plenty of people who dive and do not understand partial pressures.

 

[MarkA]

Partial pressure is a scientific term. Someone can also probably explain this far better them me but partial pressure is the pressure exerted by an individual gas in a mixture. You are correct that the pressure contained within a container is the total pressure however each specific gas in the mixture also exerts a partial pressure.

In your car tire example the total pressure may be 40psi as you stated but the each gas in the mixture has a partial pressure that makes up the total pressure.

Let's think about this in terms of your tire pressure example. 40psi is approximately 2.7atm.
(40 psi) / (14.7 psi/atm) = 2.7atm

Where did I get 14.7psi? The weight of the atmosphere at sea level exerts a pressure of 14.7 pounds per square inch of force or 14.7psi. We call this 1 atm (atmosphere of pressure).

partial pressure = (total absolute pressure) × (volume fraction of gas component)

At 2.7atm or 40psi here are the partial pressures of air in your tire.
.209 x 2.7 = .5643 partial pressure of oxygen
.78 x 2.7 = 2.106 partial pressure of nitrogen
.01 x 2.7 = 0.027 partial pressure of trace gases
-------------------
This adds up to 2.697atm (2.7) or 40psi. The total pressure in your 40psi tire.

Think of Dalton's Law (actually called Dalton's law of partial pressures) which states that: 'The total pressure of a gas mixture equals the sum of the partial pressures that make up the mixture."

Some "light" reading for you..

Dalton's law - Wikipedia

en.wikipedia.org

I'm over simplifying this here but partial pressure refers to the pressure (maybe it would help you to think of this as ratio?) of a specific gas in a total mixture. For example, air is a mixture of 20.9% oxygen, 78% nitrogen and ~1% trace gases. Each of these gases exerts a partial pressure. That would add up to 1.0

So if you use regular air as an example at sea level these are the partial pressures of the gases in air.

.209 oxygen
.78 nitrogen
.01 trace gases
---------
1.0

This is important in diving terms because the partial pressure of gases changes the shallower or deeper you go.

Regarding your question about oxygen. Yes partial pressure also comes into play it's simply easier to a calculate as it's a single gas. Oxygen is 100% oxygen (I could be pedantic and say that it's technically 99.999% but let's skip that for now).

At sea level the partial pressure of oxygen is 1.0. That's it. There are no other gases in the mixture.

In the tyre example. The 40PSI would be gauge pressure, so would need to add atmospheric pressure to get absolute pressure.

 

[macado]

In the tyre example. The 40PSI would be gauge pressure, so would need to add atmospheric pressure to get absolute pressure.

Thanks! I was trying not to explain gauge pressure vs. atmospheric pressure to make it less confusing but you are correct. I edited my above post to add a brief explanation.

 

[inquis]

Even if I pass it, I will only use the Nitrox if I feel 100% comfortable using it, and I fully understand what's involved in it.

Good choice on the nitrox class. That uses elevated oxygen fractions: typically 32% or 36% compared to the 21% in normal air. (Thus the "enriched" part of the EAN moniker.)

Because it's richer, you won't have to worry about a PO2 going too low, but you will have to worry about it going too high. Since the fraction doesn't change as you breath a tank down, only the ambient pressure can increase the PO2. This leads to a maximum safe ambient/total pressure and therefore a maximum depth. Always analyze your tank to find the O2 fraction contained, compute the Maximum Operating Depth (MOD), and write that on a piece of tape stuck to the tank for reference.

Just stay above the MOD, and you'll be good to go.

To tie this back to your original question, your tissues absorb nitrogen at a rate dependent on the partial pressure of nitrogen. Since the oxygen fraction is higher, the nitrogen fraction and therefore it's partial pressure must be lower. Slower nitrogen absorption means you can a) stay down longer for the same DCS risk, or b) lower risk for the same bottom time as air, or c) some combination of the two.

 

[PeterRabbit]

Good choice on the nitrox class. That uses elevated oxygen fractions: typically 32% or 36% compared to the 21% in normal air. (Thus the "enriched" part of the EAN moniker.)

Because it's richer, you won't have to worry about a PO2 going too low, but you will have to worry about it going too high. Since the fraction doesn't change as you breath a tank down, only the ambient pressure can increase the PO2. This leads to a maximum safe ambient/total pressure and therefore a maximum depth. Always analyze your tank to find the O2 fraction contained, compute the Maximum Operating Depth (MOD), and write that on a piece of tape stuck to the tank for reference.

Just stay above the MOD, and you'll be good to go.

To tie this back to your original question, your tissues absorb nitrogen at a rate dependent on the partial pressure of nitrogen. Since the oxygen fraction is higher, the nitrogen fraction and therefore it's partial pressure must be lower. Slower nitrogen absorption means you can a) stay down longer for the same DCS risk, or b) lower risk for the same bottom time as air, or c) some combination of the two.

One thing I'm curious about too, is once I open the tank valve and start breathing Nitrox for the first time, does it make you feel any different when you start inhaling it? Like does it make you feel like you are light-headed, hyperventilating like you're almost getting too much air, any dizziness, or the such? Or you don't really notice any symptoms at all?

 

[MarkA]

Thanks! I was trying not to explain gauge pressure vs. atmospheric pressure to make it less confusing but you are correct. I edited my above post to add a brief explanation.

Understood.

I was attempting to write a simple concise response when you posted yours. I was struggling.

Feel lucky to have been trained with the metric system where bar is the default measure of pressure. Makes things a little easier.