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!!!

 

[inquis]

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?

Nope. The reality is, even normal air has far more oxygen molecules than we can use in a single breath.

 

[rongoodman]

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?

No different than air.

 

[macado]

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.

Everything is certainly easier in bar and meters. I agree.

 

[boulderjohn]

Yeah and I'm still trying to wrap my head around why the term "partial" is used.

I will attempt a very simple explanation.

The air around us has pressure, and at sea level we say that pressure is equal to one atmosphere. That air that makes up that one atmosphere has several parts--principally nitrogen, oxygen, and argon. The percentage of each part of that total pressure is the partial pressure--oxygen 0.21, nitrogen 0.78, and argon 0.01. Those partial pressures together add up to one atmosphere.

In salt water, every 33 feet equals another atmosphere of pressure, and when you breathe, you need more molecules to fill the lungs, so the total pressure increases with depth. At 33 feet of salt water, you are now under two atmospheres of total pressure. Because the percentages of the gases in the air remain the same, their partial pressures also double. Oxygen's partial pressure is now 0.42.

At Homestead Crater, the atmospheric pressure is roughly 0.80 because of the altitude. That means the partial pressure of oxygen is 0.8 * 0.21 = .168. Your computer considers that to be too low, and it gives you a warning. I have the same problem with my Shearwaters, so I go into the settings and change the setting for that warning to a lower partial pressure.

Even though the atmospheric pressure at Homestead is less, the pressure of the water doesn't change. For fresh water, every 34 feet is equal to one full atmosphere at sea level. That means that at 34 feet, the total pressure is now 1.8 rather then 2 (0.8 for the air and 1 for the water).

 

[rongoodman]

At Homestead Crater, the atmospheric pressure is roughly 0.80 because of the altitude. That means the partial pressure of oxygen is 0.8 * 0.21 = 1.68. Your computer considers that to be too low, and it gives you a warning.

You meant .168.

 

[boulderjohn]

You meant .168.

Yes. Fixed it.