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!!!

 

[Blackcrusader]

My dive computer was 100% set to AIR, not Nitrox. My gas O2, was set to 21%. My MOD PPO2 was set to 1.40, and MOD set to 188ft. I never went deeper than 64 feet.

When diving at above sea level you need to account for this. You need to read up on altitude dives for your DC.

At sea level your pressure will be 1015 more or less depending on weather. Low pressure storms can change it.

This shot taken at 3275m or 12220ft altitude.

 

[PeterRabbit]

Did you set your altitude for a 6000ft dive?

When diving at above sea level you need to account for this

I read the owners manual and it says there is no need to adjust altitude on the Peregrine TX, and that it would do so automatically.

 

[Blackcrusader]

I read the owners manual and it says there is no need to adjust altitude on the Peregrine TX, and that it would do so automatically.

Yup edited that out of my post.

 

[Blackcrusader]

Did you change to a fresh water setting?

I read the owners manual and it says there is no need to adjust altitude on the Peregrine TX, and that it would do so automatically

 

[PeterRabbit]

Did you change to a fresh water setting?

No I didn't, so perhaps that had something to do with it too.

 

[Endler's]

Shearwater Perdix did same at Lake Tahoe. Found older thread on how to defeat the warning for Perdix:

Error low pp02 on recreational air, Post #2.

Should be similar for Peregrine, if not exact.

 

[Blackcrusader]

Why would I have gotten a PPO2 alarm? Sorry for the seemingly stupid, and ignorant question from a new SSI student.

This is a good question to ask as a new diver when you do not understand. It's a learning experience to dive at altitude when most people dive at sea level. Also to understand your DC and when to use fresh water or salt water settings.

Shearwaters offer a lot of information and yes, RTFM and doing a lot of research on these things if good for you.
I just went diving with a friend from the UK who is a Padi instructor and also TDI ANDP deco mixed gas certified. He bought a Perdix AI. He had never used it before we went to Maldives together this month.

I showed him how to use the Tec Mode and introduced him to gradient factors setup, M values, updating to latest firmware, Surf GF and other things he never had on his previous DC. He's an experienced diver over 1500 dives. We dive togther usually two trips a year around 5 - 6 weeks in total so around 65 - 70 dives in a year together. So we do not accept when a dive op tries to separate us as dive buddies which has happened. We also use the same dive settings. One thing in Maldives, we hear other divers alarms going off. We get back on the boat. I told my dive buddy don't worry those were not our audio alarms. He asks why not? I tell him the perdix AI does not have audio alarms only the on screen warnings for low NDL or depth which you can set. On many dives we had NDL minutes showing as we used GF 45/95 and other divers DC's were in deco already.

We did some light back gas deco dives and I could show him on the Perdix where the graph showed being in deco and how we cleared our deco oblligation before we got to our safety stop and why the Perdix in tec mode does not tell you to do a safety stop. You manage that yourself and the Perdix shows you are clear to ascend to surface. There are many other settings as to how the shearwaters work. So you need to download and study the PDF and asl read these forums. Let others with Peregrines and other shearwater devices chime in.

Asking questions is the correct thing to do.

 

[inquis]

Perhaps the alarm was not necessarily "false", but at the same time, I am going to assume the alarm was given in an effort to be "conservative"

Correct. The computer is always computing the partial pressure of oxygen (often abbreviated ppO2 or PO2). This is equal to the ambient pressure times the current fraction of oxygen you've set in the computer. Higher altitudes have lower ambient pressures, and therefore lower PO2s for a given fraction.

Why does a dive computer care? A PO2 that is too low is bad, as you'll pass out. As a diver ascends, the ambient pressure and therefore the PO2 of the breathing gas drops. The default warning is set to 0.18 to give a diver enough time to halt their ascent before the PO2 drops too far and they pass out & drown. This typically only happens when going well past the recreational diving depth limit (a.k.a., technical diving). In order to continue the ascent, these technical divers would switch to a richer gas (higher O2 fraction, raising PO2 to a safe level) to continue the ascent. You're not doing that type of diving, so no, you don't have to worry about it.

Fortunately, the warning level is configurable. The PO2 while breathing air (fraction of 0.2095) at 8000 ft (0.752 bar ambient pressure) is 0.158. If you set the Minimum PO2 on the computer to 0.15, you'll not receive the warning.

 

[rongoodman]

Al

Also, one other question I had was, does it matter at what altitude the cylinder was actually filled? For example, I know for a fact that the cylinders I was breathing off of, were filled at an altitude of 2800 sq ft, then transported up the the altitude we were diving at. Does that have absolutely anything to do at all with the percentage of oxygen inside the cylinder? What if the cylinder had been filled at the same altitude as the dive site, would anything have changed?

The percentage of O2 in air does not change, it's the same on the summit of Everest as at sea level. What changes is the pressure, and thus the amount of O2 present, which is why we talk about partial pressures and not percentages.

 

[PeterRabbit]

The percentage of O2 in air does not change, it's the same on the summit of Everest as at sea level. What changes is the pressure, and thus the amount of O2 present, which is why we talk about partial pressures and not percentages.

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?