The Low Setpoint myth (or misunderstanding)

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Hello,

It is pretty much what the others have said in various ways.

Distilling it down, what your instructor has displayed is a profound lack of understanding of the difference between inspired fraction of oxygen and inspired pressure of oxygen. From a physiological perspective, the body only cares about the inspired pressure of oxygen. It does not care how you achieve it.

For example, we function well with an inspired pressure of oxygen of 0.21 atm. You can get that breathing air (21% oxygen) at the surface (1 atm ambient pressure), or you could achieve it breathing 1% oxygen at 200m (21 atm ambient pressure). I would not recommend the latter in diving for other reasons, but from an oxygenation perspective you would be fine. Put another way, 1% oxygen breathed at 21 atm pressure is not a hypoxic gas. It would be, however, if you tried to breathe it at the surface!

So yes, as you descend on a functioning constant PO2 rebreather the inspired fraction of oxygen falls, but the inspired pressure of oxygen remains constant.

Simon M
 
WTF over?! That dude's an INSTRUCTOR?! That's a scary level of misunderstanding.
 
Hello,

It is pretty much what the others have said in various ways.

Distilling it down, what your instructor has displayed is a profound lack of understanding of the difference between inspired fraction of oxygen and inspired pressure of oxygen. From a physiological perspective, the body only cares about the inspired pressure of oxygen. It does not care how you achieve it.

For example, we function well with an inspired pressure of oxygen of 0.21 atm. You can get that breathing air (21% oxygen) at the surface (1 atm ambient pressure), or you could achieve it breathing 1% oxygen at 200m (21 atm ambient pressure). I would not recommend the latter in diving for other reasons, but from an oxygenation perspective you would be fine. Put another way, 1% oxygen breathed at 21 atm pressure is not a hypoxic gas. It would be, however, if you tried to breathe it at the surface!

So yes, as you descend on a functioning constant PO2 rebreather the inspired fraction of oxygen falls, but the inspired pressure of oxygen remains constant.

Simon M

Dear Simon,

I feel honoured to receive a reply from you. (As well as from the others by the way). Thank you for confirming physics didn't fail me, I didn't fail physics and that I didn't lost my sanity in the process.

Paul
 
I've been using the forum for reading up on stuff, but this is my first post. I hope it counts.

So a bit of background about myself: I've been diving for 10 years. I'm a PADI MSDT & SSI Instructor, SSI Twin Fundamentals and Limited Trimix trained. Currently doing Normoxic Trimix OC (SSI). I am also Recently did my TDI CCR Trained Helitrox Diluent Decompression Diver (so Air Diluent + Trimix) on an Dive Rite O2ptima. I am building hours and have a love hate relationship with the unit. Might change to a JJ but that is out of the scope of my story here.

All the smart and experienced people here have given you the right answer, but I'll just add a +1 from a fellow newbie.

As was mentioned upthread, assuming that your eCCR keeps working, the problem that you have if you don't remember to switch to high setpoint at depth is that you load inert gas much quicker, so your NDL drops and your deco obligation rises.

I switch at the bottom since PO2 spikes on descent, so no need to risk riding a high PO2 on the way down, that segment doesn't add much to your deco. Maybe your instructor was thinking of the risk of a quick ascent while diving at low setpoint? Reading that formula that you referred to suggests that there was a confusion between PO2 and FiO2? Was your instructor telling you to set high setpoint at the surface (impossible to maintain)? Or at some shallow depth on descent?

I'm also TDI Helitrox, and I have about the same number of dives as you but not your credentials. I have a love-love relationship with my JJ. Feel free to message me if you want me to try to talk you into buying one… :)
 
Low Set Point = 0.5
High Set Point = 1.3 (but not important for now).

Now to finish off, I do understand that
1. Diving at Low Set Point at depth, doesn't make any sense and;
2. If you (for what ever reason) make a rapid ascent the PPO2 will potentially drop to dangerous levels

It sounds like either your instructor has a fundamental misunderstanding regarding how the machine he is teaching about functions or there might be some sort of misunderstanding going on. From your discussion it sounds like you are diving your unit in automatic mode and allowing the computer to maintain a constant set point. So even without a switch from low set point to high, the set point controller is going to do its job and maintain a constant set point. Even if it is low, .5 in your example, it is plenty to keep you conscious. It is only the partial pressure of inspired oxygen that is important in determining if you would have a problem maintaining consciousness and .5 is a little over double what you see at the surface breathing air.

Of course .5 is not terribly efficient from a decompression standpoint so your no decompression limit would be shorter than it would be had you made the switch (or you decompression longer). One real concern is that if you have cut tables that factor in a set point switch and then don't make the switch you do run a serious risk of DCS.

As to your second point, making a rapid ascent is going to cause a dangerous drop in PPO2 regardless. It is just straight physics. Starting from a higher set point at the bottom would afford you a little more leeway but not by much in a practical application.
 
As to your second point, making a rapid ascent is going to cause a dangerous drop in PPO2 regardless. It is just straight physics. Starting from a higher set point at the bottom would afford you a little more leeway but not by much in a practical application.

Practical is a relative term, though, isn’t it? If you are at 1.3 at 165 feet (6 ATA), and you ascend without adding any O2, you will be a bit above 0.21 at the surface. In practice somewhat less due to the O2 you metabolize in ascent (so the faster the ascent, the greater the surface PO2 in that “experiment”) but still there is an advantage of high setpoint at depth...
 
All well understood. This was clearly about during the decent / bottom part of the dive. Really we are all on the same page here. My best guess is that they also are mixing Partial with Fraction.

As mentioned earlier I am really happy to see I wasn't in the wrong. Currently contemplating on the idea for him to draw it out for me and see where that brings us. On one side I think it's important to have a healthy discussion about it, but as we are all well aware, some people are difficult to have a debate with.

Dived yesterday and logged 7 hours now. :D

I don't think I need much convincing in getting a JJ
 
Just back from some nice diving and, I had a chance to have a long conversation with the certifying instructor on the matter. I must say; it was an insightful one, and I'll start of by stating I miss understood him.

How you may ask? Well I am not 100% sure about it myself. I am pretty sure it was a combination of a plain misunderstanding and just wrongful information that unfortunately does float out there. After the conversation I'm confident enough to say, this wrongful information didn't come from him, yet it somehow ended up in the story. I did speak with more people on the matter...

Now apparently, my query, made it's round on the internet and came right back at me. Which is absolutely fine, as I can honestly say I was seeking clarification, nothing more.

During our conversation I asked him - just as I asked in the opening post - where this confusing from other rebreather divers comes from. Since these aren't his students it's a wild guess. I am sure some interesting discussions will follow.

I will also post an update in the opening post because I do think this information should be available.

Question everything!

Happy diving.
Paul
 
This thread really upsets me. I truly hope you misunderstood your instructor and not that he has such a poor understanding of Dalton's law and basic physiology.
I teach my students to stay on low setpoint for rapid descents because as the ambient pressure increases the partial pressure of the diluent increases. If you were descending from the surface to say 5 ata the oxygen in the air alone would result in a PPO2 of 1.05 ata. That is a very breathable gas. I usually start a dive at 0.5 at the surface and will descend as fast as my diluent will alow. Even though my setpoint is set low my PPO2 will climb well beyond that and I will often have to slow my ascent to prevent spiking betong 1.6.
Depending on the dive profile I will leave my setpoint low and manually adjust my PPO2 to around 1.3.
 
If your set point is at .5, it's .5 on the surface and .5 at 100 feet. If you have air dil, your PPO2 if you do a dil flush is .84. Hypoxia occurs below .21. So you're not going do die from insuffient oxygen, but you'd be better off with a tank of air and a reg in terms of decompression obligation.
 
https://www.shearwater.com/products/perdix-ai/

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