The Low Setpoint myth (or misunderstanding)

[paulemous]

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.

I've got over 1100 dives. I teach and do weekly fun dives and/or Deco/Rebreather dives.

I have access to highly trained and knowledgeable people (really, this is important to keep me sane, you will understand later...) and never stop to enjoy enriching myself with more material and not just sticking with the standard course material.

So why all this info you may ask?

Well, without cutting the story short: I ran into a discussion with my instructor about swapping from Low to High Set Point during diving (decent), and the potential danger of NOT switching to the High Set Point. The "danger" was you breathing a hypoxic mix, essentially making it lethal to breather on the Low Set Point at depth.

Again, I might misunderstood what he tried to explain. But from my recollection it was as follows:

Example data: (for calculation purposes, no debate on what the setpoints should be now...)
Low Set Point = 0.5
High Set Point = 1.3 (but not important for now).

So at the surface you are breathing a PP02 of 0.5 Bar = 50% O2

All good so far.

Now when descending the ADV is open and we are leaving it open. Loop volume gets kept replenished by the ADV, happy days. We drop to 20m, 3 ATA.

So what ever reason we don't swap to our High Set Point and this is where the discussion started. I was told that not swapping to your High Setpoint is dangerous and due to the low O2 in the mix (0.5/3 = 16.667%) = hypoxic, you fall a sleep, end of the story.

In order to fix that we swap to High Set Point. Fixed. Simple right?

Now I had difficulty swallowing this information (this way) but during the discussion I was being stone walled: "Low Setpoint and going deep and you die from hypoxia". PERIOD. I wanted to finish the training and build my own experience and extent my knowledge, so during the training I swapped to High Setpoint no questions asked.

After my certification, I dived with other rebreather divers at this dive centre. The Low Set Point argument came up again. Those rebreather divers are under the same impression that diving at Low Set Point at depth: "Makes you go to sleep and you die" again no discussion possible; Reasoning "The instructor said so"

At this point you might understand my frustration about this. Because if you ask me, this is utter [fill in blank].

I don't want to state the obvious here, (unless I am completely wrong here, please tell me - HOWEVER I want to understand where this misinformation comes from). But electronic closed circuit rebreathers now a days maintain a constant PPO2, so regardless at what depth the PPO2 will be remained.

In other words diving on PPO2 = 0.5 at 20m is safe. diving on PPO2 = 0.5 at 40m is also fine (disregarding PPN2 when DIL is air, making you narked and giving you a lot of deco. etc etc.)

Now going back to the 20m Low Set Point 0.5. (Not switching to High Set Point) eventually your loop volume would decrease (due your body that is metabolising the O2 and CO2 being scrubbed) and the ADV would add Diluent (21%) so your controller would read somewhere in the region of 0.63 (0.21*3). Since its not dropping the solenoid wont fire O2 and you would burn Dil instead of O2. Although not an efficient way of diving this won't kill you right?

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

I look forward to hear your ideas and opinions about this.

 

[John Bantin]

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

I look forward to hear your ideas and opinions about this.

Back in the early days of leisure diving CCR, I took an Inspiration to Cocos. (Now as an early user of all CCR including prototypes, I made plenty of mistakes that I freely published but got pilloried for later! This was one of them.) The first Inspirations needed a manual set-point switch (thanks possibly to me, auto set-point switching became standard soon after!) . On one dive, I thought I had switched over to the high set-point but distracted by a whaleshark and my camera I obviously didn’t visually check the display. After a long dive at around 30 (100ft) deep, I started to ascend and it was only when I came to switch back to the low set-point that I realised I'd done the whole dive at 0.7bar. This meant my deco tracking (on a then brand new Buddy Nexus) was totally wrong. I switched over to 100% and continued on the CCR at the surface until they could drop me in somewhere at 6m without any current where I did an hour, thus avoiding a DCS incident. So diving at the low set-point means you absorb more nitrogen than is necessary.
It doesn't kill you then but it could kill you later!

 

[paulemous]

Back in the early days of leisure diving CCR, I took an Inspiration to Cocos. (Now as an early user of all CCR including prototypes, I made plenty of mistakes that I freely published but got pilloried for later! This was one of them.) The first Inspirations needed a manual set-point switch (thanks possibly to me, auto set-point switching became standard soon after!) . On one dive, I thought I had switched over to the high set-point but distracted by a whaleshark and my camera I obviously didn’t visually check the display. After a long dive at around 30 (100ft) deep, I started to ascend and it was only when I came to switch back to the low set-point that I realised I'd done the whole dive at 0.7bar. This meant my deco tracking (on a then brand new Buddy Nexus) was totally wrong. I switched over to 100% and continued on the CCR at the surface until they could drop me in somewhere at 6m without any current where I did an hour, thus avoiding a DCS incident. So diving at the low set-point means you absorb more nitrogen than is necessary.
It doesn't kill you then but it could kill you later!

Completely understood. But as per my instructor I already died of low o2 to begin with....

 

[kensuf]

PO2 is a function of pressure and fraction of O2 in the mix. If your pressure drops significantly (rapid ascent), and for whatever reason you can't get additional O2 in the loop (loop is full?), you'll have a PO2 drop. But if your PO2 is stable at 0.5, you're not at risk of hypoxia, you are just going to absorb more inert gas than you would if you were running a higher setpoint.

 

[RainPilot]

I think the only scenario where a low set point would lead to hypoxia is if one did a rapid ascent without keeping up. In this case there would be less leeway to get it wrong. That however is not what the instructor was talking about IMO

I think this is a case of an instructor passing along info they received and maybe didn’t quite understand or it was not understood by their instructor and so on.

A LOT of broken telephone stuff in RB instruction for sure. Always question everything. Everyone gets to be wrong sometimes no matter how good.

 

[paulemous]

I think the only scenario where a low set point would lead to hypoxia is if one did a rapid ascent without keeping up. In this case there would be less leeway to get it wrong. That however is not what the instructor was talking about IMO

I think this is a case of an instructor passing along info they received and maybe didn’t quite understand or it was not understood by their instructor and so on.

A LOT of broken telephone stuff in RB instruction for sure. Always question everything. Everyone gets to be wrong sometimes no matter how good.

Quite possible. Not digging any graves here, but that is pretty weird that the simple understanding of Low Set Point is so miss understood...

 

[tbone1004]

real simple.
If the rebreather is maintaining a setpoint at or above .16 *preferably .18*, you will not fall asleep, end of discussion.

I don't know why your instructor seems to think this is different than on open circuit *ppO2 of .5 is equivalent to air at 80ft*, or why he seems to think that we live at the surface breathing at .21 for our entire lives but somehow more than doubling that will make you fall asleep, but it appears he is lacking in the basic fundamentals of diving physiology and it is apparent to me that he would benefit from retaking a basic rebreather course from a QUALIFIED instructor....

sorry, I'm in a bit of a pissy mood right now, and hearing information like what you have unfortunately witnessed is a MASSIVE problem in the technical diving world where there are instructor trainers who don't actually have a clue about what they're teaching

 

[broncobowsher]

Sounds like you have a better understanding of partial pressures than the local instructor, who has also taught the others around you.

Go back to open circut basics. Run some depth profiles looking at PPO2 at depth for air, a couple of nitrox blends (say 32 and 50%) and pure oxygen. Also run that again with a hypoxic trimix blend, say 10%. Now see where your setpoints would fall into these mixes. I think that will give a clean understanding of what is happening verses what was interputed and regergated as teaching.

I have issues with a lot of education which is nothing more then regurgatating what was said without actually understanding what is actually happeneing. Including professors that don't know the subject but claim to. And they are the professor, so they are always correct.

 

[paulemous]

Sounds like you have a better understanding of partial pressures than the local instructor, who has also taught the others around you.

Go back to open circut basics. Run some depth profiles looking at PPO2 at depth for air, a couple of nitrox blends (say 32 and 50%) and pure oxygen. Also run that again with a hypoxic trimix blend, say 10%. Now see where your setpoints would fall into these mixes. I think that will give a clean understanding of what is happening verses what was interputed and regergated as teaching.

I have issues with a lot of education which is nothing more then regurgatating what was said without actually understanding what is actually happeneing. Including professors that don't know the subject but claim to. And they are the professor, so they are always correct.

I am really happy to see you guys are all agreeing with me.

 

[RyanT]

And just to add to what has been said... Assuming you maintain the same fraction of O2, a surface PO2 = 0.5 will NOT give you 0.167 at 3 atm. It gives you 0.5 * 3 = 1.5.