My AN/DP/Helitrox course

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Coming from someone who is curious about taking AN/DP/Helitrox at some point... Is the recommended ppO2 of 1.2 only for tech or is the general consensus that all diving should start switching to 1.2? What do you seasoned techies do when diving a benign dive above NDL?
 
@rjack321 and @kensuf

I was writing some long defense of 1.4 and wanting a better explanation of why 1.2 and bam there it is. The TDI books just refer to dialing back the PO2 if expecting heavy work or multiple dives over multiple days - no real tie of PO2 and He and Ox Tox........

Interesting reading and a good enough reason for the decrease.

I have been told by physicians that are significantly smarter than me that there has never been a CNS event at a PO2 of less than 1.3. I know of at least two fatalities involving heavy exertion / work and a PO2 of 1.4 -- one of them was a friends wife.

Here's a couple of other reasons to run a working PO2 of 1.2 with a MOD of 1.4.

1. The oxygen sensor that you use to analyze your gases is really only good to within 1 or 2% either direction. It's an analog chemical fuel sensor, not a digital device. So what you think is 32% could potentially be 30% or 34% (realistically it'll be closer to 31-33%, but...). That means if you think you're diving 32% at a 1.4 MOD (110'), there is a possibility you're really diving 34%, giving you a more elevated PO2. Now what happens if you have a tits up problem that causes you to drop beyond your planned working depth? You have no buffer.

2. O2 toxicity is exacerbated by CO2 (heavy work) and cold temperatures. Why push doses at your working portion of the dives? Oxygen Toxicity - StatPearls - NCBI Bookshelf

3. There's an insignificant decompression advantage. Buying into the "best mix" argument for a brief moment, the decompression advantage of using a "working PO2" of 1.4 over a working PO2 of 1.2 is short. Conducting a 30 minute dive to 140' using 26% (1.4 PO2) with 50% deco gives a total runtime of 59 minutes. Conducting the same dive on 23% (PO2 of 1.2) gives you a runtime of 63 minutes. This is with a GF 40/75 (pretty conservative). The total difference between the two schedules is 4 minutes, but now with the 23% you also have a safety margin on O2 exposure in case your analysis falls within that grey area and a deeper MOD in case something goes tits up and you wind up accidentally hitting a contingency depth.

4. Buying into the "standard gas" argument for a brief moment, 21/35 would give you a deeper MOD, significantly less narcosis, and guess what, it also has a runtime of only 63 minutes using the same profile and same GF's. Less than 5 minutes more deco for less narcosis, less work of breathing, less O2 exposure, and a safety margin MOD in case something goes south, sign me up.

The idea of running higher PO2's comes back to the idea of shorter decompression exposures and trying to shoot for the quickest way to get out of the water. This is partially mindset, so change your mindset -- your deco isn't a penalty for doing a dive, it's part of the dive itself.
 
Coming from someone who is curious about taking AN/DP/Helitrox at some point... Is the recommended ppO2 of 1.2 only for tech or is the general consensus that all diving should start switching to 1.2? What do you seasoned techies do when diving a benign dive above NDL?

For recreational exposures, "it depends" but a max PO2 of 1.4 is a hard limit. I've been that ******* that asked for 28% in the keys because I was doing the Spiegel.

Remember, O2 exposure has a time component as well. Am I doing a short reef drift dive? Am I doing a long heavy current swim? Is it cold?
 
Coming from someone who is curious about taking AN/DP/Helitrox at some point... Is the recommended ppO2 of 1.2 only for tech or is the general consensus that all diving should start switching to 1.2? What do you seasoned techies do when diving a benign dive above NDL?

On open circuit dives, AN/DP/Helitrox offers a max certification depth of 45m / 148ft the recommendations are:

1. Use standard mixes, 21/20 is for example the recommended one from TDI, 21/35 is also a good one.
When choosing a gas also have in mind the gas density. Above 5.2 g/lt is too much for the novice tec diver so a bit of more helium isn't a bad idea.

2. Dive max ppO2 1.4bar

3. Deco max ppO2 1.6bar

4. Deco gas EAN50. For a dive to 45m / 148ft it makes no big difference to deco on O2

For deeper dives, things are different. The title of this discussion is still AN/DP/Helitrox right?
 
your deco isn't a penalty for doing a dive, it's part of the dive itself.
This. Oh heavens, this. I really wonder how many lives have been damaged because of not getting this.

Thanks Ken.
 
The working portion of a dive is not necessarily the bottom part. I've done several dives where the deco portion is much more physically demanding than the bottom portion.
Example: Empress or Ireland -> no current on the wreck at depth but 2.5 to 3 knot current above 50 feet. Drift deco here would be a VERY bad idea as fog is common and the boat would never see your SMB. So you have to come up the line.
 
If you want to run a ppO2 of 1.4 for bottom gas knock yourself out. I gave you a dramatic case of how that can bite you in the butt. I was taught 1.2 for the working phase over 15 years ago (and multiple times since then). 1.6 for deco/at rest.

21/17 @ 150ft (ppO2 1.16) for 20mins has 41 mins runtime (GF 50/80 on EAN50)
25/17 @ 150ft (ppO2 1.38) for 20mins has 39 mins runtime

Your choice saves 2 minutes on the ascent but comes with more risk.


This is where the real value of this discussion comes together. Seeing people attacking a problem from different angles and providing valuable perspective gives me, a novice tech diver, the insight that I am craving. Having been preparing for my AN/DP/Helitrox course for the last year, my first class was last night and it was awesome. Stuff like the above is exactly Why I come to school board.
 
It seems to me that when one gets into the tech realm, there's a hazy line between standards and personal preferences. Some agencies still teach deep stops. Experienced divers' personal preferences are often grounded in literature they have read and their own experiences. Your instructor's and Richard's "preferences" versus TDI's "standards" .... It's all grounded in more than whim and all worth taking in ... at some point.

this is great advice.
It’s good for me. It’s good for all scuba divers. It’s good for America.

The reality is that there is some science, some pseudoscience, some anecdotal evidence, and some preferences involved with this endeavor we called technical diving. We are fooling ourselves if we believe it’s 100% science. The fact is we just don’t have enough data to support all of the theories, and many standards are based on theories that are based on interpretation of a paucity of data. A good instructor will talk about standards versus reality versus what is known versus what is believed, and the fact that the hard scientific truth lies someplace in this hazy milieu—one that we certainly don’t see clearly through.

Acceptance of this uncertainty is critical in diving. As a newbie myself, and as a scientist, or More accurately, a physician who relies on science, it is hard for me to do a thing without “knowing it with complete certainty”, However the thing itself is shrouded in uncertainty. I have to accept that. And it’s corollary is that other individuals will have different standards, preferences, ideologies, theories. I have to accept that as well.
 
The working portion of a dive is not necessarily the bottom part. I've done several dives where the deco portion is much more physically demanding than the bottom portion.
Example: Empress or Ireland -> no current on the wreck at depth but 2.5 to 3 knot current above 50 feet. Drift deco here would be a VERY bad idea as fog is common and the boat would never see your SMB. So you have to come up the line.

Dives like this are inherently problematic as your "what if" plans suck.

The best choice is to just accept that foggy days mean you cancel and go someplace else. But but but we would almost never get to dive it... The charter was $$$... We drove a long way... yup yup yup. Treat it like a winter ascent in the Adriondacks or White Mountains or ice climbing in the Gaspe - sometimes you get all prepped to go and you bag it because it's not good today. And there are more bad days than good annually - that's the price of admission.

You can also bring a scooter to partially mitigate your exposure (tell your spouse its a "safety" device). And/or bring a deco mix like EAN50 that keeps your ppO2s modest once you're in the worst of the current. But you are still screwed if you get blown off the line for whatever reason. And even a minor problem like a leaking power inflator or free flowing reg there is a good chance you are going loose the upline - nevermind a major issue like an air share or lost deco gas. When sheet goes big time sideways its almost a whole bunch of cascading failures sending you down the rabbit hole.
 
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