High altitude decompression diving

[boulderjohn]

Does anybody have good resources for deco at elevation? I have targets in the 8000' (2500m) range and trying to further my understanding before going for it. I'm not aware of any altitude deco classes and resources are limited so I would appreciate any suggestions.

You opened a 9-year old thread in which I was participating. I have done lots of high altitude deco dives since then, and I have some articles that may help on my resource page.

What elevation range has Buhlman actually been tested to?

After the Swiss military had some problems diving at altitude, Buhlmann worked on altitude adjustments. They then tested it successfully at Lake Titicaca (12,500 feet).

What changes if any to mixes and MODS do you make? I'm not asking for a class on nitrox here. I mean for a dive at 8000' , which roughly corresponds to an atmospheric pressure of .75atm, would you increase the MOD of 50% deco gas to 80' or would you bump up the FO2 to ~55% so the MOD remains the same? My gut tells me to not mess with mix or depths for consistency but wondering if anyone does this for optimization.

In our diving in New Mexico, we do adjust MODs based on altitude, but primarily for bottom mixes. That is critical because we are often mixing gases in relatively primitive conditions, and we don't get the mixes we want every time. So once we get a blend well mixed and analyzed, we will confer with my altitude adjusted MOD table to make sure the mix we have is good for the planned depth. For deco gases, we are more likely to stay closer to conventional depths.

When do you start worrying about a travel gas taking into account decreased PO2 associated with decreased ambient pressure? At 8000', 21% is down to about the .16 PO2 range, 18/45 is about a 13.5 PO2 by my math. I'd be on CCR anyways but I feel like that should be considered.

We often use a combination travel/early deco gas mix, something like 21/35. We will start the dive with an 80 of a mix like that, go to back gas at around 180-200 feet, and then switch back to that at the same depth for the ascent. That has an additional benefit of saving a lot of money--it's way cheaper than the deep back gas.

My primary warning on diving at 8,000 feet is not to be in a hurry to get in the water. The US Navy tables have a bold faced, bright red type warning about diving above 10,000 feet, and they say the reason is acclimatization. I believe the fatality in Peru was primarily due to that reason. Give your bodies plenty of time to adjust to that altitude before you dive.

 

[lostsheep]

You opened a 9-year old thread in which I was participating. I have done lots of high altitude deco dives since then, and I have some articles that may help on my resource page.

After the Swiss military had some problems diving at altitude, Buhlmann worked on altitude adjustments. They then tested it successfully at Lake Titicaca (12,500 feet).

In our diving in New Mexico, we do adjust MODs based on altitude, but primarily for bottom mixes. That is critical because we are often mixing gases in relatively primitive conditions, and we don't get the mixes we want every time. So once we get a blend well mixed and analyzed, we will confer with my altitude adjusted MOD table to make sure the mix we have is good for the planned depth. For deco gases, we are more likely to stay closer to conventional depths.

We often use a combination travel/early deco gas mix, something like 21/35. We will start the dive with an 80 of a mix like that, go to back gas at around 180-200 feet, and then switch back to that at the same depth for the ascent. That has an additional benefit of saving a lot of money--it's way cheaper than the deep back gas.

My primary warning on diving at 8,000 feet is not to be in a hurry to get in the water. The US Navy tables have a bold faced, bright red type warning about diving above 10,000 feet, and they say the reason is acclimatization. I believe the fatality in Peru was primarily due to that reason. Give your bodies plenty of time to adjust to that altitude before you dive.

I’m kinda sorry about bumping the 9 year old thread but I think it’s better than starting new ones.

The links on your page are good

I understand why acclimatization prior to diving is needed but do we have any data on actual times needed? Next time I ascend I’m going to watch my shearwaters tissue graphs, maybe that will provide a sense of time needed. My gut tells me it can’t be that long because the magnitude of the pressure gradient just isn’t that high compared to tissue loading from an actual dive. I.e., sea level to 8000’ is a change from 1atm to .75atm whereas a deco dive is clearly many times greater.

To that acclimatization point, has anyone given any thought to the difference between elevation induced hypoxia and hypoxic mixes? My personal anecdotal experience is class 4 climbing at 12,500’; that’s a PO2 of about .12 and yet I was still able to function. I’d be paying attention to a 12% mix. Does slowly decreasing PO2 decrease the deleterious effects of hypoxia?

 

[boulderjohn]

I understand why acclimatization prior to diving is needed but do we have any data on actual times needed? Next time I ascend I’m going to watch my shearwaters tissue graphs, maybe that will provide a sense of time needed. My gut tells me it can’t be that long because the magnitude of the pressure gradient just isn’t that high compared to tissue loading from an actual dive. I.e., sea level to 8000’ is a change from 1atm to .75atm whereas a deco dive is clearly many times greater.

I really don't have any specific information. It might be useful to make contact with mountain climbers about this.

I visited Mount Cotopaxi in Ecuador years ago prior to diving the Galapagos Islands. We got out of a bus at about 14,000 feet, and I was immediately winded. Two women on the bus next to us passed out when they entered the parking lot. We then hiked up a switchback trail for another thousand feet of elevation. For the first half of that hike, I had to rest every hundred feet or so. By the end I was closer to normal walking.

At that altitude we were at the building where the mountain climbers stayed for acclimatization before ascending to the peak. I forget how many hours they typically stay there.

In the Peru diving incident, the divers went in as soon as they could after reaching that level (by horseback). Their problems on the dive were caused primarily by the unexpected rapid rate at which they went through their air. I have to believe that was an effect of acclimatization, but I don't know for sure.

To that acclimatization point, has anyone given any thought to the difference between elevation induced hypoxia and hypoxic mixes? My personal anecdotal experience is class 4 climbing at 12,500’; that’s a PO2 of about .12 and yet I was still able to function. I’d be paying attention to a 12% mix. Does slowly decreasing PO2 decrease the deleterious effects of hypoxia?

At the altitude at which I have done almost all of my high altitude deco diving (about 5,000 feet), the normal air PPO2 is below the supposed threshold for hypoxia. If my Shearwater is set to air, it gives me a hypoxia warning while I am breathing the surface air. I changed the threshold level for hypoxia warning so I didn't have to look at that warning, but that is all I do.

If you are diving deep with 12% oxygen in your mix, by the time you are at 20 feet, you at at an acceptable PPO2. We do our final stop on pure oxygen, meaning it doesn't really matter at what depth above the MOD we do it. We have no concerns about doing our air breaks on bottom gas by staying near the 20 foot level.

 

[lostsheep]

FWIW I have dived in Tahoe (6200’) a decent number of times but it wasn’t until recently I started doing deco dives there. My recent outings brought up a lot of questions I hadn’t really considered before because I know more now then when I was just doing NDL dives. I have other targets that are even higher and am trying to get my facts straight before I do something stupid and end up hurting myself; help can be a long ways off in the high sierras.

 

[Norwegian Cave Diver]

If my Shearwater is set to air, it gives me a hypoxia warning while I am breathing the surface air. I changed the threshold level for hypoxia warning so I didn't have to look at that warning, but that is all I do.

How do you change the hypoxia warning? I have a Petral 2. I am not deep tech trained.

 

[inquis]

I understand why acclimatization prior to diving is needed but do we have any data on actual times needed? Next time I ascend I’m going to watch my shearwaters tissue graphs

There's two types of acclimatization. One is tissue off-gassing to the reduced surface pressure (i.e., starting "clean"), as you seem to be considering here -- only hours for a rapid ascent/drive. The other is physiological changes to better deal with low PO2 -- about a week, judging from soccer tournaments in my youth. Teams coming from sea level to almost 7000 ft were at a distinct disadvantage.

The latter is the one that hits tourists on mountain top visits. However, you said you've been fine at 12,500 ft (which is PO2 of 0.131 by my math). Using that as the threshold, a 12% BO gas (if I understand your intent correctly) won't work above 12 ft at an 8000 ft site (interesting coincidence). Striving for an "always acceptable" initial BO choice would be preferred, but I don't know if your configuration would allow that.

 

[boulderjohn]

How do you change the hypoxia warning? I have a Petral 2. I am not deep tech trained.

Off the top of my head, I don't recall how I did it, and I don't have it with me to check at the moment.

 

[inquis]

How do you change the hypoxia warning? I have a Petral 2. I am not deep tech trained.

Settings, Advanced Configuration 2, OC Min PO2

 

[lostsheep]

There's two types of acclimatization. One is tissue off-gassing to the reduced surface pressure (i.e., starting "clean"), as you seem to be considering here -- only hours for a rapid ascent/drive. The other is physiological changes to better deal with low PO2 -- about a week, judging from soccer tournaments in my youth. Teams coming from sea level to almost 7000 ft were at a distinct disadvantage.

The latter is the one that hits tourists on mountain top visits. However, you said you've been fine at 12,500 ft (which is PO2 of 0.131 by my math). Using that as the threshold, a 12% BO gas (if I understand your intent correctly) won't work above 12 ft at an 8000 ft site (interesting coincidence). Striving for an "always acceptable" initial BO choice would be preferred, but I don't know if your configuration would allow that.

Given that we can choose PO2 levels of our gasses I think for divers the primary concern is acclimatizing WRT off gassing nitrogen.

If anyone wants to report back on what their Shearwater tells them on a trip from sea level to elevation I would be very interested in what data we can get, I will do the same the next time I go to the mountains.

I don't want to insinutate that my past experiences in the mountains will have ANY affect on my judgement when it comes to selecting O2 levels in my diving gasses; I just find the discrepancy interesting.

 

[inquis]

for divers the primary concern is acclimatizing WRT off gassing nitrogen.

The impact of that will depend on the ascent/setup time and intended profile. The fast tissues will have acclimated after a 2 hr drive from 0-8000 ft and a 2 hr setup time. The slowest tissue has only off-gassed 21% of the way to the 8000 ft pressure. Whether that residual loading plays a role in the subsequent dive just depends.

I would suggest looking at this in the Subsurface planner using water equivalents at an 8000 ft elevation. The pressure change from sea level pressure to 8000 ft is equivalent to 8.75 ffw, so plan a 1440 minute (24 hr) dive at 9 ft. That saturates tissues to the sea level pressure. Tack on the ascent to surface in the profile in accordance with the drive time. Add a second dive to represent your actual dive starting at an appropriate time after surfacing (representing setup time).

You can examine the impact of non-acclimation on the deco dive by adjusting the start time. (Push it out a few days for the "clean" version and save a Copy to work with.)

My understanding is that the Shearwater will assume you're acclimated (without an actual prior dive that it's still tracking). As such, it can be useful to bump up the conservatism of the clean/acclimated dive to be comparable to the partially acclimated profile. Use that adjusted GF on your actual dive.

Normally, I'm fairly well acclimated as I arrive the day before (or actually descend in elevation), but you now have me considering a dive at substantially higher altitude that would be a drive & dive situation. Thanks for the thread bump!