Ascent To Altitude From Santa Rosa

[J-Vo]

Sunday I did a 90 minute dive with average depth of 30 ft at ~5500' elevation. I finished the dive with GF99 on my Perdix at about 40%. By the time I was home an hour later GF99 was 0%. About 3 hours after surfacing I rode in a car up to ~8000' elevation while monitoring my Perdix closely. GF99 never moved above 0% and the TLBG for all of the compartments stayed well below ambient pressure the entire time although I did notice them increase slightly.

 

[boulderjohn]

I have written a brief blog about altitude diving and diving to altitude after diving. You can find it here.

The main purpose of the blog is to introduce two longer and far more detailed articles, which you can find on my resources page here.

 

[J-Vo]

This weekend a group of us where diving in Santa Rosa at Rock Lake. On Sunday we successfully completed a 20 minutes at 275' for a total of around a 110 minute dive.

When I left Santa Rosa, my Shearwater displayed my GF99 (Leading Supersaturation) at 29% theoretical maximum with with probably the 5th slowest compartment leading. Upon cresting the hill just past Las Vegas, NM on I-25 it displayed GF99=16% with the 3rd slowest compartment leading. I did not note the surface time at this point. As I was driving through Raton, I watched the GF99 display drop form 1 to 0% with the 2nd slowest compartment leading. This was at about 3:20 surface time. When I crested Raton Pass, the GF99 was still 0% at 3:29 surface time.

As Raton Pass is higher elevation than Denver this should be the worst case supersaturation for the drive home and based on my Shearwater should have been perfectly safe, even after a deep Trimix dive at Rock Lake.

That said there is no science showing the body reacts the same on the surface as in the water and everyone has to make their own personal decision on the risk. I personally was breathing Oxygen for about 85% of the drive from Santa Rosa to Raton Pass.

Edit: Shearwater assumes you are breathing air when not in dive mode. This means the Oxygen I was breathing during my drive was not taken into account for the stated gas loading above.

 

[boulderjohn]

This weekend a group of us where diving in Santa Rosa at Rock Lake. On Sunday we successfully completed a 20 minutes at 275' for a total of around a 110 minute dive.

When I left Santa Rosa, my Shearwater displayed my GF99 (Leading Supersaturation) at 29% theoretical maximum with with probably the 5th slowest compartment leading. Upon cresting the hill just past Las Vegas, NM on I-25 it displayed GF99=16% with the 3rd slowest compartment leading. I did not note the surface time at this point. As I was driving through Raton, I watched the GF99 display drop form 1 to 0% with the 2nd slowest compartment leading. This was at about 3:20 surface time. When I crested Raton Pass, the GF99 was still 0% at 3:29 surface time.

As Raton Pass is higher elevation than Denver this should be the worst case supersaturation for the drive home and based on my Shearwater should have been perfectly safe, even after a deep Trimix dive at Rock Lake.

That said there is no science showing the body reacts the same on the surface as in the water and everyone has to make their own personal decision on the risk. I personally was breathing Oxygen for about 85% of the drive from Santa Rosa to Raton Pass.

This is excellent information! Thanks!

Just a note for people who have not studied decompression diving--it includes a surprising contrast to recreational diving. In recreational diving, when you surface from a dive and go through a surface interval, all your tissues are off-gassing. That is not true for a serious decompression dive like the one Josh completed that day. On his last two decompression stops, he was breathing pure oxygen for longer than most recreational dives. As a consequence, his fastest tissues had nitrogen saturation lower than ambient pressure. That means his fastest tissues actual on-gassed during the first part of the surface interval. As he noted above, it is the slower tissues that are most at risk and which must be protected.

 

[boulderjohn]

Another factor that I've learned during my research is acclimation. The Indian and Bolivian Navies reported that acclimation was an important factor for high elevation diving.

I know this is 4 years old, but I was recently reminded of this post. I am now very much of the opinion that acclimation is a critical factor in high altitude diving. I have recently advised people planning to dive at lakes over 10,000 feet to arrive well ahead of time and take your sweet time getting in the water.

 

[CycleCat]

I would assume in my case descending from 7600 at home to dive at 5000 for a dive would provide some margin of safety vs. going to a higher altitude.

 

[rsingler]

Sunday I did a 90 minute dive with average depth of 30 ft at ~5500' elevation. I finished the dive with GF99 on my Perdix at about 40%. By the time I was home an hour later GF99 was 0%. About 3 hours after surfacing I rode in a car up to ~8000' elevation while monitoring my Perdix closely. GF99 never moved above 0% and the TLBG for all of the compartments stayed well below ambient pressure the entire time although I did notice them increase slightly.

Since this thread has come alive again, I think it's worth commenting on how Shearwater computes GF99. I cannot get Shearwater to comment on this in their forum, perhaps for liability reasons. But I believe that GF99 is computed against the barometric pressure which is registered at the beginning of the dive. That means that during the drive to altitude, GF99 is dropping because of offgassing compared with dive altitude. However, the gradient might actually increase in some slower compartments if dissolved N2 could be compared with ambient at the higher altitude reached by car.
I think, upon reading the manual carefully, that there is a workaround. If you turn your Shearwater off, the computer tracks time and remembers your calculated tissue saturation. When you then turn your computer back ON, the computer registers a new barometric pressure consisting of the highest pressure obtained in the last 15 minutes (which it samples every 15' even when "off"). Comparing tissue saturation against a new barometric pressure at higher altitude while driving, may show a higher GF99 than had the computer been left on all the time. This is the GF99 that should concern us during the drive.
One way to test this (which has been on hold since COVID) would be to look carefully at your tissue graph after a week's diving at a resort. After waiting your required 24 hours before flight, there might still be some visible N2 in a slow compartment. If you turn on your Shearwater after your plane reaches cabin altitude, I believe that bar of the tissue graph might show a slight increase. I also believe that your wife's Shearwater, which you kept on during takeoff and ascent, will not show the increase, because the computer is still comparing against the ground reference baro pressure.

I hope to soon evaluate this better by taking a pair of computers from a pressure pot dive, to altitude immediately after the dive when even faster compartments are still relatively full. I think that the computer turned on at altitude will show the bump in GF99 that is most equivalent to diver risk.

 

[boulderjohn]

I would assume in my case descending from 7600 at home to dive at 5000 for a dive would provide some margin of safety vs. going to a higher altitude.

Yep.

 

[Scott]

I am now very much of the opinion that acclimation is a critical factor in high altitude diving. I have recently advised people planning to dive at lakes over 10,000 feet to arrive well ahead of time and take your sweet time getting in the water.

Good reminder John.
Early in my diving career we frequented a lake at 9600’ with a pass slightly above 10,000. It was not uncommon for us spend the night before onsite.
I know a number of divers that arrive at this site and enter the water as soon as they can get geared up.
I’m sure our overnight stays were more conservative than necessary. Still a lot of unknowns, just as how long is taking their sweet time.

 

[boulderjohn]

Still a lot of unknowns, just as how long is taking their sweet time.

Yep. We know a relative thimbleful about high altitude diving. I met one of the true experts, a man who does not dive at those altitudes but consults on high altitude decompression for NASA and the Pentagon (U2 flights). He had never planned a true high altitude decompression dive, but he indicated that there were only a handful of people in the world with the ability to do so.

That is for decompression diving. The only high altitude studies I know of for standard diving algorithms were by Buhlmann, and there has been precious little research done on that. If you go to a number of desktop software programs and plug in a very high altitude dive, they will spit out a profile for you. I wonder on what research that profile is based.