Altitude and diving

[AnthonyLM]

Hi,

I am getting my PADI OW cert in a few weeks and have a few dives planned in April when we head out to Maui. During that vacation we will also visit a ZipLine tour. Our current plan is

Tuesday - 2 tank boat dive
Wednesday - Catamaran/snorkel/Snuba (down to 20' for 15-20 mins)
Thursday - ZipLine tour
Friday - 2 tank boat dive

Since my daughter is a junior we are restricted to 40' on our dives and honestly I have no issue if we don't even get that deep and just focus on getting more experience with our gear at a shallower depth. I've reached out to the ZipLine company to find out what their altitude is. According the the dive company I should not go above 1,000' with 18 hours of diving.
So based on our current itinerary we would be well beyond 18 hours from the Tuesday boat dive.
Would the Snuba session on Wednesday be considered a dive?
Would our Friday dive be cutting it too close to the ZipLine tour if it is indeed above 1000'?

Thanks in advance for your help!

- Anthony

 

[tbone1004]

I believe that the restriction for altitude is generally accepted at 2,000ft, and above that has a 12 hour wait per DAN. You can call to verify, but I would take their advice over that of the dive operator.
I see no issues with that itinerary regardless of what the altitude is. You aren't snorkeling at night, so you're going to be done by 5pm, and I assume you're not getting up at the crack of dawn to go ziplining, so you're probably going to have 18 hours between those events regardless.

SNUBA even for 2 hours is not considered a "dive", because of how shallow it is. You could presumably stay there for 24 hours and immediately fly with no issues.

If you are concerned still, then just call DAN's hotline and ask them, they're quite helpful. I would recommend having DAN insurance if it will make you feel better to know that even if there is a problem, they'll cover the costs.

 

[kmarks]

The Friday dive is of no concern because you only need to worry about altitude after diving, not before.

Wednesday's snuba can sort of be considered a dive (for pressure/residual nitrogen purposes, not for logging), but it's basically just an extended safety stop. As a prospective new diver, you may have seen some brief mentions of things like Haldane's approximation, which was later refined by several different researchers and studies, most notably those of Workman and Bühlmann. Basically, JS Haldane approximated that a person could become fully saturated with N2 at any depth/partial pressure and be perfectly fine coming up to a depth/partial pressure of half that. This research led to the development of decompression tables. Other research reduced the ratio from 2:1 to about 1.5:1, which is why we do a safety stop at 15-20 feet. So doing snuba, even though it's on compressed air, at only 20 feet, basically doesn't count.

 

[boulderjohn]

Let's glance at the US Navy dive tables for a comparison.

If you dive to 35 feet for 32 minutes, you will end the dive in pressure group C.

The US Navy also has an ascent to altitude table. According to that table, a diver finishing a dive in pressure group C can immediately ascend to 8,000 feet. That diver must wait 1:48 to ascend to 9,000 feet and 6:23 to ascent to 10,000 feet.

 

[Lorenzoid]

Good to see you here, @boulderjohn . You have posted a lot of great information about altitude and diving over the years.

 

[AnthonyLM]

Thanks everyone for the feedback and info. The ZipLine company got back to me and we will be at about 700' above sea level. Based on that and the info here I feel pretty confident with our plan.

 

[spoolin01]

Another simplified way to look at it is that each 2000 ft change in elevation from sea level is about 1psi air pressure difference. So 1000 ft is not much compared to 14.7 psi per 33 foot in salt water, equivalent to just an extra foot or so in depth. That's just a pressure comparison. I don't know if there are any physiological wrinkles to complicate that analysis but I assume it's largely informative.

As to elevation changes before or after diving, if you are acclimated to high altitude prior to diving, wouldn't that also have to figure in to a pro forma analysis of NDL (of a sea level dive)?

 

[boulderjohn]

Another simplified way to look at it is that each 2000 ft change in elevation from sea level is about 1psi air pressure difference. So 1000 ft is not much compared to 14.7 psi per 33 foot in salt water, equivalent to just an extra foot or so in depth. That's just a pressure comparison. I don't know if there are any physiological wrinkles to complicate that analysis but I assume it's largely informative.

You make it seem so little by using terms not normally used in scuba. Diving at 6,000 feet is a mere 3 PSI less than sea level, so no big deal, right? Think of it in atmospheres and it sounds different. Diving at 6,000 feet is only 80% of atmospheric pressure (ATA) at sea level, and that is significant.

As to elevation changes before or after diving, if you are acclimated to high altitude prior to diving, wouldn't that also have to figure in to a pro forma analysis of NDL?

Here is a link to a resources page that has information on diving at altitude that might be helpful to you.
Deep Adventure Scuba Colorado Technical Diving - For Those Looking to Take Diving to a New Level

 

[lermontov]

Here is a link to a resources page that has information on diving at altitude that might be helpful to you.
Deep Adventure Scuba Colorado Technical Diving - For Those Looking to Take Diving to a New Level

some good stuff here

 

[spoolin01]

You make it seem so little by using terms not normally used in scuba. Diving at 6,000 feet is a mere 3 PSI less than sea level, so no big deal, right? Think of it in atmospheres and it sounds different. Diving at 6,000 feet is only 80% of atmospheric pressure (ATA) at sea level, and that is significant.

Here is a link to a resources page that has information on diving at altitude that might be helpful to you.
Deep Adventure Scuba Colorado Technical Diving - For Those Looking to Take Diving to a New Level

I was coming at it the other way. What's the difference in NDL for a diver who lives at sea level, vs a diver who lives at 2000 ft. and descends to sea level for a dive? The comment that a trip to altitude prior to a dive might not matter (briefly, one day earlier in this case) got me wondering about the importance of baseline altitude acclimation for NDL estimations. Surely you can't just assume the same baseline at sea level for everyone, independent of what altitude they are acclimated to?

The kind of question the OP asked - going to Hawaii then traveling to 1000 or 2000 ft elevation at some short interval after diving - comes up a lot, often with real concern expressed. If one can look at two options for a 30 minute dive - 50 ft vs 52 ft and make the reasoned assessment that the DCS risk is not much different between the two, does it not similarly follow that a 30 minute dive to 50 ft, followed by a fairly prompt drive to 2000 ft elevation is closely in the same ballpark of indifference?