Diving at Altitude, Compensate or not?

[amascuba]

Every recreational and even some technical training agencies that I’ve encountered state that when diving at altitude you should compensate for the dive, with an altitude corrected depth (aka deeper depth), because it will take your body longer to off-gas absorbed nitrogen in your body then it would at sea-level.

Being the type of person that I am, I have questioned this logic and I wonder if there is more to diving at altitude than the general one liner that the training agencies have in their training manuals. Here are my thoughts:

Is the material based on people diving with analog depth gauges that don’t compensate for altitude? I’m aware that some depth gauges are calibrated to sea-level and as you rise in altitude their is less pressure on the gauge to make the needle move. So when making a dive at altitude it will take more pressure on the gauge to make the needle move from the 0 mark to a depth. This is because depth gauges are nothing but pressure gauges. Using this concept I can see having to compensate for altitude, but how many analog depth gauges do you see in this day in age? Why wouldn’t the training agencies update the material to compensate for updates in technology?

Is it as simple as the training agencies make it out to be? Does diving at altitude take longer for your absorbed nitrogen to off-gas?

Here are two scenarios:

1. A person lives at an elevation of 4000ft above sea-level and makes a dive at that same elevation to 80 feet. Does this diver need to compensate for altitude? If so, why?

If you don’t compensate for altitude the depth is: 80ft.

If you do compensate for altitude the depth is: 92.8ft.

I’m no expert, but my answer would be no. At sea-level the pressure of the atmosphere exerts 14.7psi (pounds per square inch) of pressure on the body. We know that the air we breath consists of approximately 20.9% oxygen, 78.2% nitrogen, and .9% other various gases. That means of the 14.7psi, 3.0723psi of the pressure is oxygen and 11.4954psi of the pressure is nitrogen. Our body is continually equalizing itself with the pressure of the atmosphere. At 4000ft in elevation the atmospheric pressure exerted is less than 14.7psi because you have less atmosphere above you. If the person lived at 4000ft above sea-level and also dives at that elevation then the person would off-gas at the same rate at a person at sea-level because their body would be acclimated to it’s surrounding atmospheric pressure.

2. A person live at an elevation of 3000ft above sea-level, but drives to an elevation of 5000ft above sea-level. Between driving, gearing up, and getting into the water it takes the person on average of 4 hours to submerge himself. Does this person need to compensate for altitude?

This is an a complex question and to which I wouldn’t necessarily feel comfortable answering with any certainty. That is because I do not know the rate as which the body will equalize itself with the surrounding pressure of the atmosphere. As a person drives from 3000ft. to 5000ft. the body is equalizing itself with the surrounding pressure. The same is true with the time spent at the dive site gearing up right up until the person submerges himself into the water. The qustion would be, has the body had adequate time to equalize? That maybe a complex answer as everybodies physiology is different.

My opinion is that a person shouldn’t necessarily need to compensate for altitude if the person waits an acceptable amount of time at the altitude of the dive site before getting into the water. What is an acceptable amount of time? I suppose you could use the current decompression models available to help determine this, because your body is essentially off-gassing until it equalizes with the surrounding pressure. Which model do you use? Do you use the tables with 24 hour models? 12 hour models? 6 hour models? 2 hour models? Being that decompression theory is far from an excact science I would determine that it would be wise to use model(s) that you are comfortable using as well as exercising good judgement.

Questions, Comments, Flames, Additions?

 

[TeddyDiver]

Yes! Compensate!
It doesn't take longer to off-gas to same level (PN2) as it would at sea-level, but becouse of the lower atmospheric pressure surfacing with same residual nitrogen at altitude would cause higher risk to DCS. Just like to take a flight after a dive...

PS edit Up here have been two (in my knowledge) incident where divers after some days of diving wrecks in Narvik started their journey home. The road climbs to about 600m (2000ft) and both got the hit (otherone already at 300m)...

 

[amascuba]

Yes! Compensate!
It doesn't take longer to off-gas to same level (PN2) as it would at sea-level, but becouse of the lower atmospheric pressure surfacing with same residual nitrogen at altitude would cause higher risk to DCS. Just like to take a flight after a dive...

Can you elaborate more than that? Because that's basically all the information that the training agencies material gives and the point of this post.

key words: lower atmospheric pressure and residual nitrogen.

Because of the lower atmospheric pressure your body starts to equalize to the surrounding atmospheric pressure, which means lower partial pressure of nitrogen in a solution in your body before you enter the water. Whether the body is equalized or not is a different story, depending on how long you have been at a given elevation.

If your body has equalized with the surrounding atmospheric pressure then how does not compensating for altitude make your chances of DCS higher? Logically thinking it would have the same effect as diving at sea-level. This example was already givin in the first scenario that I layed out.

 

[amascuba]

PS edit Up here have been two (in my knowledge) incident where divers after some days of diving wrecks in Narvik started their journey home. The road climbs to about 600m (2000ft) and both got the hit (otherone already at 300m)...

What were the dive profiles like? There could be so many other contributing factors to them getting a DCS hit that it's not funny. It doesn't mean that driving to altitude didn't contribute partially or fully to their DCS hit, but it also doesn't mean that it did contribute. Besides, How would compensating for altitude in this situation made a difference? They may have dove a little shaller or made the dives a little shorter, but it still wouldn't make a difference if they decided to drive home to an altitude with out properly off-gassing.

How long did they wait before they made their trek home to altitude?

 

[Charlie99]

Going all the way back to JS Haldane, much of decompression models and algorithms are based upon pressure RATIOs. The "Cross" correction of the US Navy is nothing more than adjusting depths such that ratios between the surface pressure and tissue pressures stay constant. Or to put it another way, if you were to convert the tables such that everything was in atmospheres of pressure instead of fsw, then the altitude correction is simply using the reduced pressure at altitude as the "atmosphere" rather than the standard 14.7psi / 33fsw used at sea level.

Take your example of treating 80' dive at 4,000 feet as a 92' dive. At sea level, 80fsw is 2.4atm. If you reduce the atmospheric pressure by 16%, then that same 80fsw would become about 2.8 of the local atmospheres. One way to account for that would be to use whatever depth is 2.8atm on a sea level table --- about 92'. This perhaps is an oversimplistic way of looking at things, but it is a pretty reasonable approximation of what one needs to do in order to dive at the same risk levels whether at sea level or altitude.


RGBM model adjusts this a bit more, but in the more conservative direction --- i.e. RGBM model requires adding in more conservatism at altitude that do the classic neo-Haldanian models. I briefly looked at VPM and noted that it had significant altitude correction, but don't remember whether they are more or less than the 4% of depth per 1000' altitude Cross correction.


Charlie Allen

 

[TeddyDiver]

Because of the lower atmospheric pressure your body starts to equalize to the surrounding atmospheric pressure, which means lower pressure of nitrogen in a solution in your body before you enter the water. Whether the body is equalized or not is a different story, depending on how long you have been at a given elevation.

If your body has equalized with the surrounding atmospheric pressure then how does not compensating for altitude make your chances of DCS higher? Logically thinking it would have the same effect as diving at sea-level. This example was already givin in the first scenario that I layed out.

Bcs the only way you could have advantage from the altitude and lower pressure comes from quicker of-gassing BETWEEN the dives (due lower PN2in the air)

However during the dive nitrogen dilutes in the body in HALFTIMES so the PN2 of the surface becomes a fraction just in minutes after starting the dive and after making, lets say a dive to 60ft 30min you would have the same amount of nitrogen as with same dive at sea-level.

While ascending imagine when you are at the same pressure as at the surface at sea (finished dive at sea level) you are still a some feet below from the surface at altitude. If you could end your dive there (having a chamber) no sweat, but you got ascending left..

 

[TeddyDiver]

What were the dive profiles like? There could be so many other contributing factors to them getting a DCS hit that it's not funny. It doesn't mean that driving to altitude didn't contribute partially or fully to their DCS hit, but it also doesn't mean that it did contribute. Besides, How would compensating for altitude in this situation made a difference? They may have dove a little shaller or made the dives a little shorter, but it still wouldn't make a difference if they decided to drive home to an altitude with out properly off-gassing.

How long did they wait before they made their trek home to altitude?

Not wery good profiles. Other one had a rush ascend bcs of malfunc BC inflator, no symptoms at surface, about 30h before the trek. Other one a bit sawtooth profile with the last dive and after few hours the trek. Anyway the point is diving altitude has same problems very similar to flying after dive..

 

[amascuba]

Bcs the only way you could have advantage from the altitude and lower pressure comes from quicker of-gassing BETWEEN the dives (due lower PN2in the air)

If the PPN2 in your body is equalized with the atmospheric pressure how is this the case? My off-gassing between surface intervals at this point would be the same as if I was at sea-level.

For Example:

At sea-level my PPN2 at the surface is roughly .79ATA. If I did a 60ft dive my PPN2 at 60ft is roughly 2.27ATA. Lets say I do my dive within the NDL time limits and make a proper ascent and safety stop. Once my body off-gases the excess nitrogen and equalizes itself to the atmospheric pressure the PPN2 returns to .79ATA.

Now lets say that your diving at an altitude and the atmospheric pressure is .8ATA instead of 1ATA. Let's say that your body is equalized to the atmospheric pressure. Your PPN2 prior to your dive would be . 63ATA. Let's say that that we did the same 60 foot dive. Your PPN2 on this dive would be 2.07ATA. After diving within NDL, doing a proper ascent, and safety stop I surface and do the same surface interval to off-gas the excess nitrogen. After the body off-gasses then the PPN2 would return to .63ATA. In essense it's the same as doing a dive at sea-level except your PPN2 at the surface before the dive is different at altitude than it is at sea-level.

However during the dive nitrogen dilutes in the body in HALFTIMES so the PN2 of the surface becomes a fraction just in minutes after starting the dive and after making, lets say a dive to 60ft 30min you would have the same amount of nitrogen as with same dive at sea-level.

You wouldn't have the same amount of nitrogen in solution during a dive at altitude because the atmospheric pressure also contributes to nitrogen loading during the dive. If the atmospheric pressure is less then the nitrogen loading would be less. The nitrogen would also offgas at the same rate at altitude or at sea-level.

While ascending imagine when you are at the same pressure as at the surface at sea (finished dive at sea level) you are still a some feet below from the surface at altitude. If you could end your dive there (having a chamber) no sweat, but you got ascending left..

Same difference here. Your body is equalized the PPN2 before you dive. You make your dive where the PPN2 in your body equalizes to the water pressure + atmospheric pressure. You end your dive and your body again equalizes to the atmospheric pressure. Pretty simple thinking IMHO. You may have a PPN2 of .79ATA at a given depth at an altitude, but if your body was already acclimated the a PPN2 of .63ATA before the dive, how does that make it any more dangerous to not altitude correct? You will surface from the dive and your body will again acclimate itself to the .63ATA PPN2.

 

[amascuba]

Not wery good profiles. Other one had a rush ascend bcs of malfunc BC inflator, no symptoms at surface, about 30h before the trek. Other one a bit sawtooth profile with the last dive and after few hours the trek. Anyway the point is diving altitude has same problems very similar to flying after dive..

Not in all cases. Take my first scenario. The body is acclimated to an altitude before the dive, therefore after the dive it would be like diving at sea-level as far as nitrogen on-gassing and off-gassing. The big question would be how long does it take for the body to become acclimated to the altitude? You could follow the no fly rules I suppose.

 

[Charlie99]

Amascuba, go back a few posts and read my post about RATIOs of pressures.

Let's take the example where the diver is still acclimated to sea level, and then does a dive where the atmospheric pressure is 0.8ata.

When he ascends at the end of the dive, he will reach 1ata when still 6.6 feet beneath the surface. The standard sea level dive tables would apply for doing a dive and then ascending only as far as 6.6 feet depth. He would have to do additional decompression to be able to have the deco ceiling clear up to the surface, which is 0.8ata.

When you go through the calculations, it turns out that if you assume that the diver has acclimated himself to the altitude, then the pressure RATIOS stay the same only if you adjust the effective depths you use on the tables by the same ratio as the local atmospheric pressure to standard sea level pressure.

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Although initially, it may seem to be unrelated, think about how long you could dive at sea level provided that, instead of returning to the actual surface, that you would return to a bell or diving chamber or habitat that was at 10'. Any dive that would have the deepest required deco stop at 10' would be become an NDL dive if you returned to the habitat.

The relationship between those excursion-from-10'-habitat dives and normal sea level dives is pretty the same as the relationship between sea level dives and altitude dives.

Or to put it another way, you could use sea level tables at altitude, but only if you ended the dive by going into a habitat submerged a few feet so that its internal pressure is 1 ata. You would than have to do additional decompression to be able to ascend from the habitat up to the local atmospheric pressure.

Charlie Allen