Diving at Altitude, Compensate or not?

[Gene_Hobbs]

I'll just throw out a few more references worth reading. All of these are made available by the Uhdersea and Hyperbaric Medical Society (UHMS) can be downloaded from the Rubicon Research Repository (RRR). http://archive.rubicon-foundation.org/ A search for "altitude" will yield 74 hits. Most seem to be abstracts or just related to altitude decompression sickness.

Diving at altitude: a review of decompression strategies.
Egi and Brubakk
Undersea and Hyperbaric Medicine, 1995
PubMed ID# 7580768, RRR ID# 2194
Abstract: Diving at altitude requires different tables from those at sea level due to the reduction in surface ambient pressure. Several algorithms extrapolating sea-level diving experimental data have been proposed to construct altitude diving tables. The rationale for these algorithms is reviewed together with the conservatism of the resulting tables and decompression computer outputs. All algorithms are based on the adaptation of critical tissue tensions to altitude. These are linear extrapolation (LEM), constant ratio translation (CRT), and constant ratio extrapolation (CRE) of maximum permissible tissue tensions (M values). Either new tables using the altitude-adapted M values were put forward or sea-level tables are to be used through an operation called correction. In this review it is shown that for a given set of M values, CRT and CRE give the same result for no-decompression-stop dives; they always yield more conservative results than LEM. When decompression stops are used, CRT is more conservative than CRE. When applied to different sets of M values, the conservatism becomes a function of bottom time, depth, and altitude. The analysis shows that the tables derived using CRT of U.S. Navy (USN) schedules and CRE Boni et al. tables give more conservative results than LEM Buhlmann tables for higher altitude, longer bottom time, and deeper dives. Aviation altitude exposure decompression sickness (DCS) data are also addressed to compare different model outputs. When applied to USN and Royal Navy tables, LEM yields an altitude DCS limit of 8,581 and 8,977 m, respectively. On the other hand, the altitude limit calculated using CRE applied to USN M values and LEM Buhlmann tables is found to be below 6,000 m.

The theory of high-altitide corrections to the U.S. Navy standard decompression tables. The cross corrections.
Bell and Borgwardt
Undersea Biomedical Research, 1976
PubMed ID# 1273981, RRR ID# 2748
Abstract: The theoretical basis for the Cross high-altitude corrections to the USN Standard Decompression Tables is derived. Providing corrections are made for depth and ascent rate and if no decompression stops are made, a dive at altitude can be transformed to a dive at sea level for which the theoretical tissue responses are mathematically similar to the altitude dive. The transformation fails if decompression stops are required due to the fact that the stop criteria used in the USN Tables do not obey the same rule of transformation. It is shown that the failure of the high-altitude correction is expected to be conservative.

Automatic compensation by capillary gauge for altitude decompression
Mackay
Undersea Biomedical Research, 1976
PubMed ID# 10897866, RRR ID# 2440
Abstract: According to simple theory, the indications of a capillary depth gauge are such that ascent rate and decompression-stop position are correct to give unaltered gammas (tissue overpressures) and a safe dive profile--independent of the density of the medium or of surface altitude--if position indications are used directly in a standard decompression table. The usual dial depth gauges must be doubly corrected for altitude but not medium before tables can be used, while distance-measuring systems should be corrected for both altitude and medium. The capillary-gauge dive profile is theoretically conservative when timed step decompression rather than continuous ascent is used if gamma increases with altitude.

Diving at diminished atmospheric pressure: air decompression tables for different altitudes
Boni, Schibli, Nussberger, and Buhlmann
Undersea Biomedical Research, 1976
PubMed ID# 969023, RRR ID# 2750
Abstract: Fifty subjects performed 106 simulated dives at a final ambient pressure of 0.7 at (3000 m above sea level). One hundred and forty-three subjects performed 278 actual controlled dives at altitudes 900-1700 m above sea level. From the experience of these dives, air-decompression tables for altitudes 0-3200 m above sea level were calculated. Tables up to 2000 m above sea level were tested on humans under wet conditions.

 

[rjack321]

If you drive up to altitude and make a SINGLE dive and then drive home you need FIVE TABLES.

Or you can just...
use nitrox,
avoid pushing no deco limits,
make reasonable ascents (with deep stops) on all your dives,
pad the stops on the altitude dives a little bit,
and I doubt you'd experience any higher incidence of DCS than the remainder of the sea level population.

This same basic issue has been debated ad naseum in the computer vs. no computer threads.

There's a wide window between "most assuredly safe from DCS" (actually that = not diving at all) and "that dive is definately going to bend you". Layering on vast quantities of highly precise calculations does not mean there's any additional accuracy to avoid DCS which is all about physiology and only a touch of physics.

 

[TheAvatar]

Well when you consider that tables have not been wet tested above 8000ft, I don't think your "just take some precautions and wing it" attitude is the best approach to a 10,000ft table dive with a 12,000ft post dive ascent. When you actually understand the science behind everything, it makes a lot more sense and your nonchalant attitude would be scary. Is it really that hard to do it the safe way? NO

We dive nitrox up here... we dive it as air while observing MOD. We do safety stops. And we follow the tables.

The only way to get to a chamber from here is to ASCEND ALTITUDE first.

 

[Dr Deco]

Hello NJMike:

Since you are driving downhill, you are increasing the pressure post dive. As you suggested, this should not pose a problem as far as DCS is concerned.

Dr Deco :doctor:

 

[rjack321]

Well when you consider that tables have not been wet tested above 8000ft, I don't think your "just take some precautions and wing it" attitude is the best approach to a 10,000ft table dive with a 12,000ft post dive ascent. When you actually understand the science behind everything, it makes a lot more sense and your nonchalant attitude would be scary. Is it really that hard to do it the safe way? NO

We dive nitrox up here... we dive it as air while observing MOD. We do safety stops. And we follow the tables.

The only way to get to a chamber from here is to ASCEND ALTITUDE first.

Where is "here". The 10k ft dive is extremely rare and the 12K pass to get home is even rarer.

Most altitude divers are at or going to 6-8k ft afterwards. That equal to about 7 ft of water. Not much.

If you are diving and doing dives/ascents in a manner which would get you bent at altitude you'd probably be bent at sea level too. Bad enough to go to the chamber?? Maybe, maybe not. Altitude would exacerbate your symptoms for sure. The issue here is that people assume their dives are "no deco" with a shallow safety stop vs. decompression to some extent on every dive. If you are treating your sea level dives conservatively then altitude is just (barely) a notch up in conservativeness.

Having a highly precise estimate of what is ultimately a coarse science is putting more faith in decompression science/understanding than is merited IMO.

 

[TheAvatar]

So are you saying that 6-8K it is ok to take a few precautions and pray but above that it isn't OK? Got any scientific studies to back that up?

10K table dive is a normal weekly dive here.

So your attitude is "the science is coarse, so lets ignore it and just take a few other precautions instead."

Our attitude is "the tables are untested theories based on good science, so let's follow them and take further precautions in case they are not conservative enough."

Food for thought, instantaneously going from 0ft to 18000ft can get you bent. I believe the USAF found that out. Haldane's theories agree. You can try to mentally minimize the issue by converting things to "mere" feet of water, but when you think that there is up to a 35% change in ambient surface pressure here versus what the tables were calculated for, it is a little differnet issue. It is the ratios that are relevant. USN calls for following these multiple tables as necessary.

You can scoff at altitude all you want but the physics behind the tables is sound enough to be paid heed and the uncertainy BEYOND following the tables is where you should apply your extra precautions.

Scientific theory supports following the tables. So do you have any good science or evidence to support ignoring them or just an attitude?

 

[O2BBubbleFree]

Okay, I'm still trying to digest all of this....

Here's a potential scenario for me, right after Christmas. I live in NJ at an elevation of approx 1000'. After Christmas (with all my new gear!) I drive down to Mt. Storm Lake in West Virginia, at an elevation of 3244' (if I can find a buddy).

I do 2 dives (water should be above 60 F) to a max of 60', then pack up and drive home.....downhill to 1000'.

Since I am not ascending after the dive, is this a problem?

Mike,

According to the rules I learned in my cert course, and based on the info you provided, your first dive will have a NDL of 27 minutes, and you will not be able to do a second dive.

The good news is that you are still above your recommended max depth for this altitude.

Your ascent should be slower than normal.

Your new recommended safety stop depth is 13'.

Like Dr. Deco said, descending after the dive is not a concern.

Having said that, please do not take my numbers and go dive it. I plugged in some numbers and applied some rules to illustrate that there is more to it than 'pad a little here,' and 'take it easy there.' I do not believe this is the place to educate a diver on using the tables and rules, so have been intentionally vague.

TheAvatar mentioned 5 tables, and I have no doubt that he is stating a fact. The course I took had two tables and 8 rules, which are probably a simplification of his tables.

 

[TheAvatar]

i didn't mean to imply 5 tables are *always* needed.... for the method i was speaking of there are of course exceptions

if the predive ascent is not significant, the predive ascent table probably isn't needed and you therefor don't need to calculate effective residual time prior to the first dive... two tables gone (you'll of course still need the surface interval/residual nitrogen table if doing repet dives) here is an online version of the table we use nto determine the initial repet group for ascent to altitude within 12hrs prior to the initial dive http://www.tpub.com/content/diving/NavyDiving/css/Dive_Man_ChangeA_269.htm

on the same token, if you aren't ascending more than 1000ft within 24 hours of the last dive, you probably don't need the post dive ascent to altitude table (which i found online here http://www.ndc.noaa.gov/pdfs/AscentToAltitudeTable.pdf )

if i dive a lake close to me once in a day, i may only need two tables: altitude-depth adjustement table and a normal dive table

lots of "may" and "probably"... interested folks can take a class and they'll learn all the necessary details and the when/why/how

 

[rjack321]

You haven't stated where these normal 10k dives are or the 12k passes you are routinely driving over.

Sure an immediate ascent to 18k can get you bent. But rather than taking the extreme and saying that illustrates your point maybe use a more logical approach.

(I took an altitude class once, when I lived in WY. I compare the advocated methods from that to current recommended ascent rates and practices, ugh. My current sea level practices are about where Padi was at altitude 10 years ago.)

Just look at something along the lines of the previous example - a 3500 ft dive. That's about 88% of 1 ATM. 0.88 x 1013 mB = 891 mB So I've plugged a pressure of 891mB decoplanner.

Required stops with 30/85 GF, gas is 32%

dive time, total stop time

Sea level
10,0
20,1
30,1
40,2
50,2
60,2
70,2
80,2
90,2
100,4
110,6

3500ft
10,0
20,1
30,1
40,2
50,2
60,2
70,2
80,2
90,6

So from a practical perspective what's changed? If you're pushing the very edge of a Buhlmann model at sea level you have a couple of extra minutes added into your ascent. You could stretch out your sea level dive by about 10 minutes longer and be at approximately the same M-values. But these are 1-1/2 hour exposures needed at 60 ft on 32% before you start seeing substantive differences in the m-values.

Let's use another example: 5500ft 81% of an ATA = 820 mB
10,1
20,1
30,2
40,2
50,2
60,2
70,2
80,5
90,9

Ok, now we have some "substantive" changes from sea level. We need to start adding in significantly more ascent time after "only" 80 minutes instead of somewhere around 100 minutes of exposure (60 ft dive) at sea level.

Personally Buhlmann isn't exactly the state of the art anymore, but its what I have available. I wouldn't do a 90min sea level dive and ascend virtually directly to the surface, and I wouldn't recommend people do that at altitude either.

But the point is that reasonable ascent profiles - like the NAUI and DAN deep stop recommendations - can easily compensate for typical altitude exposures by stretching out the ascent. + Nitrox is definately the right gas for the job.

If you live someplace like the Tibetian plateau you more pressing issues than altitude (like no water).

 

[Charlie99]

Personally Buhlmann isn't exactly the state of the art anymore, but its what I have available.

If you reran the profiles in RGBM, you would see slightly more compensation needed for altitude. BRW, the author of RGBM has posted that the altitude corrections in RGBM are slightly more than the classic Bullhman or Cross corrections. IIRC, VPM corrections were pretty close to that of RGBM, which makes sense since RGBM seems to have VPM as it's core.

As I noted in an earlier post, just doing a simple depth correction of 4% per 1000' of altitude is a pretty simple and reasonably accurate altitude adjustment. In other words, for diving at 3500', just use sea level tables, but assume your depth is 3.5x4%=14% deeper than actual depth. Easy.

Or use nitrox that gives you that much EAD advantage or more, and then use air tables. Even easier -- no calculations.