Diving at altitude

[Dr Deco]

Hello Scuba Board Readers:

Where Did That Come From?

Someone asked about my recent post on this thread. I was referring to the comment alluded to above by Mike Ferrara “Now, I think there are some that suggest that since decompression isn't an exact science anyway, and, if you are following certain procedures, that the relatively small difference between actual depth and theoretical depth can be ignored unless at very high altitude.”

Because of the relatively large safety margin for most divers in a decompression schedule, probably some of these differences would not be noted. Remember, for every 1 per cent of the divers that are ‘hit’ on a schedule, 99 per cent made it with no problem and could probably do so again. The ‘problem’ is that you do not know where you stand on the DCS sensitivity scale. You must, then, always be prudent and dive as if you were one of the ‘sensitives.’

All substances, including nitrogen, act in the body in a specific fashion. This will vary from person to person and one reason why potent medications are controlled by physicians and the dose is adjusted for each patient to give the desired effect. If the medication is found to be unacceptable, another drug is chosen. Thus it is with nitrogen; the ‘dose’ is diver dependent. Humans, and lab animals, do not yield the same decompression response with a given ‘dose’ of inert gas.

We must have the gas present to assist us in lung inflation (because oxygen alone is too toxic), but we can reach the point of unacceptable side effects, viz, decompression sickness. Variability must be an accepted outcome of decompression. Exceeding the limits is often accompanied without undue consequences and diving within the table will often yield a ‘hit.’

Dr Deco :doctor:

 

[DA Aquamaster]

I think the degree you can ignore your theoretical depth depends on the altitude and how deep you are diving.

At 1000 ft a dive to 150 feet yields a theoretical depth of 155 ft which could be argued is not all that big a deal. But the same 150 ft dive at 5000 ft will yield a theoretical depth of 180 ft which is a 30 foot difference that I take care not to ignore.

A 101 ft dive at 2000 feet yields a theoretical depth of 108 ft. and you at least have to think about that, but if you are using a table where you are using the next interval of 110 ft. anyway it still makes no practical difference.

BUT what I always wonder about are the recommended stop depths on high altitude correction charts. For example the theoretical deco stop depths at 5000 ft are 8, 17, 25 and 33 feet, instead of the usual 10, 20, 30 and 40 feet. I understand the theory but I still always wonder if I really want to make my 40 ft stop at 33 ft. It just seems sort of sinful. My computer seems to agree as it sticks with the normal 10 ft intervals.

 

[DepartureDiver]

Better late than never ... but still wanted to chime in. While there are different ways to view altitude, the general consensus is that corrections are definitely necessary. The fact that the pressure of the water is the same (virtually) makes no difference. The issue is the lesser pressure the diver is surfacing to when at altitude. For example, a 2:1 ratio is reached at 33’ in the ocean. However at an altitude of 6200’ such as Lake Tahoe, a 2:1 ration is reached at a depth of 26’. This is the principle upon which the most common altitude corrections are based, i.e. the Cross/ratio corrections. While these corrections are very easy, it should also be pointed out that these simple ratio corrections also appear to be conservative, i.e. while a correction needs to be done, it may not need to be as great as the formula calculates. This gives even more credence to DA Aquamaster’s comments that one may not need to worry at the lower altitudes (assuming the diver is acclimated at altitude). Fortunately, the deco stop depth issues can be virtually ignored in today’s diving world unless one is diving off printed tables for calculating deco times. This would only be an issue for a recreational diver that accidentally exceeded a no-stop time. The reason I say this is if one is doing recreational dives within no-stop time limits, then the depth of a safety stop need not be exact and can easily be performed at a 10’ increment or at 15’. As it pertains to decompression stop diving, if one is using diving software to plan a dive, it will be given in terms of the common stop increments of every 10’. Also, if a deco dive is being performed, the norm is to use some deco gasses. For example, if a nitrox blend is being used, then the nitrogen pressure is less than that of air (obviously), which also means it is less than that of an air deco stop. In other words, a 10’ stop on a nitrox blend can have less nitrogen pressure (and better offgassing) than an 8’ stop on air and thus offgassing is still properly occurring. The best example of all is a 20’ stop on oxygen. There would simply be no reason to correct this for altitude at all such as performing it at 17’. Plus the beauty of altitude is that a 20’ stop on oxygen will result in a lower ppOxygen than that at sea level due to the lesser atmospheric pressure.

 

[devilfish]

Some of the basic compensation basis for altitude diving.

Fresh water lake at sea level 14.7 psi, 2 ata is 34 ft where gas absorbtion is double.
14.7psi/.432 = 34 ffw

Fresh water lake at altitude of 11 psi example, 2 ata is 25.5 ft where gas absorbtion is double.
11psi/.432 = 25.46 ft where pressure is double.

If for example a safe ascenet pressure reduction is 30 ft/min at sea level, at an altitude where pressure is 11 psi, the same rate of pressure reduction during ascent should be 25 ft/minute.

 

[DepartureDiver]

Some of the basic compensation basis for altitude diving.

Fresh water lake at sea level 14.7 psi, 2 ata is 34 ft where gas absorbtion is double.
14.7psi/.432 = 34 ffw

Fresh water lake at altitude of 11 psi example, 2 ata is 25.5 ft where gas absorbtion is double.
11psi/.432 = 25.46 ft where pressure is double.

If for example a safe ascenet pressure reduction is 30 ft/min at sea level, at an altitude where pressure is 11 psi, the same rate of pressure reduction during ascent should be 25 ft/minute.

Thanks for the math ... it's actually close to what we do here. Tahoe's pressure is 11.67 psi. You are right, when wanting a table to work exactly as it was designed, the ratio method is what does it. While this may add a slight conservatism factor to the no-stop times as compared to the reduced times that would really be allowed, the surface intervals for a repetitive dive won't work any other way since they are strictly based upon tension-pressures to atmopsheric pressures (i.e. the ratio). This also holds true for the ascent rates. However, it is a minor difference in my opinion and if a table uses a 30 fpm ascent rate at the ocean, it will work fine at altitude. The ascent rate issue is really the rate of pressure reduction and not a ratio for calculating dives. In other words, any dive should use a good ascent rate based upon the pressure reduction rate whether at sea level or altitude.