Altitude dives and Narcosis

[Wijbrandus]

Why would an altitude diver be narced at less depth than in the ocean? It seems to me that if narcosis is caused by breathing gass at 4 ata or more, then altitude would not have anything to do with this.

I'm asking because on my AOW dives, we did 60' that adjusted to 90 for the altitude, and then the DM had us do our little math narc test. I passed just fine. The only reason anyone had a problem at all was because they couldn't read what he wrote clearly.

Now that I think about it, I'm beginning to doubt I was even narced at all. It seems that if narcosis [usually] only hits you after you pass 100', then a theoretical depth doesn't mean a hill of beans and it's only the true ata that matters.

Am I right? Or am I totally screwed up now?

 

[DandyDon]

I look forward to the answers from the experts on this one, but while we wait...

Narcosis does not "...[usually] only hits you after you pass 100'..." The narcotic effect begins as soon as you increase pressure, as soon as you submerge, and gets stronger/better as you go deeper. This is one of the reasons we enjoy Scuba, I think, the slight high we get under pressure. But as it increases, different people have different reactions, somewhat like different people have various results from other narcotics.

Like you, I do not think that Altitude of the dive would have any affect...

 

[H2Andy]

the effect of diving at higher altitudes is similar to diving deeper. for example,
some agencies recommend that at 1200 feet, divers add 10 ft to their bottom depth to allow for the effect of altitude. an additional 10 ft. is added if divers are not acclimatized to that altitude. so a 100 foot dive is treated as a 120 foot dive. that's just at 1200 feet.

i wonder since narcosis gets worse with depth, and you are effectively diving "deeper" at altitude, maybe diving at altitude does "increase" the narcosis.

however, i wonder how this could be. isn't narcosis brought on by the effects
of nitrogen under pressure? so i'm having a hard time conceptualizing how
diving at altitude would "increase" the pressure on the nitrogen at all...

hmmmm.....

 

[Wijbrandus]

H2Andy,

In altitude class we are taught that the increased elevation is giving us a pre-existing pressure group. This takes time to work off, and is effectively negated by staying the night before the dive at the same altitude you will be diving. There is also a 4% adjustment to your effective depth for altitude (per 1000'), but the bottom line is I don't see how this can affect the pressure of 100' of water above me.

So why would we do a dive to 60' and expect to see any narcosis? It was my understanding that narcosis really begins to manifest at a depth of 100' (4 ata), and I don't see how an altitude adjustment would have any bearing on this.

These are the things you wish you'd thought to ask in class.

 

[DandyDon]

These are the things you wish you'd thought to ask in class.

And hope your instructor knew...?!

When I took my Nitorx class, we had to do 2 dives - from 4600 feet elevation. He insisted that I had to stay above 70 feet to prevent OxTox from the 32% EANx?!

I suggested that the two were not connected, and while the adjusted depth for the 84 foot hole would be 110 for nitrogen loading purposes, that was still less than 111 MOD. He wouldn't accept that.

Okay-fine, I just wanted to do my dives, get my card, and get. Trying to give him the benefit of the doubt, though - I tried a trip to Roatan with his group. More evidence that this fellow needs to stick with his full time job...!

 

[jagfish]

Interesting thread...
I'll be interested to hear experts on this one as well, but I'll throw in a guess. I would suppose that most posters here had the reason for the addition of extra depth to the actual depth is to take into consideratoin that an existing pressure group exists, there is already a fair bit of nitrogen in the body.

However, I would also agree that this would seem to have litle to do with the effects of pressure. If anything, it seems to me that at a given depth at altitude, there would be slightly less pressure than at sea level. While the weight of the column of wter above you wold be the same, the "extra" one ata of pressure from the weight of the sky above you would be reduced by a factor determined by the altitude you are at.

Just a guess...

JAG

 

[rmediver2002]

Nitrogen becomes narcotic due in large part to the partial pressure (or depth of the dive) (Henry's law), these effects can be increased through action of other disolved gasses (CO2 and in some references O2 / NITROX)

So the short answer is... No the altitude diver will not be narced at a shallower depth than when in the ocean.


For altitude diving Assuming the project is at:

8000 ft = 2438.3999839065604 m

Atmospheric pressure is 0.719 ATA or 10.57 psi this is much different from the sea level pressure our bodies are under (14.7 psi) this gives us two items to consider.

1. When we initially ascend to altitude from sea level we begin to off-gas from our normal saturation pressure of 14.7 psi.

2. For the dives we make at altitude, it is going to take us some depth of water just to return to 14.7 psi. This in combination with the fact that we have switched from the density of salt water to fresh water...(the numbers are going to appear exceedingly higher in large part due to the fact that we will be returning to a much lower ambient pressure to conduct out "off-gassing" than when at sea level. Instead of developing new tables most agencies have elected to modify the existing tables for use at altitude)

For item number one - Upon arrival at the project site the divers must be assumed to be dirty, this is to account for the nitrogen remaining in their bodies from sea level. At 8000 ft the divers would be considered F divers (Navy table).

After 12 hours the divers will reach equilibrium with the altitude, more time will be required for acclimatization. The divers tissues will be nearing ambient partial pressures of gas but the body needs much more time to effectively adapt to living at the decreased ambient pressure...


For item number 2 - Cross correction formula for depth:

Depth of dive ___ x (pressure at sea level/pressure at altitude) = equivalent depth for decompression

Example:

100 fsw dive x (1 ata/.719 ata) = 1.390 ata) = 139.021 equivalent depth, round up to 140 fsw

This figure is used to compute decompression obligations and does not translate directly to narcosis (Henry's Law) nor to air consumption (Boyle's law) for that matter...


Jeff Lane

 

[DA Aquamaster]

I am kind of amazed that altitude diving is not more commonly understood. It is SOP here becuase the lake where we do most of the OW training is at 4600 feet but I assumed it was at least touched in other OW classes as well.

I idea of an unstructor suggesting that the theoretical depth at altitude (based on the lower atmospheric pressure at the surface) had any bearing on ox tox concerns with nitrox (based on absolute pressure) is really scary. He or she has no business teaching an OW class let alone a nitrox class.

A 130 ft dive at 5000' altitude will have a theoretical depth of 156 feet but the partial pressure of the in nitrox 32 will still be 1.58 and O2 toxicity will not be any larger a concern than it would be at sea level at the same 130 ft actual depth.

In the same fashion nitrogen narcosis is also dependent on absolute pressure and the actual depth is the relevant factor, not the theoretical depth.

Theoretical depth is however a concern for N2 loading and offgassing as you are returning to a pressure less than 1 standard atmosphere so no decompression limits and deco schedules are based on the deeper theoretical depth.

Around here we haver never bothered adjusting the deco stop depths and I have never met anyone who does. A 10' stop at 5000' could theoretically be done at 8' and a 20' stop could be done at 17' but the 2' to 3' difference is not significant. A 40' stop could be done at 33' and that is signifcantly shallower but I think you are still better off stopping at the deeper depth. Also if I am diving any profiles requiring relatively deep stops, they are genrated either with Dplan or VPM and they both specify standard stop depths or they are done using a dive computer that again specifies standard 10 ft stop intervals.

 

[Wijbrandus]

I am kind of amazed that altitude diving is not more commonly understood. It is SOP here becuase the lake where we do most of the OW training is at 4600 feet but I assumed it was at least touched in other OW classes as well.

DA, that is exactly why I'm asking all these questions on the matter. I refused to do any local diving with my buddy without guidance until we were certified AOW because we had no exposure to the altitude adjustments needed to make it safe.

We certified, I now know how to use the tables, and I feel confident enough in my abilities to question some of the things we were told. I think altitude adjustments only apply to theoretical depth for nitrogen loading relative to your air pressure, but we were told to use the theoretical depth for everything, including narcosis and oxygen toxicity. That's where I started to disbelieve what we were being told.

We also did a "nitrox adventure dive". Supposed to show us the benefits of nitrox. We were diving 32%, and we were told not to descend below 40'. The reason was listed as oxtox, but when I did the partial pressure in my head I figured out that 32 should be safe to at least 100'. Unless there was a training limit by PADI, I don't understand why they would have told us that. It definitely made some people a little more nervous thinking they could tox out if they went below 40'.

 

[fdog]

When depths in fresh water are converted to sea water equivalents, I haven't noticed any significant differences between sea level and altitude (here at Lake Tahoe).

Purely subjective.

All the best, James