Nitrox Diving at Altitude

[ianr33]

From the thread on the Lake Mead accident:

Originally Posted by Dive-aholic

That's not true. Diving nitrox requires you adjust your actual depths to a shallower level. In Lake Mead, a dive to 110', the MOD for 32%, is equivalent to the pressure you'd get at about 115', so you would have to adjust your actual depth down to 100'.

I still contend that you can dive nitrox deeper at altitude than at sea level.

The pressure at 5500 ft is about 1/2 ATM (Atmospheric pressure - Wikipedia, the free encyclopedia)

Normal max pO2 allowed for bottom gas is 1.4

At sea level ,using 32% the MOD will be :
0.32X Total Pressure = 1.4 ATM
Total Pressure = 1.4/.32 = 4.375 ATM
Take off 1 ATM for the Atmosphere and you get 3.375 ATM due to water pressure

3.375 X 33 = 111 feet


At 5500 Feet pressure is only 0.5. ATM

Total allowed pressure is still 4.375 ATM
Take off 0.5 for the Atmospheric Pressure leaves 3.875 ATM of water pressure

3.875 X 33 =128 feet

So the MOD of 32% nitrox at an altitude of 5500 ft will be 128 feet.

The NDL's will be reduced as the diver is surfacing to a lower pressure,but the MOD for any particular mix will be increased.

Am I correct here ??

[in_cavediver]

As far as I can tell, you're dead on. MOD is based solely of PO2's which does makes it deeper at altitude since the surface pressure is lower. You still have to do the same munging of depths to compensate for altitude for NDL though.

Now, the really cool thing - If you do an altitude dive at 8000', what is the time to fly?

hint - airplanes are pressurized to around 7000'

Originally Posted by ianr33

From the thread on the Lake Mead accident:

I still contend that you can dive nitrox deeper at altitude than at sea level.

The pressure at 5500 ft is about 1/2 ATM (Atmospheric pressure - Wikipedia, the free encyclopedia)

Normal max pO2 allowed for bottom gas is 1.4

At sea level ,using 32% the MOD will be :
0.32X Total Pressure = 1.4 ATM
Total Pressure = 1.4/.32 = 4.375 ATM
Take off 1 ATM for the Atmosphere and you get 3.375 ATM due to water pressure

3.375 X 33 = 111 feet


At 5500 Feet pressure is only 0.5. ATM

Total allowed pressure is still 4.375 ATM
Take off 0.5 for the Atmospheric Pressure leaves 3.875 ATM of water pressure

3.875 X 33 =128 feet

So the MOD of 32% nitrox at an altitude of 5500 ft will be 128 feet.

The NDL's will be reduced as the diver is surfacing to a lower pressure,but the MOD for any particular mix will be increased.

Am I correct here ??

[DivingCRNA]

Originally Posted by ianr33

From the thread on the Lake Mead accident:

I still contend that you can dive nitrox deeper at altitude than at sea level.

The pressure at 5500 ft is about 1/2 ATM (Atmospheric pressure - Wikipedia, the free encyclopedia)

Normal max pO2 allowed for bottom gas is 1.4

At sea level ,using 32% the MOD will be :
0.32X Total Pressure = 1.4 ATM
Total Pressure = 1.4/.32 = 4.375 ATM
Take off 1 ATM for the Atmosphere and you get 3.375 ATM due to water pressure

3.375 X 33 = 111 feet


At 5500 Feet pressure is only 0.5. ATM

Total allowed pressure is still 4.375 ATM
Take off 0.5 for the Atmospheric Pressure leaves 3.875 ATM of water pressure

3.875 X 33 =128 feet

So the MOD of 32% nitrox at an altitude of 5500 ft will be 128 feet.

The NDL's will be reduced as the diver is surfacing to a lower pressure,but the MOD for any particular mix will be increased.

Am I correct here ??

Looks right, but you only gain 17 feet. It is not like it is a huge difference.

[Dive-aholic]

You're approaching it wrong. The ATM pressure at the surface is what causes divers to have to shorten their dives due to shortened NDLs. The reason you have shortened NDLs is because when you surface, nitrogen will not off gas as fast at high altitude as it does as sea level.

Let's take nitrox out of the equation for a minute. A dive to 110' is theoretically equivalent to a dive to 134'. What that means is when you dive to 110' the nitrogen uptake on your body is equivalent to the nitrogen uptake on a dive to 134' at a sea level location. Here's a link to the table commonly used in altitude diving.

So now we add 32% nitrox to it. With an MOD of 110', we need to stay shallower than a theoretical depth of 110', which according to the table is 90'. If you go beyond 90', you'll hit pressures equivalent to greater than 1.4 ATM.

And 17' is a huge difference. Would you breathe 100% oxygen at 37'? Personally, I won't breathe mixes that give my a ppO2 greater than 1.2 when diving trimix or doing working dives. With easy dives, I'll go 1.4.

[texdiveguy]

For me personally if using nitrox as a back gas at altitude I simply calc. my dive depth/NDL as if my bg was air and dive those numbers based on my altitude charts. Taking advantage of the higher O2 value as a benefit to my physiology at altitude.

[ianr33]

Originally Posted by Dive-aholic

Let's take nitrox out of the equation for a minute. A dive to 110' is theoretically equivalent to a dive to 134'. What that means is when you dive to 110' the nitrogen uptake on your body is equivalent to the nitrogen uptake on a dive to 134' at a sea level location. Here's a link to the table commonly used in altitude diving.

Agreed but I am not talking about NDL's

Originally Posted by Dive-aholic

So now we add 32% nitrox to it. With an MOD of 110', we need to stay shallower than a theoretical depth of 110', which according to the table is 90'. If you go beyond 90', you'll hit pressures equivalent to greater than 1.4 ATM.

Sorry Rob but that makes no sense to me at all.

Originally Posted by Dive-aholic

And 17' is a huge difference. Would you breathe 100% oxygen at 37'?

Actually I probably would provided I was at an altitude sufficient to make the pO2 no more than 1.6 ATM. (Approx 5000 feet)

[Puffer Fish]

Originally Posted by Dive-aholic

You're approaching it wrong. The ATM pressure at the surface is what causes divers to have to shorten their dives due to shortened NDLs. The reason you have shortened NDLs is because when you surface, nitrogen will not off gas as fast at high altitude as it does as sea level.

Let's take nitrox out of the equation for a minute. A dive to 110' is theoretically equivalent to a dive to 134'. What that means is when you dive to 110' the nitrogen uptake on your body is equivalent to the nitrogen uptake on a dive to 134' at a sea level location. Here's a link to the table commonly used in altitude diving.

So now we add 32% nitrox to it. With an MOD of 110', we need to stay shallower than a theoretical depth of 110', which according to the table is 90'. If you go beyond 90', you'll hit pressures equivalent to greater than 1.4 ATM.

And 17' is a huge difference. Would you breathe 100% oxygen at 37'? Personally, I won't breathe mixes that give my a ppO2 greater than 1.2 when diving trimix or doing working dives. With easy dives, I'll go 1.4.

I think you are mixing Oxygen tox with nitrogen issues. The MOD would increase. 1.4 is an absolute value, and if you started with .5 atm, it would indeed make the MOD 17 feet deeper (a ot of good that would do).

The only other way would be to say that .5 atm now equals one ATM, and then the MOD would be 55 ft (which is obviously not correct).

The same applies for Nitrogen tox., it would take place (as it also is an absolute value) deeper.

However, DCS concerns rule the day with high altitude diving, so it would be unlikely that there would be much benefit to the greater MOD.

[OTGav]

Originally Posted by Dive-aholic

The reason you have shortened NDLs is because when you surface, nitrogen will not off gas as fast at high altitude as it does as sea level.

Is this right? I thought that with less dense air at altitude that the pressure gradient was higher - making you off gass a little faster in fact.

I also thought that shortened NDL is down to reduced air pressure increainsg the chance of bubble formation at the surface for a lesser ammount of nitrogen in solution in the blood than at sea level? Rather than anything to do with off gassing.

I could be getting my gass physics confused though.

[Dive-aholic]

ian and PF, I think you might be confusing theoretical ocean air depth, which is deeper, for MOD. The TOAD is used to either calculate the safe mix to use on the dive or to calculate your decompression schedule. It is not an MOD as I understand it.

Originally Posted by OTGav

Is this right? I thought that with less dense air at altitude that the pressure gradient was higher - making you off gass a little faster in fact.

I also thought that shortened NDL is down to reduced air pressure increainsg the chance of bubble formation at the surface for a lesser ammount of nitrogen in solution in the blood than at sea level? Rather than anything to do with off gassing.

I could be getting my gass physics confused though.

If less dense air caused you to off gas faster then we could all fly immediately after diving. We need to wait 12-24 hours after diving to fly because cabin pressure is at 8000'. Going to that pressure too soon after diving could cause us to take a hit.

[in_cavediver]

I believe there are a few things getting confused.

Start with MOD. The max po2's are fixed. The equation itself is simple. We add the surface pressure in atm to the water depth pressure in atm and multiply it by the FO2. The only altitude dependent term is the surface pressure. It goes down so the the water pressure can go up. Hence deeper MOD.

Now, on NDL's and off gassing. At altitude, there is a larger gradient, due to the lower pressure at the surface. The pressure of water depth is the same - 1atm/33ft. What that means is that you can't absorb as much nitrogen on a dive and still do a 'no stop' dive as you could at sea level. That's the physics of it. Now, to make it easier on divers, a relationship was developed to use standard sea level tables and 'munge' the depths to work at altitude. This munging only has basis in the nitrogen uptake/off-gassing realm. It isn't valid for END's or PO2 limits. Those are strictly pressure based.

Lastly, about off-gassing. Yes at 8000', you will off-gas faster. So fast in fact that the sea level limits would get you bent. This is why I mentioned the time to fly for an 8000' altitude dive. That time is zero since you not changing pressure. From sea level, that time is 12-24hrs.