How soon can I move to 9000ft after diving?

[yohnis]

I did 2 tank dives at the depth of average 45ft, 70mins each.
Then waited 24hrs, following the standard rule. My dive computer said I was desaturated at that time.
Then I started to drive to 9000ft mountain. At around 8000ft, residual nitrogen level went from 0 to 70%, with desaturation time of 15hrs. I got scared and turned around, stayed at sea level for additional 16hrs.
At 40hrs after the dive, I started driving up again, then at 9000ft, nitrogen level went up to 70% again. It was zero at the sea level.

I do not understand what is going on. My dive computer is wrong? Is there any guidelines how long I need to wait before going up to 9000ft?

I attach a pic of my dive computer, showing high level of nitrogen after 40hrs of surface time.
Comments appreciated.

 

[fdog]

• When you are at sea level (when not even diving), your body contains nitrogen at sea level pressure. This is also know as saturated.
• You're driving to altitude, 9000', where the pressure is ~30% less.
• The computer is displaying this pressure change.

To put it another way, this is the same pressure ratio change as if you were to stay at 13 fsw for 24 hours (saturate), and the computer would behave in much the same way.

We dive at sea level alot, then have to drive over a 7000' pass to get home. Depending on the profile that we dove that morning, our computers occasionally will display a change in desaturation times.


All the best, James

 

[fdog]

I did 2 tank dives at the depth of average 45ft, 70mins each.
Then waited 24hrs, following the standard rule. My dive computer said I was desaturated at that time.
Then I started to drive to 9000ft mountain. At around 8000ft, residual nitrogen level went from 0 to 70%, with desaturation time of 15hrs. I got scared and turned around, stayed at sea level for additional 16hrs.
At 40hrs after the dive, I started driving up again, then at 9000ft, nitrogen level went up to 70% again. It was zero at the sea level.

I do not understand what is going on. My dive computer is wrong? Is there any guidelines how long I need to wait before going up to 9000ft?

I attach a pic of my dive computer, showing high level of nitrogen after 40hrs of surface time.
Comments appreciated.

Oops, sorry, didn't get to this question for you -

You may wish to peruse the NOAA Ascent to Altitude tables. Although simple to use, there is fine print that you should pay attention to. Also, you can't refer to PADI or NAUI tables for the pressure group, you have to use the NOAA Dive Tables to get that.

All the best, James

 

[yohnis]

Thank you very much for the tip.
According to the NOAA table, I got out as L diver. Then required surface interval to 9000ft is 24hrs.
(In fact, none of the required interval goes beyond 24hrs.)

Does this mean it was ok to go up to 9000ft at initial attempt at 24hrs, even though my dive computer showed some residual nitrogen? Actually nitrogen level was at 70%, not 100%.

 

[TSandM]

Remember that you have nitrogen in your bloodstream at sea level, too. The fact that your computer showed the exact same thing at 24 hours and at 40 hours makes me wonder if it's simply looking at the sea level value and comparing THAT to "saturation" at 9000 feet.

Dive computer algorithms generally work by diving the theoretical body into time compartments. Each compartment has a half-life, which is the time that it takes to bring the compartment halfway from where it starts to the saturation value at a given depth. So, if you are at 30 feet, it takes five minutes to bring your "5-minute compartment" to 15 feet of pressure; 30 minutes to bring the "30-minute" compartment to 15 feet of pressure, and so on. Compartments are generally considered to empty at the same rate, and after six half-lives, are considered to have equilibrated with the pressure where you are. So, after 40 hours, compartments with half-lives up to 6 hours will have equilibrated to sea level. Most models don't use compartments a lot longer than that (since the loading in such compartments is really only relevant to multiple-dive multiple-day scenarios) and such compartments would have very minimal loading after two dives as shallow and brief as you describe.

So I think your computer was telling you that any time you drive to 9000 feet, you're undergoing a degree of decompression!

 

[bleeb]

Does this mean it was ok to go up to 9000ft at initial attempt at 24hrs, even though my dive computer showed some residual nitrogen? Actually nitrogen level was at 70%, not 100%.

This doesn't directly answer your specific question, but for background information, it can take days for all the excess N2 to clear out, especially from multiple deep dives, even if all the dives were within NDLs. I'm not familiar with how your computer handles things or what the % represents exactly, but on mine, it separately tracks No Fly time and saturation. The former will clear within a day while the latter is still non-zero.

 

[fdog]

Thank you very much for the tip.
According to the NOAA table, I got out as L diver. Then required surface interval to 9000ft is 24hrs.
(In fact, none of the required interval goes beyond 24hrs.)

Does this mean it was ok to go up to 9000ft at initial attempt at 24hrs, even though my dive computer showed some residual nitrogen? Actually nitrogen level was at 70%, not 100%.

Yes, it means it was OK.



As I had said in post #2:

• When you are at sea level (when not even diving), your body contains nitrogen at sea level pressure. This is also know as saturated.
• You're driving to altitude, 9000', where the pressure is ~30% less.
• The computer is displaying this pressure change.

I would expect this behavior anytime your computer:

(A) Is functioning in "Dive computer mode", e.g. still has time remaining on the no-fly or desaturation clock, or, gets wet
-and-
(B) ascends to a dramatically higher altitude

Your computer is attempting to protect you from DCS while performing an additional dive at altitude, by compensating for residual nitrogen. This residual comes from being stored at sea level (higher pressure than 9000').

All the best, James

 

[Charlie99]

This doesn't directly answer your specific question, but for background information, it can take days for all the excess N2 to clear out, especially from multiple deep dives, even if all the dives were within NDLs.

As both FDOG and TS&M have pointed out, you will NEVER get rid of the N2 from the 1ata pressure at sea level, no matter how long of a surface interval you have after your dives.

That 70% the OP is seeing on his computer isn't the N2 from the dives. It is the loading from breathing air at sea level.

 

[String]

1atm is a permanent saturation dive. Decrease the pressure by going high and those tissues are always going to offgas to find a new, lower saturation point at the new ambient pressure

If that rate of offgassing is more than the compartment can tolerate (if you subscribe to tissue models) then you will bubble.

The 70% is going to be his computer telling him he's off-gassing nitrogen at a 70% of maximum allowable rate for that particular pressure/70% on its way to maximum tolerate overpressure. Go higher it'll get 80,90, 100 and so on as the pressure differential increases.

 

[bleeb]

As both FDOG and TS&M have pointed out, you will NEVER get rid of the N2 from the 1ata pressure at sea level, no matter how long of a surface interval you have after your dives.

You've misunderstood. That wasn't what I was referring to. I'm talking about the nitrogen loading people acquire while diving, and it takes days to clear that if you've been diving a lot, irrespective of the 1 ata most everyone lives with. As TSandM explained, a model's theoretical 6-hour compartment will take 40 hours to effectively empty i.e. equilibrate to the saturation level determined by the local atmospheric ambient pressure. For SIs before ascent to altitude, one would hope that any decent computer is including this in its calculations, although as others have commented in this thread, the OP's dives were probably short, shallow, and few enough that they probably had little effect in this particular case after the time delay mentioned.