How do dive computers calculate nitrogen absorbtion and clearance when using nitrox?

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

Do computers only adjust for nitrox by calculating the EAD and tracking time and depth for that adjustment or are they doing something more complex?

Do they give credit for faster offgassing when using nitrox on ascent and at the safety stop or are they just using air calculations (at EAD) for less accumulation?

I am a nitrox recreational diver so please limit answers to that level.

I like the above question and the interesting and informative answers.

And if I dive really shallow (less than 15 feet) with 40% nitrox, my residual nitrogen would be lower than if I had stayed on the surface because the partial pressure of nitrogen during my dive would be lower than air at sea level. Would the computer remember that and carry that over to my next dive?

I think you'd need higher O2 (less N2) or a shallower depth-10' would do it.

Check my math.
.79 ata N2 at the surface breathing air.

.87 ata N2 at 15 feet breathing 60% N2
(15+33) /33 X .60 = .87
 
I think you'd need higher O2 (less N2) or a shallower depth.
Math checks. I can’t show my work like you did, but my math says that at 40%, you equal the partial pressure of surface air at 10 feet.

Ending up with tissues cleaner than you went in is practically only going to happen with pure oxygen.

And again, only in the fastest tissues: those that take on and push out nitrogen the fastest. So, within minutes, you’ll be very close to normal levels even if you were able to flush all the nitrogen out! :)

(ETA: I believe the fastest compartment is five minutes. in 6 halftimes you reach 98+ percent saturation. So that means in 30 minutes you would go from 0% to >98% of saturation.)
 
but my math says that at 40%, you equal the partial pressure of surface air at 10 feet.
:) I was just editing my post as you wrote that.
 
Actually, I live at 2000 m and could be home 90 min after a shore dive, but I have always stayed at sea level overnight before going home. I would use the USN Ascent to Altitude Table if I ever went home the same day.

This brings up another thing about dive computers. I really wish they would tell me what my residual nitrogen or repetitive group was at the end of a dive. The only thing I can get out of the few that I have experience with is the NDLs using planning mode. I guess I could use those numbers to guesstimate my repetitive group.
This sounds like you'd use your dive computer's "repetitive group" to help you use the Navy tables. I hope you are aware that not all tables use repetitive groups that are the same as the Navy tables, and dive computers would be even worse. One of the fundamental rules is to NOT transfer information between tables or dive computers
 
This sounds like you'd use your dive computer's "repetitive group" to help you use the Navy tables. I hope you are aware that not all tables use repetitive groups that are the same as the Navy tables, and dive computers would be even worse. One of the fundamental rules is to NOT transfer information between tables or dive computers

Yeah, I get that. I would have to use the USN Tables retroactively to figure out what my rep group is in order to use the Ascent to Altitude Table.

It sure would be nice if the computer would tell me what my nitrogen load is at the end of the day in a way that I could use to figure out my own safe ascent to altitude plan.

I'll have to save some dive plans from the computer right after I finish some dives and see how they compare to repetitive groups from the tables. I can see how things may not match up well with the computer tracking 16 compartments and the tables not doing that.
 
This brings up another thing about dive computers. I really wish they would tell me what my residual nitrogen or repetitive group was at the end of a dive. The only thing I can get out of the few that I have experience with is the NDLs using planning mode. I guess I could use those numbers to guesstimate my repetitive group.
My Garmin tells me my residual N2. I dived 8 hours ago and my N2 is at 17% (which includes driving home to 260m). I'm not sure how to use that information in planning my next dive but it's there.

My Perdix shows my tissues in a graph.
 
That's very interesting. What if you breathed O2 pre dive and washed out all your nitrogen?

How much benefit would that be and how much longer would your NDL be?

It's not that simple: gas exchange is driven by delta-pressure. Start with less N2 in the tissue and it'll be on-gassing faster b/c of greater dP. So it depends on your bottom time and tissue half-time, and on "multi-level" profiles: on other tissue compartments in your model too.

Simple case: a square profile no-stop dive would be controlled by the fastest tissue, if your computer has it at 5 minutes: 5*6=30 minutes will saturate it regradless of its starting PPN2. But for RGBM's 2.5-minute TC that's only 15 minutes.
 
Yes, this is true, but I wish Shearwater would take it a step further.

Once you are out of the water, it assumes you are breathing air. You don't have to be breathing air, though. You can be breathing nitrox or oxygen instead with the goal of increasing the offgassing gradient in those middle tissues. I wish we had the option of telling the computer what we are breathing during the SI.
I've wondered if people ever do that--breathe O2 or Nitrox on the surface after a dive. Even if there's no easy way to calculate how much it shortens your required surface interval, it could add a margin of safety with repetitive dives that are pushing the limits.
 
what about switching between air and nitrox? Like diving nitrox in the morning and then having a 3 hour lunch break and diving air in the afternoon?
 
what about switching between air and nitrox? Like diving nitrox in the morning and then having a 3 hour lunch break and diving air in the afternoon?
Dude, you are beginning to worry me. You are looking at DM? Your quesion is in the basic Nitrox class. You set the computer for the gas you are going to use on a dive. So your morning dive you set the computer to (say) 32%, then you set it to 21% for the afternoon dive. The computer knows how much nitrogen is in you after the first dive, and calculates how much is off gassed during your SI.
 

Back
Top Bottom