Diving Nitrox with Computers?

[eponym]

Other thing with Suunto...By doing the simple calculations, we all know that MOD for 32% @ 1.4 is 111.375 ft ...if my memory serves me right. Suunto parameters set at P0 and A0 will give you an MOD of 107 ft. While it may not be much, you could now be safely diving at 110 ft with a computer giving you a big lip on and chirping like there is no tomorrow. Not being a big fan of lip on...I will set the PO2 at 1.5 instead and remain fully aware of my 111.375 MOD

1.4 divided by .33 yields the 107 MOD. The Suuntos always add 1 to the fraction of Oxygen you enter. From the manuals:

The dive computer will not accept fractional percentage values of oxygen concentration. Do not round up fractional percentages! For example, 31.8% oxygen should be entered as 31%.

So if I find by analysis exactly 32% I would be within my rights to enter that as 31%. That would be confusing, of course, but at least the MOD the dive computer reports would be the correct one for the mix.

In practice I usually find by analysis some fractional fraction () and just enter it as called for in the manual excerpt above.

-Bryan

 

[driebel]

Wow, exciting times while I was in class, thanks. The original question was totally in the context of recreational diving, in which "only" EANx has an MOD, and there is no changing of mixes, certainly not on the fly.

Also, some of the discussion seems to get sidetracked by assuming I'm insisting on this display as a necessary safety precaution. No no. I'm asking this: As a computer designer, you notice you have prime screen real estate just left of center, in big font. You have two choices with which to fill that screen space, EANx percentage or MOD. Which do you choose? Suutno chose percentage, I respectfully disagree and would have gone with MOD. Is there a clear argument for the percentage? I think RTee and gcbryan gave some reasonable arguments for the percentage. It's the choice between those two I'm interested in. Not an argument for/against the MOD in isolation, but the compare/contrast with percentage specifically. Why is/is not the raw EANx percentage more useful than the MOD?

 

[Blackwood]

Percentage is a more flexible number. It feeds directly into multiple calculations, including MOD and deco (i.e. NDL math).

MOD does not. Also, MOD is a function of more than just FO2. FO2 OTOH stands alone.

This is an odd disussion which seems predicated on the inability to remember a single number. So I think and argument can be made for displaying the independent and more flexible of two directly related quantities.

Another possibility is that FO2 looks good in marketing photos of a nitrox computer. MOD would just look like the depth displayed by any old computer. Consider that scubapro put retaining pins in their 2nd stage diaphragm caps such that the S always faces up for photos. I wouldn't discount reasons independent to diver usage.

 

[watboy]

I don't know which version of the Vyper the OP is using, but on my Vyper Air, which I believe is the same interface as at least the Vyper 2. The lower right and left buttons toggle the information of the lower right and left displays. On my VA, you can toggle the lower right display to show your current PP02 instead of your cumulative oxygen percentage. I rather see my present PPo2 than my MOD and I usually set it to show PP02 throughout the dive.

The annoying thing is that the right display has the choice of PPo2,OLF%,Temperature and Dive time while the left shows actual time,o2% of your mix, max depth. I would prefer being able to show both PPo2 and dive time at the same time.

 

[Jax]

Not an argument for/against the MOD in isolation, but the compare/contrast with percentage specifically. Why is/is not the raw EANx percentage more useful than the MOD?

This is an odd disussion which seems predicated on the inability to remember a single number. So I think and argument can be made for displaying the independent and more flexible of two directly related quantities.

Agree . . . I'm not sure why people keep jumping on "inability to remember a single number." The query sounds more like an interest in "why do the computer designers decide to show one static number (Fraction of Oxygen or FO) instead of the other (Max Op Depth or MOD)?"

Here's another fly in the ointment . . . Instead of the FO or MOD, why don't the computer designers display a running Partial Pressure of O2? (PPO2)

If I were designing the computer, I would choose the FO over the MOD because it is a setting the user puts in while the MOD is calculated. However, if I had a choice, I'd rather display the PPO2 . . . .

Another possibility is that FO2 looks good in marketing photos of a nitrox computer. MOD would just look like the depth displayed by any old computer. Consider that scubapro put retaining pins in their 2nd stage diaphragm caps such that the S always faces up for photos. I wouldn't discount reasons independent to diver usage.

Best darned thought I've heard yet!

 

[watboy]

Here's another fly in the ointment . . . Instead of the FO or MOD, why don't the computer designers display a running Partial Pressure of O2? (PPO2)

If I were designing the computer, I would choose the FO over the MOD because it is a setting the user puts in while the MOD is calculated. However, if I had a choice, I'd rather display the PPO2 . . . .
!

The Vyper does show PPO2

 

[Blackwood]

Agree . . . I'm not sure why people keep jumping on "inability to remember a single number."

That's how I interpret the clarified question in post seven ("Assume the user has screwed up and forgotten the plan. Isn't the MOD the more important reminder").

 

[DiveNav]

. . . I'm not sure why people keep jumping on "inability to remember a single number." ....

Because we are getting old ??

Mhhhhh .... where did I leave my glasses ?

Alberto (aka eDiver)

 

[Charlie99]

You have two choices with which to fill that screen space, EANx percentage or MOD. Which do you choose?

I think that many of the people that lean towards showing the MOD have the misconception that the MOD is somehow a magic depth at which something changes.

O2 CNS loading is a function of both time and depth, the same way that N2 loading is a function of depth and time. CNS loading calculations may be more non-linear than the N2 calculations, but it is still a time-depth dosage, not a magic depth above/below which there is some change.

That's why some use 1.4ata ppO2 to calculate MOD. Some use 1.6ata, some use 1.2 or even 1.1ata. Your CNS clock depends not just on the ppO2, but also the time you are exposed to the ppO2.

So MOD is a fuzzy number. It makes only a little more sense to show that number than figuring out what depth corresponds to an NDL of 45 minutes and displaying that depth on your computer as the "Deco depth limit".

 

[Charlie99]

That's how I interpret the clarified question in post seven ("Assume the user has screwed up and forgotten the plan. Isn't the MOD the more important reminder").

The important numbers regards O2 are the percentage of CNS clock, and the "O2 time remaining at this depth".


The "O2 time remaining at the depth" is the equivalent to an "NDL time remaining at this depth", but calculated for O2 dosage rather than calculated for N2 like the NDL.

I would imagine (but have not verified this) that there are some air integrated computers out there that show the lowest of three calculations .... remaining air to a user configured ascent pressure using current at depth consumption rate, remaining NDL time at this depth, and remaining CNS clock time at this depth.