Suunto ppO2 MOD Question

[Selachimorpha]

Hi there,

I was looking at my girlfriends new Suunto D4 the other day and when you change the Nitrox O2% it displays the Maximum Operating Depth for the given mix, according to the partial pressure you have selected (in this case 1.4). Setting it to 21% the MOD for 1.4 displayed by the computer is is 56.7 m. This seems correct to me according to my calculations (1.4/0.21 = 6.67 ata or 56.7 m).

I have a Suunto Gekko and it is also set to a partial pressure of 1.4. However when I switch it to 21% the MOD displayed is 54.1 m.

So my question to everyone is why the 2.6 meter difference? The Gekko is an older model than the D4 and I no longer have the instructions. A change between the two models is that the Gekko uses Suunto RGBM, whereas the D4 uses Suunto RGBM DS (DS stands for Deep Stops). However I don't see how this would affect the MOD for a given mix?

Maybe it is simply set to be more conservative on the Gekko. Neither computers are in personal adjustment or altitude mode. The D4 is set to 100% RGBM (on the newer Suunto's you can select between 100% or 50%, the 50% setting makes deco calculations less conservative).

Would be very interested to hear your theories, especially from people who own other Suunto models. Maybe Suunto has changed the calculations between the "old" and "new" or "newer" computers.

old = Gekko, Vyper, Vytec, Stinger, Cobra, Mosquito

new = D4, D6, D9, Vytec DS and everything that came after those eg Vyper Air, Cobra 3 etc
(no idea about Vyper 2 and Cobra 2)

Any ideas?

 

[Selachimorpha]

Anyone?

 

[RJP]

Is one set for SW and one for FW?

 

[Atom]

I have a Vyper Air so this might not apply but here's how it's explained in the manual.

When you set the % of O2 in your nitrox mix you're supposed to set the measured % rounded down to the next integer. If you have 31.9% O2 you set it to 31% in the computer, this is to be conservative since if you set it to 32% you'll be absorbing more N2 because the mix is not the one the computer thinks you have. This makes sense for N2 absorption but it's not conservative for ppO2 (the conservative setting is more O2 than what you have in the tank).

So to solve the issue the Vyper Air will compute the displayed ppO2 and MOD with O2%+1 thus providing more conservative limits.

A MOD of 51.4m for a ppO2 of 1.4 would fit a O2% of 21.84, which corresponds roughly to 20.95 + 0.9. Which might be what the computer does, since there's no need to add a full % since if this was the case you'd be using the correct N2 % value. The manual might state it does +1 just because it easier to explain.

I've check the Gekko manual (http://media.suunto.com/media/suunto/manuals/en_US/gekko_manual_en_v3_m56577569830658673.pdf) and it explains it does the same thing in the nitrox section.

 

[Selachimorpha]

Is one set for SW and one for FW?

Nope.

 

[Selachimorpha]

I have a Vyper Air so this might not apply but here's how it's explained in the manual.

When you set the % of O2 in your nitrox mix you're supposed to set the measured % rounded down to the next integer. If you have 31.9% O2 you set it to 31% in the computer, this is to be conservative since if you set it to 32% you'll be absorbing more N2 because the mix is not the one the computer thinks you have. This makes sense for N2 absorption but it's not conservative for ppO2 (the conservative setting is more O2 than what you have in the tank).

So to solve the issue the Vyper Air will compute the displayed ppO2 and MOD with O2%+1 thus providing more conservative limits.

A MOD of 51.4m for a ppO2 of 1.4 would fit a O2% of 21.84, which corresponds roughly to 20.95 + 0.9. Which might be what the computer does, since there's no need to add a full % since if this was the case you'd be using the correct N2 % value. The manual might state it does +1 just because it easier to explain.

I've check the Gekko manual (http://media.suunto.com/media/suunto/manuals/en_US/gekko_manual_en_v3_m56577569830658673.pdf) and it explains it does the same thing in the nitrox section.

Yeah but then why does the D4 do this differently to the Gekko? What MOD does your Vyper Air display for 21% O2 at 1.4?

 

[Atom]

The D4 manual states it does it like the Gekko/Vyper Air, I don't have my computer with me right now but I'll check this when I'm back home. In any case this is rather strange.

 

[Atom]

On my Vyper Air, 21 % at ppO2 1.4 gives a MOD of 186ft so 56.69m but at 32% I have a MOD of 106 ft so 32.3m, this would be 33.75 according to the formula. A MOD of 32.3m is equivalent to a mix of 33%. So maybe the rule +1 O2% rule only applies to mix greater than 21% on the computer....

 

[Antarctic-Adventurer]

My Suunto D6 displays the max operating depth in both air and Nitrox 21 modes as 56.7 metres as expected. Perhaps the algorithm changed when Suunto moved to newer models?

 

[Scared Silly]

I noticed this "feature" with my Vytec DS (reads the same as a Cobra). It is a difference of about 5%. The difference between fresh water and salt water is around 3%. However, if you go to the Red Sea it is around 4%. So my guess is that Suunto added in a fudge factor and made it and "even" 5%.