MOD of Scubapro computers at 21%

[Wibble]

Yes, indeed, it is all a bit arbitrary and doesnt make much difference in any real world scenario, but I am trying to figure out the calculation they use and the variables. All my other computers calculate exactly, so it is purely a mathematical curiosity.

You’ve not seen Suunto then. They also calculate it incorrectly adding in some proprietary fiddle factor.

 

[Nick Ruberg]

You’ve not seen Suunto then. They also calculate it incorrectly adding in some proprietary fiddle factor.

I have seen Suunto. They take your oxygen reading and simply add 1% to it and then calculate the MOD. From experience (my Suunto is at home and I am at the office), it works out perfectly if you just add the 1%. I once Googled it and they do that for safety to bring your MOD down - they reckon when you analyse (let's say you get 32.4% and enter 32% into the computer) , they calculate your MOD on 33% for safety and perform the calculations of bottom time on 32%.

 

[Dan]

Like @lowviz says, you are splitting hairs. You use Oxygen partial pressure of 1.4 (only 2 significant figures, not 1.41 or 1,39 or 1.399). So, mathematically your depth accuracy would also be in 2 significant figures. The decimal point in the depth calculation would be the errors of the estimation. The difference between your head and the dive computer on the wrist is about a foot, which is within the error of calculation.

 

[Dan]

Everyone’s tolerance on MOD and how long you are exposed at that MOD without experiencing Oxygen toxicity that would be fatal to you would also vary. I was at 1.7 for 5 minutes without any problem (145 feet or 44m depth at sea with 32% Nitrox, taking a photo of Zebra shark in Palau, below).

So, whether the calculation shows 57.8m or 58.3m, it’s the same and within the errors. I call it 58m +/- 1m.

 

[Wibble]

I have seen Suunto. They take your oxygen reading and simply add 1% to it and then calculate the MOD. From experience (my Suunto is at home and I am at the office), it works out perfectly if you just add the 1%. I once Googled it and they do that for safety to bring your MOD down - they reckon when you analyse (let's say you get 32.4% and enter 32% into the computer) , they calculate your MOD on 33% for safety and perform the calculations of bottom time on 32%.

It’s ridiculous and wrong (Suunto not you!). It’s complete and utter meaningless CYA horse puckey on Suunto's part.

Worse than that it won’t allow you to switch to oxygen at 6m/20ft without ascending above your ceiling because of this broken nonsense.

 

[Nick Ruberg]

Guys, I am merely trying to understand their formula. This isn't about the tolerance to oxygen toxicity or a debate on the salinity of the oceans, purely a question about: how do they calculate it? Shearwater - works out perfectly. Simple calculation. Suunto i can work out (one may or may not agree with their reasoning, but one can calculate it). Scubapro - Can't work out their logic.

 

[broncobowsher]

None of the formulas are correct. That is why there are so many of them. Trying to capture variables, but none catch them all. Nobody even knows what all the variables are. So they work them in the best way they can. You lean them toward conservative, you make safe computers that divers hate because they have more deco time than others and the others are not getting divers bent, or go aggressive that make divers happy until they get bent. People like to think that it is a sharp line, one side is safe, the other you die. But it isn't a sharp line, it is a big fat gray zone. Which side of the gray line to play on?

 

[lowviz]

Scubapro - Can't work out their logic.

It appears to me that you adjusted the ambient pressure to give you better agreement across your O2 concentrations.

Would it not make more physical sense to adjust the 10m? This number will change with temperature and salinity. Metre sea water - Wikipedia

Even worse (from link above):
"In the metric system, a pressure of 10 msw is defined as 1 bar. Pressure conversion between msw and fsw is slightly different from length conversion between metres and feet; 10 msw = 32.6336 fsw and 10 m = 32.8083 ft."

 

[tursiops]

There is a note on p61 of the G2 manuals:

NOTE: Air (21/0) gives a different MOD than, for example, trimix 21/10. The reason is, the G2 uses a more precise value for the oxygen in the air, which is 20.7%​

It looks to me like the G2 may actually using the 855mb it is measuring -- which is 0.844 ATA -- for its calculations, but is adding in a safety factor (like Suunto does) to calculate the MOD, like 0.3-0.5%, which would allow it to mitigate someone rounding the "wrong" direction in an O2 analysis.

​

 

[Wibble]

It appears to me that you adjusted the ambient pressure to give you better agreement across your O2 concentrations.

Would it not make more physical sense to adjust the 10m? This number will change with temperature and salinity. Metre sea water - Wikipedia

Even worse (from link above):
"In the metric system, a pressure of 10 msw is defined as 1 bar. Pressure conversion between msw and fsw is slightly different from length conversion between metres and feet; 10 msw = 32.6336 fsw and 10 m = 32.8083 ft."

Isn't it so easy. 1 atmosphere is 10 metres. Simple as that.

PPO2 / percentage = MOD
1.4 / 0.21 = 6.66666667 atmospheres = which is 5.66666667 metres (the first ATA is free)

What is it in cubits per squirrel? (Says the Brit!)


Edit -- yes, decimal point madness -- it's 56.6 metres!!!

Thanks @Dan and @tursiops