MOD of Scubapro computers at 21%

[tursiops]

Aha. That 20.7% now explains a bit. That is what I backward calculated. Only question now remains why when the barometric pressure was reading 855 millibars (as per the G2) It appeared to use 920 millibars.

also, why 20.7%?

Your location at Johannesburg explains the 855 mb nicely. 855 mb is about 0,844 atm. You should be using atm in the calculation, not mb. Go through your calculations again, letting the FO2 be the variable, and fixing 0.844 atm and using the G2 calculated MOD. See what percentages it corresponds to.

 

[lowviz]

Go through your calculations again, letting the FO2 be the variable, and fixing 0.844 atm and using the G2 calculated MOD. See what percentages it corresponds to.

This is clearly an altitude diving 'what if' exercise. Yet again, I question the use of 10 meters per atmosphere in this particular MOD calculation...

Edit:
@Nick Ruberg, this may help: Nitrox and Diving at Altitude – Utah Scuba Divers

 

[Nick Ruberg]

Your location at Johannesburg explains the 855 mb nicely. 855 mb is about 0,844 atm. You should be using atm in the calculation, not mb. Go through your calculations again, letting the FO2 be the variable, and fixing 0.844 atm and using the G2 calculated MOD. See what percentages it corresponds to.

You’re quite right in that atm(not mb) should be used. However, 0.844 makes my calculation worse. If I use that on the 22% I work back to an FO2 of 22.2%

 

[tridacna]

Johannesburg, South Africa. We’re pretty high here.

Ek se. Move to Durbs and you won't be so high.

 

[lowviz]

@boulderjohn, would you mind clearing this up? It shouldn't be anywhere near this hard to figure out what SP is using in its calcs...

 

[tursiops]

You’re quite right in that atm(not mb) should be used. However, 0.844 makes my calculation worse. If I use that on the 22% I work back to an FO2 of 22.2%

Nick, 22.2% and 22% are the same in scuba diving.
If you use 22 vs 22.2, but keep 0.844 atm, what are the two MODs that you get? Both results, rounded off, give 55m.
By the way, going backwards from 54.4 gives 22.3%, not 22.2%; you've got to round off.
I still think ScubaPro is adding a "safety" factor of 0.3 to 0.5% to whatever percentage O2 you put in, to give you a slightly smaller MOD. Note that -- using partial derivatives -- a change in FO2 of 1% give a change in MOD of about 3m.

 

[inquis]

The main issue I see is taking the G2 numbers as reference because they seem wrong to me. (It doesn't make much sense to think about the Scubapro until that's resolved.) Looking at the 22% and 32% numbers for the G2:
Surface pressure P0 is 855 millibars (measured) = 0.844 atm (calculated)
At 22% MOD = 54.4 (stated)
At 32% MOD = 34.6 (stated)
It's not clear whether the G2 is set to saltwater from the OP, but it should be one of 10.0 m/atm (salt), 10.3 m/atm (fresh), or 10.13 m/atm (EN13319, which equates to 10 m/bar). Call this W (for Water column).

Since MOD = (1.4 / FO2 - P0) * W, there is no combination of the above possibilities that give the two stated MOD values. The closest combination is using a value of 0.844 for P0 and 10.0 for W, but evaluate to nearly a meter too high.

Additively inflating the FO2 (ala Suunto) by 0.5% gives agreement at the 32%, but is 0.6m off from the 22%. Inflating by a multiplicative factor of 1.5% (i.e., 1.015x) is off by 0.1m on both, so I could see this being down to roundoff error in the stated MODs. On the flip side, the use of multiplicative inflation seems strange to me in spite of it achieving the best match.

Without more data, it's hard to guess what the G2 is doing any further. However, the original question is how do things relate to the ScubaPro, and it's very likely that it should NOT match to sub-meter precision. You may get further by contacting Garmin and ask them what exactly goes into that calculation, as it clearly not the "standard" equation.

 

[inquis]

One other possibility: The G2 MODs for 22% & 32% are matched within roundoff of the stated values by using a partial pressure of 1.38 instead of 1.40 (and surface pressure converted to atm and salt water 10.0 m/atm, as one would hope, and no FO2 inflation). With more data points, you might reverse-engineer it, but it's easier just to round to the nearest meter and move on.

 

[Nick Ruberg]

Thanks for your input, inquisit. Well taken. I am using salt water just to make the values easier (10m = 1 bar). Yes, I know it doesnt make sense to dive salt water at altitude. This isnt a diving question but maths.
Just had another thought.
Today the surface pressure is different but the MOD is the same, so I thought that likely SP is taking into account the fact that I am at altitude class C1 and according to their G2 manual (pg 53) the switch point from C0 to C1 is 905 mbar, so I think it doesnt take the surface P into account when giving the MOD, but rather just a fixed value of 0.92 - which makes sense otherwise your MOD will be changing slightly all the time.

Now, to equate 0.92 ata to what? at 905 mbar (SP switch point to C1), that works out to 0.893 ata

 

[inquis]

I am using salt water just to make the values easier (10m = 1 bar)

That's not "salt" water for most computers with which I'm familiar. (No idea about SP, though.) That's some in-between made up thing (EN13319 standard) because salinity varies in the real world. "Salt" is 10.0 m = 1 atm for everything I've seen. EN13319 is 10.0 m = 1 bar or 10.13 m = 1 atm. Not important in the real world, but that's not your question.