MOD of Scubapro computers at 21%

[tursiops]

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.

Bingo! Good catch. And the 905mb switchpoint corresponds to 0.893 atm.
Thus: (1.4/0.32 -0.893)*10=34.82m. You got 34.6.
You are hung up on surface pressure being 0.92 atm (932mb), but I see no justification for that other than getting the answer you want...which could also be done (by SP) by increasing the FO2.
If SP is so smart, they will also not accept "salt" for a dive at C1 altitude. So 10m/atm ought to be 10.3 or 10.4, depending on your druthers.

 

[boulderjohn]

@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...

I just happened to look at this thread and found this post. I got no notification of the reference to me from ScubaBoard.

To be honest, I read the stuff about figuring out ScubaPro's calculations as something of a blur because I frankly don't care about ScubaPro's MOD calculations. I do most of my diving at roughly the same altitude as the OP, and I use 0.83 ATA for surface pressure. There was an earlier reference in this thread to an article from Utah about this, and they make the mistake of using 33 feet for the ATA changes of water, which is for salt water. I have done the calculations for our altitude and freshwater (34 feet) and created a MOD chart, and we use that chart for planning our dives.

For us, that planning is critical. Yesterday the student with whom I am working came by, and we mixed gases in my driveway. We had some issues when he accidentally added too much oxygen to his doubles, and with the price of helium being what it is, we did a lot of moving things around to get him an acceptable mix without sending precious helium into the stratosphere. When we were done, we got the mix down to 16% oxygen, which my chart tells me is good to 270 feet. We are planning to go to 250 feet, so that looks good. We will, of course, check again before we dive because it can take a while to mix completely.

So what do our computers say?

Well, we use Shearwaters, and I can't say that I have done a detailed analysis, but they seem to be pretty close to my MOD chart. When we are ascending at the end of the dive, and we approach a gas switch, it lights up the mix we are currently using to remind us that we have a gas in the system that we can switch to at that point, and it does so just about when we are expecting it. The nice thing about it is we can switch to O2 well before the 20 foot stop, and if we drop a foot or so during that stop, it is not going to get all excited and issue a high PPO2 warning, which is what annoyingly happens at sea level.

I am sure you have noticed by now the number of times I used vague terms "just about." That is because I know the precise numbers are estimates that don't need to be all that precise. At depth I stay enough above the 1.4 and 1.6 limits that I don't have to worry about things and don't have to care about carrying out calculations to the 4th decimal point.

If you are interested, here is my article on diving at altitude. There is a specific chapter on MOD.

 

[Nick Ruberg]

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.

I’ve been looking for the definition in the SI units and 10msw is defined as 1 bar (not 1 atm) according to the US navy diving manual and to Wikipedia.

 

[inquis]

I see v7 of the Navy Diving Manual stating 33 fsw per atm (only 2 significant figures). That equates to 10 m/atm or 9.9 m/bar.

Regardless, though, what matters is what YOUR computer is using. Suunto (arguably the market leader) and Shearwater computers, as well as Subsurface, Techdeco, and I'm pretty sure MultiDeco dive planners use 10.0 m/atm for salt. Hardly exhaustive, but a pretty wide swath through available tools. I only mention it because your question involved several decimals of precision that are typically ignored. Odds are that 10.0 m/atm is what SP uses. (It also seemed that way from the regression on your provided data points.)

Does the SP manual give any info about their concept of salt water? You might also check things with a fresh water setting (as someone mentioned above) to reduce the number of unknowns and focus on the surface pressure question.

 

[Nick Ruberg]

Indeed true. I mentioned it because I was also under the impression that 10m=1 atm. SP says nothing in The manual other Than that 10msw corresponds to approximately 10.3m fresh water.

I see v7 of the Navy Diving Manual stating 33 fsw per atm (only 2 significant figures). That equates to 10 m/atm or 9.9 m/bar.

Regardless, though, what matters is what YOUR computer is using. Suunto (arguably the market leader) and Shearwater computers, as well as Subsurface, Techdeco, and I'm pretty sure MultiDeco dive planners use 10.0 m/atm for salt. Hardly exhaustive, but a pretty wide swath through available tools. I only mention it because your question involved several decimals of precision that are typically ignored. Odds are that 10.0 m/atm is what SP uses. (It also seemed that way from the regression on your provided data points.)

Does the SP manual give any info about their concept of salt water? You might also check things with a fresh water setting (as someone mentioned above) to reduce the number of unknowns and focus on the surface pressure questioni

 

[Nick Ruberg]

I think I may have worked it out:
Sea level mbar: 1010
33 feet of sea water per atm and 3.28083 feet per metres gives us 10.05843 msw per atm and this gives us 9.95884 metres per bar of pressure
Altitude in bars at C1 (as per Scubapro manual) is 0.905 bar
PpO2 FO2 PAbs PAbs minus surf P MOD (calc) MOD (G2)
1.4 20.7% 6.76 5.8583 58.34 58.3
1.5 35% 4.286 3.3807 33.67 33.7
1.6 50% 3.2000 2.2950 22.86 22.9
1.4 70% 2.00000 1.0950 10.90 10.9

I've just chosen some arb values, but on my spreadsheet i have "tested" far more. I also get it to work out perfectly when i set it to fresh water.

 

[tursiops]

Best to run your checks for freshwater, because there is no ambiguity on meters of fresh water for 1 bar or 1 atm. 34 mfw is still an approximation; 1.0000 atm = 33.9324 mfw = 1.01325 bar.

Any conversion to msw depends on what you mean by "salt water." Shearwater uses:

The EN13319 (European CE standard for dive computers) value is between fresh and salt and is the Perdix default value. The EN13319 value corresponds to a 10m increase in depth for pressure increase of 1 bar.​

​

The density value used for each setting is:​

• Fresh Water = 1000kg/m³
• EN13319 = 1020 kg/m³
• Salt Water = 1030 kg/m³

 

[Nick Ruberg]

I think that the Scubapro calcs precede EN13319 ?? Not sure though, but here is the fresh water side of the calc:
10.3 metres per atmosphere
1010 mbars at sea level (same as above)
10.19802 metres per bar
PPO2 FO2 PAbs PAbs minus Surf P MOD (calc) MOD(G2)
1.4 20.7% 6.7633 5.8583 59.74 59.7
1.4 32% 4.375 3.47 35.39 35.4
1.6 50% 3.2000 2.2950 23.40 23.4
1.4 70% 2.00000 1.09500 11.17 11.2
1.6 100% 1.600000 0.6950 7.09 7.1