MOD of Scubapro computers at 21%

[Nick Ruberg]

Hi guys,
Does anyone know why there is a bit of an anomaly in calculating the MOD at 21% with SP computers. Mathematically, all the way from 22% upwards, the calculation works But only if I use a surface P of 0.92 - problem is that the computer is giving me a pressure of 0.855 bars.

to get the same figures as my G2 I need to enter an O2 of 20.74% at 21%

here’s what I mean:
PPO2. = 1.4
salt water
The G2 says surface pressure is 855 millibars but for the calculation I will use 920 millibars
At 21% MOD = [ (1.4/0.2074)-0.92] * 10 = 58.3 (same as G2 using 21%
At 22% MOD = [ ( 1.4 / 0.22)-0.92] * 10 = 54.4 (same as G2)
At 32% MOD = [ (1.4 / 0.32)-0.92] * 10 = 34.6 (same as the G2)

two questions:
1. If the G2 gives me a surface pressure of 855 millibars, it appears to be using 920 millibars in its calculation
2. why use 20.74% as the oxygen in 21% (should be around 20.94% to be correct)

thanks,
nick

 

[lowviz]

Obviously you are splitting hairs. None of it means anything in real world diving, but you do raise a scientifically valid (but inconsequential) point.

Since you are playing with four places of precision, your results are easily skewed by the local salinity of salt water. First global map of soil moisture and ocean salinity

EXACTLY what is your G2 using for this value?

 

[Wibble]

MOD is normally PPO2 divided by Mix = Pressure in ATA
So for 21% it's 1.4 / 0.21 = 6.66 ATA which is 56.6 metres (multipled by 3.3 for 188.1 feet).

There's theory and there's practice.

Theoretically there's loads of decimal places you can worry about.
Practice says don't bother: it's all approximate as soon as a person gets in the water as they're all different.

The important thing is seawater / freshwater or even diving in a vat of alcohol makes no difference as it's the pressure that counts. That if you brought a tape measure (calibrated of course) with you to measure to the millimetre how deep you are, you've missed the point. It's the 6.66ATA which counts, setting the PPO2 of the 21% at 6.66 atmospheres; if it's 57 metres in seawater, it'll be about 2% deeper = 57+1.14= 58.14 metres in freshwater. Who cares!

OK, some may care. For example if you're diving in a cave (freshwater) and surveying it. Unless you're doing something like that, it doesn't matter.

 

[tursiops]

If he is going to track so many decimal places, he needs to take into account that his conversion from ATA to depth is wrong because he is assuming sea level and an arbitrary kind of "salt" water. He should do the comparison in fresh water, and if he G2 is correct, not assume he is at sea level.

 

[lowviz]

And then there is salinity...

https://link.springer.com/content/pdf/bbm%3A978-3-319-18908-6%2F1.pdf

 

[broncobowsher]

Good luck reading O2 to that many decimal points as well. Many of us don't even bother with the 0.1 and just call air 21%.

 

[lowviz]

Getting close.

Your G2 online manual gives:

"2.2.5 Water type

The G2 measures pressure and converts it to a depth value by using water density as a constant. A depth of 10m/33ft in salt water corresponds approximately to 10.3m/34ft of depth in fresh water."

Note the lack of four significant digits or any mention of standard salinity. Your calculations appear to be within SP's experimental error...

 

[BoundForElsewhere]

This is going to be one of "those" threads, isn't it?

I can read these things three times and still not understand a lick of it.

 

[Nick Ruberg]

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.

 

[Nick Ruberg]

If he is going to track so many decimal places, he needs to take into account that his conversion from ATA to depth is wrong because he is assuming sea level and an arbitrary kind of "salt" water. He should do the comparison in fresh water, and if he G2 is correct, not assume he is at sea level.

The G2 works on 10m of salt water to 1 bar pressure (stated in the manual). The conversion is based on what the G2 reads at the surface (since it gives a pressure in millibars).