Cressi Leonardo - MOD vs PO2 discrepancy

[Angelo Farina]

The point is that rounding the O2 percentage UP means being more safe in terms of exceeding the limit of oxygen toxicity, but it is LESS SAFE for Nitrogen assumption and release...
So the safe rule should always be to round DOWN the Oxygen percentage, which means rounding UP the Nitrogen percentage, which is safe for deco.
So at that point you would not be fully safe for Oxygen toxicity. Hence the need to add 1% "internally", for safety.
I think that the Cressi approach is the correct one, for being safe BOTH regarding Nitrogen (deco) and Oxygen (toxicity).
In practice, your round down for deco and round up for toxicity. A win-win case!

 

[tursiops]

You must round off your analysis, and do so in the safest direction. That is NOT the same direction for N2 absorption and O2 toxicity. For N2, you round down, so 31.5 becomes 31. For O2 you round up, so 31.5 becomes 32. Unfortunately, you can only set one number in your computer, so you set the N2-safe number (31) in your computer, and stay 5 ft above your MOD to compensate for the correction needed. Suunto tries to do this for you, not entirely successfully IMHO.

 

[delta-v]

Thanks Angelo and Tursiops for the additional info. The rationale is not in dispute here.

For e.g. we rebreather divers (esp. on extreme exposures) use a lesser PO2 setpoint for deco calculations and a higher setpoint for CNS loading calculations. The PO2 spread looks like 1.25 / 1.30 / 1.35.
The rationale is to compensate for PO2 fluctuations in the breathing loop which is never constant.

The core issue here is the fact that a precision diving instrument is deliberately (and secretly) manipulating the results of a fundamental calculation which should be universal given the same inputs.

Thus 2 + 2 = 4, except in Cressi-land where 2 + 2 = 3.

 

[Esprise Me]

... unless it, being Italian, is computing in those units them Yurrupeans are using, and then back-converting to (a round number of) "standard" units. Ask google what 32.4 metres is in feet, you may get enlightened.

Like Ken says, rounding's fun.

I actually did the calculation in bars initially before converting to feet. I'll admit I used 1 bar = 33 feet as a shortcut, knowing it's not exact. But doing it again as a precise calculation, I get a MOD of 110.728 feet--much closer to 111 than 106. What's the significance of 32.4 meters, besides being roughly 106 feet?

ETA: I confirmed that my Nitrox training manual says to round off to the nearest number, not to round up or down, and the Cressi manual doesn't say otherwise.

 

[dmaziuk]

I actually did the calculation in bars initially before converting to feet. I'll admit I used 1 bar = 33 feet as a shortcut, knowing it's not exact. But doing it again as a precise calculation, I get a MOD of 110.728 feet--much closer to 111 than 106. What's the significance of 32.4 meters, besides being roughly 106 feet?

Try 33% in e.g. this one and see. (And note where he rounds the corresp. dead king boot sizes.)

 

[dmaziuk]

I confirmed that my Nitrox training manual says to round off to the nearest number, not to round up or down, and the Cressi manual doesn't say otherwise.

PS. legal dept. says to always round toward "safe". Obviously. 32.?%: to 33% and 106.29': to 106'.

 

[Bigbella]

Actually Suunto appears to round down:

WARNING: THE DIVE COMPUTER WILL NOT ACCEPT FRACTIONAL PERCENTAGE VALUES OF OXYGEN CONCENTRATION. DO NOT ROUND UP FRACTIONAL PERCENTAGES! Rounding up will cause nitrogen percentages to be understated and will affect decompression calculations.
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Thanks for that. I recollected just the opposite; it's been a while since I futzed with them. Gotta hit those manuals again. No huge loss, since most consumer O2 analyzers are not terribly accurate. "Rounded up for MOD; down for nitrogen debt," was what I was always taught.

It was the physical tables themselves, before computers, where we rounded to the closest whole percentage . . .

like everything in this sport, you round to the more conservative number for safety. In the case of O2 percentage it is always up for MOD calculations, and down for decompression calculations. Rounding up for MOD will give you a shallower depth, though it is splitting hairs at that point, but still good to round up, and rounding down for decompression calculations will give you a slightly longer run time, but you'll come with a bit less nitrogen loading.

This btw should have been covered in your nitrox class....

 

[delta-v]

O2% + 0.8 ?

Spot on!
See reply from Cressi below.

 

[Manatee Diver]

Thus 2 + 2 = 4, except in Cressi-land where 2 + 2 = 3.

That is true for really small values of 2, with really large values of 2: 2 + 2 = 5

 

[delta-v]

A Cressi rep replied to my email; pretty decent response time. Reproduced verbatim:

Air PO2 is universally considered to be 21% so, when Air mode is set., PO2 = 21% is used in Max Depth calculation.
When using a Nitrox mix (not Air) the set PO2 is a measured and rounded quantity so, in order to take that into account and to have a larger safety margin, the Max Depth calculation is done using (PO2 = set PO2% + 0,8%).
Hope this can help.
Merry Christmas and best wishes for a happy new year.

This officially explains the discrepancy and confirms that the Leonardo displays a modified MOD.

I have suggested to Cressi that this modification ought to be clearly documented in the user manual.