Cressi Leonardo - MOD vs PO2 discrepancy

[delta-v]

Student is using her Cressi Leonardo. PO2 set to 1.4.

When entering EANx mix, the max depth displayed by the computer is shallower than expected.
E.g. EAN28 gives a max depth of 38.7 metres.
But EAN28 @ 1.4 PO2 = 40 m

Have reviewed the user manual but cannot find an explanation for this behaviour.

Even the graphic illustrations in the manual show shallower depths than expected!
E.g. air (21%) @ 1.4 PO2 is 54.2 m (should be 56.6 m)
E.g EAN32 @ 1.4 is 32.6 m (should be 33.75 m)

The discrepancy is not a fixed constant. It appears the computer is internally using a PO2 less than 1.4 to determine max depth for the given mix.

Searched online but could find no precedents.
Any ideas what may be causing this 'error'?

 

[Manatee Diver]

On Suuntos they round up to the next percentage, maybe Cressi does the same.

 

[Andy in Gap]

There are conservatism settings on the Leonardo called "Dive Safety Factor" in the manual. Its at either 0, 1 or 2. Its possible that the safety factor modifies the PPO2 the computer considers to be safe by some percentage, or doesn't allow you to go to full MOD. I used mine on 0 (least conservative) and never remember having this issue.

 

[delta-v]

On Suuntos they round up to the next percentage, maybe Cressi does the same.

Thanks for the reply, I don't have a Suunto handy to check, but with EAN28 there's no rounding up/down.
At 1.4 PO2, EAN28 is 5 ATA (40 metres / 132 feet).
Very strange!

 

[delta-v]

There are conservatism settings on the Leonardo called "Dive Safety Factor" in the manual. Its at either 0, 1 or 2. Its possible that the safety factor modifies the PPO2 the computer considers to be safe by some percentage, or doesn't allow you to go to full MOD. I used mine on 0 (least conservative) and never remember having this issue.

Yes, so we disabled the Deep Stop and the Dive Safety Factor at the outset.
Following your reply, I asked the student to change SF back to 1 and 2 and run the PLAN mode again.
Unfortunately the max depth does not change - there's still a significant discrepancy.

I've also ruled out saltwater/fresh water conversion (the Leonardo doesn't have this as an option).
Will write Cressi directly.

 

[Manatee Diver]

Thanks for the reply, I don't have a Suunto handy to check, but with EAN28 there's no rounding up/down.
At 1.4 PO2, EAN28 is 5 ATA (40 metres / 132 feet).
Very strange!

I know for a fact that the Cobra 3 and the Zoop Novo rounded up, it is even in the manual.

From Page 68 of the Cobra 3 manual:
As a safety precaution, the oxygen calculations in the computer are made with an
oxygen percentage of 1% + set O2%.

 

[delta-v]

I know for a fact that the Cobra 3 and the Zoop Novo rounded up, it is even in the manual.

From Page 68 of the Cobra 3 manual:
As a safety precaution, the oxygen calculations in the computer are made with an
oxygen percentage of 1% + set O2%.

Oh okay, got you now.
That may be exactly what's happening here, although the Leonardo manual doesn't mention this anywhere.
It seems excessive nanny-ing, considering the diver is required to set the PO2. If someone selects 1.4 PO2, they should get 1.4 PO2.
Oh well, I'll get an official confirmation from Cressi, but thank you for the insight!

 

[Esprise Me]

I've noticed this on my Cressi as well and was mildly annoyed by it. It's one thing to have a conservative algorithm for something with as many variables as decompression, but calculating max depth for a given PO2 is just simple math. If you let me choose my PO2 and some nebulous safety factor (which I set to zero), why do you then pad the numbers?

 

[KenGordon]

why do you then pad the numbers?

You measure some gas as 28.9, 28.5, or 28.4, what do you tell the computer? 28 or 29? I guess you tell it 28 and it expects that or it will have to do the deco numbers based on of whatever you put in less one. So your entering 28 means anything from 28.0 to 28.9.

Rounding rules...

 

[Esprise Me]

Interesting. The math sort of checks out, but more like they think I'm an idiot than like they need to account for rounding. If I measure the O2 at 32.9%, rounding down to 32 would be wrong-- doesn't everyone know you round up for .5 and above? So it would only have to account for up to .4 difference--say, I tell my computer 32% but really it's 32.4%.

If I'm diving with exactly 32%, I can dive to 111.375 feet without exceeding po2 of 1.4. The tables I've seen round this down to 111, which is fine by me. But my computer tells me 106 is my limit. I calculate that as one foot shallower than the max depth for 33%. That's not accounting for rounding; that's treating me like I either can't measure gas within a full percentage/don't know how to round, or can't control my buoyancy within 5 feet of accuracy.