Cressi Leonardo - MOD vs PO2 discrepancy
- Thread starter delta-v
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
OP
My sentiments exactly.
Moreover, it sets up a conflict b/w the instructor (and training materials) versus the dive computer on the student's wrist... thus confusing the student about a calculation which is otherwise very straightforward.
Taking this one step forward, let's say the student/diver uses the Cressi to determine max depth to mark/label the cylinders... you can see the confusion this would cause on the boat where divers with the same mix (e.g. EAN32) would have different MODs on their cylinders.
Really silly and pointless in my opinion.
We could not go diving due to bad weather, but I am now very keen to find out at what depth will the Cressi sound the alarm for max depth at set PO2.
We plan to take EAN28 to 40 m / 131 feet. Will it sound off as if we're diving EAN29?
Rooster59
Contributor
O2% + 0.8 ?
... 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 think when you put in PO2 of 1.4 it is actually using 1.35 for the calculation for max depth. Safety factor only shortens the NDL time, not the depth. It probably also uses 1.25 for 1.3, and 1.45 for 1.5. I’ll check my computer against calculations later this week to see.
When I first used nitrox in the 1990s, we were always told to round up; never deal in fractions, on those early tables; and the Suuntos instruct you to do the same . . .
Actually Suunto appears to round down:
NOTE: When you have analyzed your gas, you should round the result down when entering it for Suunto EON Core. For example, if the analyzed gas is 31.8% oxygen, then define the gas as 31%. This makes the decompression calculations safer. Oxygen calculations (pO2, OTU, CNS%) are also kept conservative, since the oxygen percent used for those are O2% + 1.
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.
The above taken from the EON Core manualWARNING: 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.
OP
I've written Cressi and forwarded this thread. Waiting for a response.
At the moment it appears the Leonardo is approaching max depth calculations in the same manner as some of the other rec computers mentioned in this thread - with the exception that its user manual makes no mention of this padding.
I feel the issue of fractions and rounding is a red herring here. The diver makes that decision after s/he analyses the mix and rounds down accordingly. The computer does not accept decimal percentages.
So if a diver inputs a specific FO2 and a specific PO2, then the computer should not interfere - esp. without a clear mention of this in the user manual.
On further reflection, this padding raises an ugly possibility. Is the FO2 + 1% only applied to the max depth calculation, or to the N2 on-gassing as well?
In other words, if the computer treats an entered EAN28 as EAN29, then does it carry this over to the real-time no-stop calculations? Thus showing the diver more NDL time (and less deco) than actual... which would be quite dangerous. I assume they wouldn't be mad enough to do that, but you can see the doubts this kind of meddling raises.
At the moment it appears the Leonardo is approaching max depth calculations in the same manner as some of the other rec computers mentioned in this thread - with the exception that its user manual makes no mention of this padding.
I feel the issue of fractions and rounding is a red herring here. The diver makes that decision after s/he analyses the mix and rounds down accordingly. The computer does not accept decimal percentages.
So if a diver inputs a specific FO2 and a specific PO2, then the computer should not interfere - esp. without a clear mention of this in the user manual.
On further reflection, this padding raises an ugly possibility. Is the FO2 + 1% only applied to the max depth calculation, or to the N2 on-gassing as well?
In other words, if the computer treats an entered EAN28 as EAN29, then does it carry this over to the real-time no-stop calculations? Thus showing the diver more NDL time (and less deco) than actual... which would be quite dangerous. I assume they wouldn't be mad enough to do that, but you can see the doubts this kind of meddling raises.
Eksilbergenseren
Registered
Just a thought, haven't done the calculations, but:
EAN28 has a PO2 of 1.4 at 5 bars. That, I guess, is undisputed. But what depth 5 bars compares to is a sum of air pressure and density of water (depends on gravity as well, but let us assue that to be constant and keep that out of it.) If the computer in question assumes a higher air pressure than 1 bar and a water density corresponding to more than 1 bar per 10 meters it will reach 5 bars at a lower depth.
While in water, on the otherhand, it should calculate depth with the same numbers, and the PO2 alarm will go off at the same depth as the other computer. (They will show different depth on the display.)
Actual seawater is denser than what corresponds to 1 bar per 10 meter, and the air pressure is often more than a bar. So the computer showing PO2 of 1.4 at less than 40 meters may very well be most correct. But again, as long as you calculate and meassure with the same computer it does not matter what these input values are.
The computer with correct input will display a more correct depth. (When compared to detph meassured by a tape meassure.)
It would be interesting to see both computers mounted on the same arm, and see if they display the same depth.
EAN28 has a PO2 of 1.4 at 5 bars. That, I guess, is undisputed. But what depth 5 bars compares to is a sum of air pressure and density of water (depends on gravity as well, but let us assue that to be constant and keep that out of it.) If the computer in question assumes a higher air pressure than 1 bar and a water density corresponding to more than 1 bar per 10 meters it will reach 5 bars at a lower depth.
While in water, on the otherhand, it should calculate depth with the same numbers, and the PO2 alarm will go off at the same depth as the other computer. (They will show different depth on the display.)
Actual seawater is denser than what corresponds to 1 bar per 10 meter, and the air pressure is often more than a bar. So the computer showing PO2 of 1.4 at less than 40 meters may very well be most correct. But again, as long as you calculate and meassure with the same computer it does not matter what these input values are.
The computer with correct input will display a more correct depth. (When compared to detph meassured by a tape meassure.)
It would be interesting to see both computers mounted on the same arm, and see if they display the same depth.
OP
That's a really interesting perspective Eksilbergenseren, thanks for sharing.
Since we're looking at the bigger picture here (from a purely academic PoV!), let's also add:
- Gas mix analysis variation (user error or EANx analyser fault)
- Dive computer pressure transducer variation (typically +/- 1 m / 10 ft)
- Salinity and temperature of the dived water
Since we're looking at the bigger picture here (from a purely academic PoV!), let's also add:
- Gas mix analysis variation (user error or EANx analyser fault)
- Dive computer pressure transducer variation (typically +/- 1 m / 10 ft)
- Salinity and temperature of the dived water
Similar threads
