NITROX computer questions

[Pinto]

My brain cramped when reading the manual on my new DC. I could use some help understanding one of the setting options. When diving Nitrox, the set up is straight forward with easy to set PO2 alarm value and FO2 setting for the mix.

There is a separate FO2 Default option which comes factory set on but allows a diver to turn it off. According to the manual, when set on, 10 minutes after surfacing, FO2 displays at 50% and all further calculations are based on 50% O2 for Oxygen and 21% O2 for Nitrogen (79% Nitrogen).

This statement was taken right out of the manual "50% O2 for Oxygen and 21% O2 for Nitrogen (79% Nitrogen).

First question= Can this statement be correct?

When FO2 is set 'on' the DC reverts to 50% after each repetitive dive unless FO2 was set prior to the dive. When set off, FO2 remains at the last set point.

My questions:
1) Does this mean that once I have surfaced, 10 minutes after the dive, calculations assume I am breathing normal air or 50% O2? I assumed the DC want to default to 21% O2/79% Nitrogen until I reset PO2 and begin my next dive?

2) Why does it wait 10 minutes after the dive to make the change?

3) If I turn this Default setting off and PO2 on the dive was set for let's say 32%, during S/I I'm not consuming 32% gas. In regards to calculating total O2 exposure, isn't the computer running the numbers the same whether the FO2 default is on or off?

 

[PfcAJ]

That's real weird. What model of computer are you using?

 

[Blackwood]

It's pretty normal (I think).

Unless you set it otherwise, it uses Air for the deco calcs and 50% for the CNS calcs. Unless you have an advanced nitrox card, this is a conservative approach, however arbitrary.

To your questions, I can only guess without reading the manual/using the unit.

1) Does this mean that once I have surfaced, 10 minutes after the dive, calculations assume I am breathing normal air or 50% O2? I assumed the DC want to default to 21% O2/79% Nitrogen until I reset PO2 and begin my next dive?

It assumes you're breathing air on your surface interval. I highly doubt it counts anything during your SIT towards oxygen loading. It's just preparing for the next dive.

2) Why does it wait 10 minutes after the dive to make the change?

Probably in case you go back down right away.

3) If I turn this Default setting off and PO2 on the dive was set for let's say 32%, during S/I I'm not consuming 32% gas. In regards to calculating total O2 exposure, isn't the computer running the numbers the same whether the FO2 default is on or off?

I imagine so. As I said above, I doubt it counts the surface interval towards oxygen at all, so whatever gas it's assuming for the next dive is irrelevant.

Again, this is just conjecture. I don't use a "dive computer" per se. Take it for what it's worth (zero).

 

[PfcAJ]

It's pretty normal (I think).

Unless you set it otherwise, it uses Air for the deco calcs and 50% for the CNS calcs. This is a conservative approach, however arbitrary.

To your questions, I can only guess without reading the manual/using the unit.

1) Does this mean that once I have surfaced, 10 minutes after the dive, calculations assume I am breathing normal air or 50% O2? I assumed the DC want to default to 21% O2/79% Nitrogen until I reset PO2 and begin my next dive?

It assumes you're breathing air on your surface interval. I highly doubt it counts anything during your SIT towards oxygen loading.

2) Why does it wait 10 minutes after the dive to make the change?

Probably in case you go back down right away.

3) If I turn this Default setting off and PO2 on the dive was set for let's say 32%, during S/I I'm not consuming 32% gas. In regards to calculating total O2 exposure, isn't the computer running the numbers the same whether the FO2 default is on or off?

I imagine so.

Its normal? I guess I'm just used to reality

 

[Blackwood]

Its normal? I guess I'm just used to reality

What, your backgas doesn't turn into a magical potion after your first dive?

AFAIK, most nitrox computers have this 'feature.'

 

[Xaryo]

is it a sherwood insight or wisdom?

it sounds like mine.

the o2 setting going back to 50% after a dive is a safety.

if you forget to reset your FO2 for the next dive, it will calculate a hypothetical gas of 50% O2 and 79% N2, a worst case scenario. you will be penalize in time and depth, but at least you'll be safe.

As for the 10 minutes-waiting, it is because if you get back in the water before 10 minutes, the computer will consider it the same dive, so no need to change the FO2.


with the FO2 setting at ON, if you set your computer for Nitrox 21%, it will stay at 21% between dive, but it will be "stuck" in nitrox mode" untill it shut down 24h after the end of last dive.


You should re-read the manuel to know exactly what your computer tells you.

 

[Puffer Fish]

What, your backgas doesn't turn into a magical potion after your first dive?

AFAIK, most nitrox computers have this 'feature.'

Oceanic, at least the older ones just do the 50% thing.. while scubapro ones just stay where you put them....and some do the after the dive.. some do it the next day. Amazing how many different ways there are to punish people for not understanding their computer..

Note The diverite duo switched at midnight, the treo does it 10 minutes after surfacing...so even different models can vary.

 

[Marek K]

This is a Pelagic-type computer, right? Like Aeris Atmos or Oceanic Veo?

Ours say the exact same thing.

I don't think this has anything to do with the surface interval. It's a default safety feature, when turned ON, that just has to do with how the computer is set for the next and subsequent dives.

If ON, and you forget to set the exact PO2 for the subsequent dive(s), the computer will default to the worst-case values of O2 (50%) and N2 (79%) -- separately. Which means that it will calculate both O2 and N2 saturations at very high rates -- rates that wouldn't both occur with any actual mix, but it's safe-siding both values. Which in turn means that it should calculate a short and shallow, but safe, dive.

You might want to set it ON as a fail-safe if you dive tanks with various PO2 values.

If you dive pre-banked Nitrox, which is pretty much going to be the same PO2 each time (like EAN 32), you may want to set the feature OFF and set the PO2 to that value. The setting won't change then between dives, but you're still going to test the gas to make sure.

*edit* Xaryo said pretty much the same thing above -- Sherwoods are made by Pelagic too, I think.

 

[Xaryo]

I think you're right Marek about who make the computer.

It is very important to read carefully the owner manuel. All these little details are inside. at any time, you should know what you computer tells you.

 

[ianr33]

Turn the stupid feature off and be CERTAIN to check the mixture before descending on each dive.

Its a "safety" feature but in practice its just annoying: Do a dive on 32%, do a second dive ,again on 32% and your computer will give you all sorts of O2 warnings because its reset itself to 50% O2 during the surface interval. Brilliant!

It will use 21% as the breathing gas during surface intervals. (Otherwise there would be lots of bent divers and lawsuits!)