Nitrox and partial pressures

[johndiver999]

Is this an Oceanic thing or do you know of other brands that do the same?

I pretty much only use oceanic for many years now, so I don't know, sorry. It is not really a problem, just a default setting that must be set ONCE when you replace the battery - or accidentally reset the default by pressing the wrong button.

 

[SelfDiver]

Hello all,

I went diving on Saturday, April 26th with Jupiter Dive Center on an afternoon dive. I was given two cylinders of nitrox with approximately 36% of oxygen each and I set my Oceanic Pro Plus 3 computer accordingly as well as setting a conservative partial pressure of 1.40 as I was taught in the nitrox class. Both dives were expected to bottom out at a maximum depth of 75 feet at deepest portion of the reef. My first dive went well. After a 1 hour surface interval, we descended once more to the reef. However, my computer was beeping at me every time that I went below 60 feet, showing me that I was reaching my partial pressure of 1.40. I tried to stay above 60 feet the rest of the dive, but I was concerned that I would not be able to descend further to get a closeup view of the reef. What would be the reason that I was reaching the PP02 of 1.40 at 60 feet, while the other divers (who i assumed were breathing the same 36% mixture as everyone had either rented the cylinder from the shop or had them filled there) were able to descend below me to about 70+ feet? This really took out the fun out of the second dive.
I would be grateful with any assistance with the mechanics behind this.

Thanks!

Hey! Live and learn.
Computers are nice to have but be prepared to ignore it I'm when it misbehaves.
We should do some math-stuff first
1.4/0.36=3.889 so about 28 meters or 90 ft
The computer decided that 70 feet is too deep. We'll, it is wrong.
At this point in the dive, you ignore the computer, maybe just use the depth and ignore deco and Oxygen calculations.

A pre dive plan would have given you a max depth+runtime combination.
Just two numbers to remember and it gives you the ability to ignore the computer and continue the dive as planned.

You did the right thing in the moment. All the RTFM people are wrong along with the "buy a Shearwater" crowd. No amount of gear can compensate for lack of training.
The computer is not essential dive gear. It is nice to have a functioning computer (that you know how to use) to make life a bit easier. If it was essential life saving gear, you would take two or three.
A Shearwater or Suunto or Garmin will not help you if you can't figure out when to ignore it.
You analyzed a cylinder and wrote "MOD=90 ft". You may have even signed your name on a clipboard at the shop attesting to this measurement. And now you are in the water at 70 ft and the computer decided that you're too deep...so what if the computer is beeping at you?

Think of it as a learning experience.
Plan your dive.

 

[BRT]

Is this an Oceanic thing or do you know of other brands that do the same?

The Oceanic's I have can be set to not go to 50% for MOD purposes.

 

[tursiops]

The computer decided that 70 feet is too deep. We'll, it is wrong.

Nonsense.
The computer was set worng...it was set to 50% O2. Its calculation was correct. The diver was wrong in his setting; and he was wrong to ignore his computer. He should have aborted. His assumption what all the other divers were on 36% was...an assumption. That could have killed him. He is lucky.

 

[broncobowsher]

Nonsense.
The computer was set worng...it was set to 50% O2. Its calculation was correct. The diver was wrong in his setting; and he was wrong to ignore his computer. He should have aborted. His assumption what all the other divers were on 36% was...an assumption. That could have killed him. He is lucky.

The problem is the computer wasn't "set" to 50%. It decided on it's own that you did not interact with the computer just before the dive and set the nitrox level at that point, and as a safety feature it defaults to 50% oxygen and Nitrogen to 79%. As conservative as possible on the 2 gasses it watches.

There was an idea behind it. If you are diving and change gasses between dives and don't update the computer with the new gas, it goes into 'fail safe' gas choices. But if doing multiple tanks of the same mix, or a double dip on the same tank with a surface interval. It just goes into the fail safe mode and is pointless.
I had one of these computers. It is OK for air. But the Nitrox implementation is horrible. I remember going through the menus once while diving air and got into the Nitrox setting, went to 21% for air. Next dive, oh, you are on 50% O2 and going to kill yourself. Didn't take long and ditched it. Did my research, Liquidvision or Shearwater. I got Shearwater, perfect choice. Today it would be Shearwater or Garmin, but probably still Shearwater.

 

[halocline]

Nonsense.
The computer was set worng...it was set to 50% O2. Its calculation was correct. The diver was wrong in his setting; and he was wrong to ignore his computer. He should have aborted. His assumption what all the other divers were on 36% was...an assumption. That could have killed him. He is lucky.

I agree that the ‘fault’ was that the diver did not know enough about using his computer to set it correctly for the 2nd dive. But I’m curious why you think that could have killed him. He presumably knew he was diving 36% (I assume he analyzed his tanks like every nitrox diver must) and he knew that he was far shallower than the MOD.

I believe the computer probably defaulted back to N2 loading calculations at 21%, meaning it was showing him more N2 loading (less time to NDL) than he actually had. So it’s not like the computer would let him ascend while he was in reality loaded enough to require stops.

 

[tursiops]

I agree that the ‘fault’ was that the diver did not know enough about using his computer to set it correctly for the 2nd dive. But I’m curious why you think that could have killed him. He presumably knew he was diving 36% (I assume he analyzed his tanks like every nitrox diver must) and he knew that he was far shallower than the MOD.

I believe the computer probably defaulted back to N2 loading calculations at 21%, meaning it was showing him more N2 loading (less time to NDL) than he actually had. So it’s not like the computer would let him ascend while he was in reality loaded enough to require stops.

You are right; I was unclear. In general, he should follow the computer, not assume everybody else was OK to follow. His computer said don't go deeper; that did not seem right, so he went deeper anyway. That was not a good decision. We don't know if he analyzed his own tanks, or trusted someone else. All we know is he thought he was on 36% and could go to 90 ft, but his computer said stop at 70. He assumed the computer was wrong; perhaps it was right, and he was not on 36%? He did not really know. His only tool -- which he was trusting to tell him depth, time, NDL -- said don't go deeper. He ignored it. If it was wrong, if it had gone bonkers, maybe the depth was wrong too, or the the time? He suddenly had no trustworthy information, so he chose the worst decision: ignore his only input and just follow the crowd, trusting in their depth, time, N2, and O2 status. Not the safe decision.

If he had RTFM, he MIGHT have realized the problem, in which case he STILL should not have gone deeper for fear of locking the computer out and for sure having it store an incorrect N2 status (in case of a 3rd dive that day).. He SHOULD have stayed at 70, or if early in the dive, surfaced, waited 10 mins for the computer to reset to a new dive, reset it to 36%, and gone down again to rejoin the group. I've had to do that, others have had to do that too.

 

[Divin'Papaw]

I agree that the ‘fault’ was that the diver did not know enough about using his computer to set it correctly for the 2nd dive. But I’m curious why you think that could have killed him. He presumably knew he was diving 36% (I assume he analyzed his tanks like every nitrox diver must) and he knew that he was far shallower than the MOD.

You are right; I was unclear. In general, he should follow the computer, not assume everybody else was OK to follow. His computer said don't go deeper; that did not seem right, so he went deeper anyway. That was not a good decision. We don't know if he analyzed his own tanks, or trusted someone else. All we know is he thought he was on 36% and could go to 90 ft, but his computer said stop at 70.

He was diving with Jupiter Dive Center using their tanks. The tanks were analyzed either by him or by a crew member in his presence. That is standard practice on all of their charters.

 

[tursiops]

He was diving with Jupiter Dive Center using their tanks. The tanks were analyzed either by him or by a crew member in his presence. That is standard practice on all of their charters.

So? If he is sure he was on 36% and he can't trust his computer, what is the prudent action? Follow the crowd? Or surface?

 

[scubadada]

I have been diving Oceanic computers since 2002, more than 2400 dives. I have never once turned on the FO2 50% default. This function is described in detail on pages 13 and 14 of the Pro Plus 3 manual. Yes, please be familiar with your computer and read the owner's manual.

There is no excuse for diving on the FO2 50% default by accident. I don't disagree, this is a particularly lame setting that has been in place for a very long time on Oceanic computers. Extremely easy to avoid, permanently