Nitrox and partial pressures

[boulderjohn]

RTFM--sometimes it helps.

I have never read the manual for this computer, so I am speaking in general of manuals I have read, and in those cases, I would call them borderline unreadable.

Because of concerns with liability, they include every blessed fact possible (with exceptions noted later in this post. The order in which that information appears is often chaotic. It would be really nice if they listed the 6-8 key things you need to know in one place, but I suspect an attorney has told them not to do that because it could be argued later on that doing that keeps people from discovering some other fact buried in all the verbiage of the rest of the manual.

I bought my first computer more than a quarter century ago, and I well remember my mind swimming with all that information scattered throughout the manual. It would not surprise me in the slightest if someone made a good faith effort to RTFM and missed the little tidbit tucked away somewhere about the computer switching the nitrox setting on its own.

 

[tursiops]

This function is described in detail on pages 13 and 14 of the Pro Plus 3 manual.

The description is quite clear and quite prominent.

I have never read the manual for this computer, so I am speaking in general of manuals I have read, and in those cases, I would call them borderline unreadable.

Yep, some are terrible. This one is not.

I bought my first computer more than a quarter century ago, and I well remember my mind swimming with all that information scattered throughout the manual. It would not surprise me in the slightest if someone made a good faith effort to RTFM and missed the little tidbit tucked away somewhere about the computer switching the nitrox setting on its own.

You are being much too generous. it is much more likely that the manual was never read. Occam's Razor.

 

[grantmac]

Any computer that needs RTFM to do a simple NDL dive on any mixture of oxygen and nitrogen is a piss poor design.
Any fool can figure out how to dive a shearwater in tech mode by simply scrolling through the menus while watching a YouTube video.

 

[scubadada]

Any computer that needs RTFM to do a simple NDL dive on any mixture of oxygen and nitrogen is a piss poor design.
Any fool can figure out how to dive a shearwater in tech mode by simply scrolling through the menus while watching a YouTube video.

Sorry, setting up a Teric completely from scratch, even in OC rec mode, requires you read the manual. After doing this countless times now, I could probably do it in my sleep.

 

[Divin'Papaw]

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?

I’m not playing along. I was simply answering the question regarding analysis. He would have known he was in fact diving 36%.

 

[broncobowsher]

My issue with the a computer is it doesn't like being setup and left.
The usual dive for me is to setup everything before hand and have it ready to go. The day or night before. Or as soon as I am out of the water I am setting everything up for the next dive. The pre-jump is as minimal as possible. I am not going to be setting the gas into the computer on the pre-jump. That's just a poor design.

The Oceanic I had was a neat wizzbang computer when I first got it. Didn't take long to find it only marginally useful. After a decade of 'better than a bottom timer and tables' I got a Shearwater. Now own 3 of them.

 

[scubadada]

My issue with the a computer is it doesn't like being setup and left.
The usual dive for me is to setup everything before hand and have it ready to go. The day or night before. Or as soon as I am out of the water I am setting everything up for the next dive. The pre-jump is as minimal as possible. I am not going to be setting the gas into the computer on the pre-jump. That's just a poor design.

The Oceanic I had was a neat wizzbang computer when I first got it. Didn't take long to find it only marginally useful. After a decade of 'better than a bottom timer and tables' I got a Shearwater. Now own 3 of them.

Oceanic has some idiosyncrasies, but they are easy to deal with. The initial nitrox setting will apply for 2 hours and would then require a reset. Once diving, the nitrox remains unchanged as long as the surface interval is less than 24 hours, otherwise would require reset.

 

[SelfDiver]

The original question is answered. The computer has a setting that resets Nitrox to 50%

The additional discussion on computer setup is irrelevant.

Let's assume that the computer was set up correctly and because of whatever, goes in to GUAGE mode. Do you end the dive or continue according to plan with your depth and time devices?
The pre-planned deco should match whatever a correctly functioning computer would give for a square profile.

Let's say there are two best-in-class Shearwater/garmin/oceanic/cressi...brandX on the diver and they are both set correctly. The computers somehow massively disagree(or one of them dies). Now what? Do you end the dive or continue according to plan with your depth and time devices?

One could argue that with three computers, you can ignore the odd one out. Plausible with a technical dive team that has two computers each.

OP has a malfunctioning computer. He knows the mix because he analyzed and wrote it down. The error happened before he went in the water. The dive plan should have had a known max time and depth. The MOD is given so the max NDL time is one number to remember or write it down. Any dive briefing should have those numbers.

Op was not diving solo. He had a whole group doing the same dives with the same or higher nitrogen mix. He can stay on level with the shallowest diver and ascend with whoever goes up first. He can also add 3 minutes to the safety stop.

If we plan this dive on paper:
Oxygen limits could be about 80-90 minutes (assuming 60-70 minutes at 1.4 on the first dive).
Nitrogen limit would be about 22 minutes if diving on tables.
1st dive- 40 min NDL 36% to 28 meters
One hour surface
2nd dive- NDL 22 min 36% at 28 meters
At a SAC rate of 20 LPM, you get about 22 minutes to 50 BAR.
So the turn time is either set by gas or Nitrogen at about 22 minutes.
What if the computer malfunctions the other way? It somehow gives you a 99 min NDL after 20 minutes at depth. Now what? Do you trust that or continue according to plan with your depth and time devices?

The additional discussion on computer setup is irrelevant. This is a planning error.

 

[maudiver216]

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.

It's funny that you mention this, because their sheet only required the 02 % and did not have a column for MOD.

 

[maudiver216]

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.

Let's be realistic here: the 1.4 PPO2 is a conservative setting. Also, my computer did not give me any problems on the first dive with the same gas mixture at the same depth. I was not in danger and I didn't say that I followed the other divers to depth.