Dive safe! A short story from a chamber operator

[Esprise Me]

I think you were responding to my post. If not, sorry! Just ignore.

My computer did not track O2 even though I had selected nitrox and a mix of 21%. It tracked tissue loading for nitrogen, as if I was on air. I dive with a ProPlus 2. Other computers may work differently.

Yes, I was responding to you. I've reread your posts and I think maybe I understand now; you dive with Nitrox (32?) but use air settings for conservatism/safety, but then to address the O2 exposure, you set your computer to Nitrox 21 instead of air, but that doesn't work because the computer doesn't track the actual oxygen exposure from Nitrox (32? or whatever.) Is that correct? I guess I was confused because I don't see that as "not working;" it's just working with the data you gave it instead of the real data.

Seems like a lot of hoops to jump through when you could just set a personal limit of beginning your ascent when you have 5 or 10 minutes of NDL, or whatever suits you. A few minutes looking over both sets of tables could probably give you an idea of how many extra minutes of NDL gives you the same buffer as diving Nitrox on air tables (how many more minutes does Nitrox 32 give you at 60 feet vs. air, etc.) Kind of like setting your watch 5 minutes ahead so you'll be on time, vs. just planning to be 5 minutes early because you know you're always late.

 

[DeepSeaExplorer]

I wasn’t really responding to any specific person, just the general theme that was developing.

But, I will add that it’s possible to dive conservatively without doing this. Your computer would no longer be tracking anything correctly: O2, N2, MOD, NDL. Then, throw in multiple dives over multiple days, and you have no idea what’s happening. Is that diving more conservatively, or adding more risk?

A better way would be to dive the computer conservatively or get computer with adjustable conservation factors.

 

[DeepSeaExplorer]

Seems like a lot of hoops to jump through when you could just set a personal limit of beginning your ascent when you have 5 or 10 minutes of NDL,..

I used to have a computer that would let you set alarms so many minutes before the NDL That’s an alternative too.

 

[ljwillia]

Yes, I was responding to you. I've reread your posts and I think maybe I understand now; you dive with Nitrox (32?) but use air settings for conservatism/safety, but then to address the O2 exposure, you set your computer to Nitrox 21 instead of air, but that doesn't work because the computer doesn't track the actual oxygen exposure from Nitrox (32? or whatever.) Is that correct? I guess I was confused because I don't see that as "not working;" it's just working with the data you gave it instead of the real data.

Seems like a lot of hoops to jump through when you could just set a personal limit of beginning your ascent when you have 5 or 10 minutes of NDL, or whatever suits you. A few minutes looking over both sets of tables could probably give you an idea of how many extra minutes of NDL gives you the same buffer as diving Nitrox on air tables (how many more minutes does Nitrox 32 give you at 60 feet vs. air, etc.) Kind of like setting your watch 5 minutes ahead so you'll be on time, vs. just planning to be 5 minutes early because you know you're always late.

You're absolutely right. As I mentioned. I've not been doing this long. After reading this thread I think it is more beneficial to set the right mix and dive more conservatively than my computer.

This has been a really interesting and educational thread. Thank you @Divetech Cayman for posting this article and for everyone who contributed. This is why I love Scubaboard!

 

[Esprise Me]

I used to have a computer that would let you set alarms so many minutes before the NDL That’s an alternative too.

I like that idea! That way you're being given accurate data in real time, plus if anything does go wrong your "black box" computer can help accurately reconstruct the accident and help you to decide on new limits going forward.

I think my resistance to the idea of using a more conservative setting is that I wouldn't know how much time I really had left if, say, my buddy was having a problem, or someone else in the group (or some rando who happened to be in the water at the same time) and I had to make a quick decision about how much personal risk I would accept to try to help them. But I'm not actually familiar with how the more conservative settings work, so maybe that concern is unfounded?

 

[scubadada]

There is confusion concerning oxygen exposure because there are two, quite different mechanisms for tracking exposure.

The NOAA oxygen exposure table has a maximum exposure over a 24 hour period. There is no credit for elimination until 24 hours after the end of a dive. With this mechanism of tracking, it is possible to exceed the 24 hour oxygen exposure limit. As an example, I have done this while diving in SE Florida when doing 5 dives per day or, by doing a 5th dive in the 24 hour window of exposure (i.e., the next morning). Five dives on 36% at an average depth of 60 feet for 70-75 minutes will give you and exposure of 350-375 minutes at a pO2 of about 1.0, easily exceeding the limit of 300 minutes. Some, perhaps only older, dive computers track oxygen this way, my 9 year old Oceanic VT3 does. I get a warning when I hit 80% and then an alarm when I exceed 100%. All pertinent dive information is still available, albeit, some on alternate screens.

Most contemporary dive computers and planning software track oxygen exposure with an elimination half life of 90 minutes. You lose half of your accumulated oxygen every 90 min. My Oceanic Geo 2, Dive Rite Nitek Q, Shearwater Teric, and MultiDeco, all use this oxygen tracking algorithm. This makes it nearly impossible/impossible to exceed oxygen exposure limits with anything considered usual recreational diving. If I did the same 5 dives described above, all in one stretch with only 1 hour surface intervals, my oxygen exposure would be only 62.8%. With 10 such dives, oxygen exposure would be at 69%

I hope this helps clear up some of the confusion regarding discussions of oxygen exposure tracking

 

[Esprise Me]

Thanks; that table above is what I was looking at when I was thinking it would be easy to do in 5 dives per day. Is the computer algorithm more generous because it gives you "credit" for the lower partial pressure as you ascend as well as your surface interval?

 

[scubadada]

Thanks; that table above is what I was looking at when I was thinking it would be easy to do in 5 dives per day. Is the computer algorithm more generous because it gives you "credit" for the lower partial pressure as you ascend as well as your surface interval?

Hi @Esprise Me

It is not particularly easy to exceed the oxygen exposure limits of the NOAA table.

Using the NOAA table assumes a square profile, just like a deco table. A computer will use your real depth. I ran MultiDeco as square profile dives to 60 feet for 75 minutes in the example of the 90 minute half life, pretty much the same as using the table. Yes, you do get credit for a portion of the dive that is very shallow and you get the elimination half life for the SI.

 

[johndiver999]

I like that idea! That way you're being given accurate data in real time, plus if anything does go wrong your "black box" computer can help accurately reconstruct the accident and help you to decide on new limits going forward.

I think my resistance to the idea of using a more conservative setting is that I wouldn't know how much time I really had left if, say, my buddy was having a problem, or someone else in the group (or some rando who happened to be in the water at the same time) and I had to make a quick decision about how much personal risk I would accept to try to help them. But I'm not actually familiar with how the more conservative settings work, so maybe that concern is unfounded?

I agree, plus you could end up violating a mandatory deco stop that was an artifact of mis-setting the computer to air, when the reality might be no violation. Why add the risk of
Locking up your computer by playing games with the input???

. Perhaps one of the few justifiable reasons might be to go through and conduct a simulated deco stop, which you know is probably a pretend one. Just to make sure you know what it looks like on the computer

 

[scubadada]

Re tracking of oxygen exposure, see this page from the Teric manual: