Oxygen Clock Question

iratnl

Hi All,

I'm off for a weeks liveaboard diving next week, and am expecting (hoping) to do up to 4 dives per day, of about 1 hour each.

Up till now, I've only ever done a max of 3, 1 hour dives per day, and always with my set point at 1.3 (ish)... This leaves me well inside the 210 mins per 24hrs allowed on the NOAA chart.

Now - 4 hours, being 240 mins means I cannot run all my dives at 1.3, so my question is, what do you normally do in this situation ?

1) Just run all dives at a lower setpoint (max time limit for 1.2 is 240 mins)
2) Mix and Match - i.e. do some dives at 1.3, others at lower set points
3) Vary the setpoint during a single dive (but of a pain to track)
4) A combination of the above !

Devils Advocate

Hi Rob,
remember that your o2 clock reduces when you are on the surface, so take that into account. Run your Po2 at 1.2 max or even 1.1 for the week. If it looks like a naff dive, skip it
I have the manual here which should be delivered by Thursday.

Aaron.

Janos

When I do Red Sea liveaboards, I run a low ppO2 of around 1.0 to 1.1. There are few square profile dives and few deep one so I'm well within no-deco limits so there's little to be gained from running a high ppO2.

There are one or two exceptions, mainly when I ask them to put a bit of Helium for some of the deeper dives, in which case I still run a low ppO2 on the bottom, but boost it right up for the last ten minutes of deco.

Janos

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iratnl

Hi Aaron,

I thought the 24hr limit was just that - i.e took no account of surface intervals ?

I'm really looking forward to next week , and thanks for sorting the manual for me.

Devils Advocate

It Doesn't, just bear EVERYTHING in mind when you are planning the dive day!
You will not gain a great deal more by running high Po2's.Like Janos said, keep it low and enoy the trip.
Aaron.

Abbo

I don't know what kind of liveaboard you are talking about, but I would guess that a lot of those dives are within recreational limits. If that is the case you don't have a problem: cut your PO2 right back. For dives at 25m or less, 0.7 is ample. Otherwise, your solenoid is working overtime trying to chase the setpoint, and to maintain buoyancy you are venting the loop and shooting all that O2 straight into the atmosphere. At a PO2 of 1.2, you have 4 hours exposure as your daily limit; at 1.0 you have 5 hours, and at 0.7 you have 9.5 hours/day as your limit. Surely the only reason for having a PO2 any higher is to manage a deco obligation. OTUs are not life-threatening: the dangerous bit is the CNS risk. I really think there's minimal risk of break dancing from a CNS hit after a PO2 of 1.3 for 4 hours.

Freef

I was taught that the exposure has a half time of 90 min. Current thinking may vary the time, but I believe that is still right.

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David.

Currently owner of two differently sized ankles.

iratnl

But doesn't that only apply to calculating the residual O2 exposure between dives. The 24hr limit is fixed and once reached, there is no more diving for the day ?

If you reach the 24hr limit after 2 dives, you should not do a third; even after (say) a 3 hour surface interval, which using the 90 min 1/2 time would (in theory) have reduced your exposure from 100% to 25%

Or am I totally mistaken ?

iratnl

Errr - a set point of 0.7 at 25m would mean I'd be breathing nitrox 20 ! Not really an ideal mix; especially as my buddy will have 15L of nitrox 32 .

Think I'll stick to 1.1 as suggested by Aaron and Janos (Thanks guys)

ScubaDadMiami

I think you are mixing and (mis)matching CNS clock and OTU related issues.

The CNS O2 clock, which deals with oxygen exposure to the central nervous system (toxing issues), does have a surface interval credit feature for less than 24 hours. Even if you are at 100 percent, after a 6 hour surface interval, you are all the way back down to 7 percent. Also, by using intervals of lower PO2 during dives (such as the equivalent of back gas breaks on open circuit), divers have managed to complete dives in which they theoretically have gone well above the 100 percent mark.

Now, I am in no way encouraging anyone to dive to anywhere near the maximum, and I certainly am not doing so for above the mark. However, I am just putting this information out for the sake of discussion and comparison.

One thing that has been drilled into me during training (thank you, John) is that you really do have to start paying attention to OTU exposure (pulmonary exposure) when you start doing longer dives and multiple diving days. There is no credit for intervals less than 24 hours when tracking OTUs like there is for CNS O2.

Keep in mind that the maximum exposures suggested contemplate being able to complete a chamber treatment after. You do not want to limit your options in that regard by pushing it in your diving.

Treating things in a conservative way, I just go with the long-term exposure limits of 300 per day, which one can tolerate pretty much forever. This is pretty easy to do if you keep the PO2 for your dives in the conservative range.

One other related issue: Myopia. You do not want to start pushing high PO2 for several days because this has led myopia issues. It seems that some divers can tolerate the exposures better than others. So, you do not want to get close to the mark by experimenting on yourself during these outings.

Really, you will find that there will not be a significant difference in run time using a lower PO2 when compared to a higher one (within reasonable limits). However, you will find that your O2 exposures will be siginicantly different. I tend to run at lower PO2 during the dive and then bump up higher during portions of decompression, and this often makes the run time on the dive just a few minutes longer than it would were I to run at a consistent PO2 throughout the entire dive.

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IANTD CCR Instructor
Miami Beach, Florida
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