Oxygen Toxicity Units on the Wisdom II

Assuming we are diving in the Nitrox mode, I have a few questions for the Sherwood rep (or whoever else can answer authoritatively):

1. Do the OTU's shown on the Widsom II O2 bar graph display directly correlate with the cumulative percent saturation? For example: no ticks displayed = less than 20%, 1 tick = 21-40%, 2 ticks = 41-60%, 3 ticks = 61-80%, etc?

2. From reading the manual, my guess is that the answer to the first question is No. Instead, it seems that OTU's are a combination of acute toxicity units (that is, PO2 due to depth) and subacute toxicity units (or daily oxygen exposure).

I presume that the reasoning for this combination is that during a dive, you want to be notified regardless of which one might be causing the alarm, and the remedy (to go UP) is the same for both.

On the other hand, after a series of dives, it would be helpful to know the cumulative O2 saturation for planning purposes. In the built-in Wisdom log we can view the max PO2 for each dive. Where can we find the cumulative O2 sat for each dive in a series?

3. My backup computer is a Genesis React Pro which, I think, is the same as the Sherwood Insight. In its log, it does display cumulative O2 sat for each dive. If that info is not available in the Wisdom log, would that be something you might add to the next software upgrade?

If you watch your O2 bargraph during the SI (or after your final dive) next time you are out diving, you'll probably find that the Wisdom uses a 90 minute halftime for CNS toxicity calculations. If you surface from a dive with the CNS clock at 50% of the limit, after an SI of 90 minutes it is assumed to be at only 25%.

In other words, although they use the "per dive" NOAA limits, they ignore the "cumulative in 24 hour" NOAA CNS limits since they use a much more rational "remove 1/2 of accumulated CNS oxtox % every 90 minutes". The NOAA 24 hour CNS limits effectively do the calcuations as if the CNS oxtox effects from any given dive fully in effect for 24 hours, then making it disappears instantly 24 hours after the dive.

All of the above comments are strictly related to the CNS oxtox calculations, and have nothing to do with the pulmonary oxtox calculations, which are quite different and have very little effect on recreational diving.

Your question is a bit confusing since you use the term OTU, which is most often associated with pulmonary oxtox, rather than the CNS oxtox. What you are seeing on the bargraph is undoubtably reflecting CNS oxtox calculations, since normal NDL or mild deco scuba dives rarely push the pulmonary limits at all.

Charlie99:

If you watch your O2 bargraph during the SI (or after you final dive) next time you are out diving, you'll probably find that the Wisdom uses a 90 minute halftime for CNS toxicity calculations...

Your question is a bit confusing since you use the term OTU, which is most often associated with pulmonary oxtox, rather than the CNS oxtox. What you are seeing on the bargraph is undoubtably reflecting CNS oxtox, since normal OC NLD or mild deco scuba dives rarely push the pulmonary limits at all.

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Thanks, Charlie. I have not noticed that.

I have never had any O2 ticks before, but this weekend in Florida we made 8 dives in less than 30 hours. Most were relatively shallow dives using EAN 36%. We noticed our O2 segments were incrementing, and we never saw them back off, even after an 11 hour surface interval (overnight).

OTU is the term the manual uses. It says:

A cautionary High O2 Mode is entered upon reaching 80% (240 OTU) of the Maximum per dive allowable O2 of 300 SOTU (Single Oxygen Toxicity Unit) or 24-hour period allowable O2 saturation of 300 DOTU (Daily Oxygen Toxicity Unit). The Oxygen Bar Graph (O2BG) shows the maximum of either SOTU or DOTU. (italics mine)

Upon reaching 80% (240 OTU), the Audible Alarm sounds, the Red LED flashes, and the message HIGH > O2 scrolls at the top of the screen (each on 1/2 second). When O2 saturation becomes equal to or greater than 300 OTU (100% limit), all O2BG segments flash in conjunction with the red LED light and sounding of the audible alarm.

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I agree that it is confusing. I also agree that pulmonary risk should be negligible, so I am trying to understand why the O2BG remained so high for so long.

If the manual mentions the 90-minute oxygen half-life credit, I missed it. Perhaps our dives were too close together to take advantage of it, but overnight should have been enough time to return it to zero ticks. For the two of us who were diving, it didn't drop at all overnight.

The section of manual you quoted does indeed sound like it is referring to pulmonary toxicity, rather than the CNS toxicity that normally controls recreational scuba.

What I described above is what happens with both my Oceanic Data Plus and Oceanic Data Plus 2 computers. In many things, all of the various Pelagic Pressure System comptuers (sold as Oceanic, Aeris, Sherwood, and probably a couple other brand names) are very similar if not identical. In this case, there seems to be a big difference.

The fact that your bargraph didn't back off, even after 11 hour SI does tend to confirm that what your bargraph was displaying at that time was pulmonary related. IF (big IF, there) the oxygen bargraph follows the same sort of logic used to handle multiple compartments on the N2 bargraph, then whichever oxygen oxtox (SOTU, DOTU, or CNS) is closest to the limit will be the one that determines what the bargraph reads.

My curiosity is tweaked. I'll download a Wisdom II manual and take a peek.

Charlie

Charlie99:

My curiosity is tweaked. I'll download a Wisdom II manual and take a peek.

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While you're at it, see if you can tell what happens IF you hit the 100% O2 limit. Does the computer shut down like it does if you skip a DECO stop, or does it just keep telling you to go up?

This question became pertinent to us. Because our O2 was so high, we decided to use AIR for our last two dives -- which then got our N2 levels cranked up, too. We were almost to the point that one more tick on either bar graph would have been a problem, and we didn't want the computers to completely shut down.

Fortunately, we got all our dives in, and when I went back through the logs, our greatest PO2 exposure was 1.25 on a single dive. Most were less than 1.0. From the Genesis backup I learned that our cumulative O2 saturation topped out at 76% after the eighth dive. (It was 67% after the 6th dive of the first day.)

So we were well within safe limits, but when you only have bar graphs to look at, you don't necessarily know that, nor can you plan very accurately how much you can do on a subsequent dive.

Thus it would be helpful (I think) to be able to equate each segment on the bar graphs with some kind of number. Or just give me the number in a log display like the Genesis React Pro does.

Oh, and tell me whether it is CNS or pulmonary toxicity we are dealing with. The former worries me MUCH more than the latter.

The Sherwood manual is inconsistent in that some places they refer to OTUs and specifically the Hamilton Repex method, and in other areas they refer to the NOAA oxygen exposure table as the limits.

If you go to the "reference material" section of the manual, page 93, you will see that Sherwood says that they use the 1991 NOAA limits to control both CNS and pulmonary exposure. Those are similar, but not really the same as OTUs.

In the back of the manual, Sherwood has a glossary where they define OTUs as a Hamilton's Repex method term for oxygen dose. The NOAA 24 hour limits are roughly 300 OTUs for 0.8ata to 1.6ata, but are only 194 OTUs for a 0.6ata exposure.

If Sherwood is to properly account for high ppO2 exposure in the 1.4 to 1.6ata range, then they definitely need to be doing something other than OTUs and a 300 otu limit, since the NOAA per dive limit at 1.6ata is only 45 minutes. 45 minutes at 1.6ata is only 82 OTUs. The NOAA per dive limits reduce by over a factor of 3 when going from 150 minutes at 1.4 ata ppO2 down to 45 minutes at 1.6ata. Meanwhile, the OTU clock only goes from 1.63 OTUs/min to 1.92 OTUs/min, a difference of only 18% (Reference on Hamilton Repex: http://www.oceanwreckdivers.com/images/tolerating_exposure_to_high_oxygen_levels.PDF and Rubicon Research Repository: Item 123456789/4866
See particularly Tables 1 and 3 of the first link)

My guess is that they are really using the NOAA per dive and per 24 hour limits rather than OTUs and that 100% "O2 saturation" (another ill-defined term they use) is probably 100% of NOAA limit. I'll also bet that Sherwood's SOTU and DOTU correspond to NOAA's single dive and 24 hour limits, with the SOTU being what is often referred to as the "CNS clock".

It is strange, though, that they specifically refer to a limit of 300 OTU, since this is what the Hamilton Repex method allows as a daily exposure on a continuous basis. (850 otu is allowed for a 1 day exposure if you have not high ppO2 exposure in the previous few days; 1400 total or 700 per day allowed for 2 day missions; going down to 300/day for continuous exposures of 10 days or more. The NOAA table is roughly the continuous exposure limit)

A question I would like the Sherwood rep to answer is how the Wisdom 2 would respond to back to back high ppO2 exposures of 45 minutes at 1.6ata, with minimal SI in between. Does the "per dive" limit just get cleared out as soon as a 10 minute SI has taken place?

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Looking into this a bit further, I see that if one is using a 300 OTU limit for pulmonary toxicity, that this will actually be the limiting factor in a lot of typical multiday recreational diving, rather than CNS, provided that on stays at ppO2 of 1.4ata and lower. This is contrary to what I believed before looking into this a bit more this afternoon --- but is consistent with my own personal experience doing very heavy multi-day repetitive diving on nitrox, where I found that after 5 or 6 days of heavy diving that I needed to back off a bit because of throat irritation and loss of breathing capacity.

Charlie Allen

Good stuff, Charlie, particularly the Hamilton article. (The Ocean Wreck Divers site also looks like a good source of info, too.)

Clearly the Wisdom manual raises as many questions as it answers on this topic.

Time for our friendly Sherwood guy to help us out...

We are at DEMA the annual trade show, in Las Vegas NV. I will have to run this by the Product Engineer on this one. I will have a chance to run this by him the first part of next week.

Thanks,
Sherwood Rep

You know, when you push the limits of your computer (even unintentionally), you learn things that you didn't know that you didn't know. Thanks to help from Charlie, I have been wading through some pretty deep technical writing from Hamilton and others with respect to oxygen toxicity.

Sounds like what we need to know from Sherwood is:

a. How does the Wisdom calculate SOTU and DOTU? Does it make an actual calculation, or does it perform a "lookup" based upon NOAA tables?

b. Is there any surface time credit given for oxygen exposure and, if so, how is it calculated? Corollary: If the surface interval is <90 minutes, is any credit given?

c. Is the 300 unit limit a flat, conservative limit that was chosen with the assumption that a recreational diver might dive multiple days?

d. When the unit displays and the manual talks about "percent saturation", is that really just a percentage of OTUs that have accumulated?

e. Do the O2 bar graph segments have any numerical correlation? Eg, if three segments are displayed, is that equivalent to 60% saturation (or 180 OTUs)?

Charlie, I do note on pictures of the Wisdom display that the last segment on the O2 Bar Graph says "CNS", so I think it must be most worried about CNS toxicity, not pulmonary.

I have an email I can forward you with some graphs. I tried to copy them into here, they will not translate. What email address can I forward use to forward this information to you?

Thanks,