OP
Sent it to you by PM.
Oxygen Toxicity Units on the Wisdom II
- Thread starter wve
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Sent it to you by PM.
tonykdr
Guest
what kind of info did you find?? I just picked up this computer and find this very interesting...would like to know the answer.
Xaryo
Contributor
Me too I'd like to know. I'm using the insight, but they are the same except for the air integrated.
OP
Sherwood answered some of the questions about the theory behind their algorithm, but there is a discrepancy between what was explained and how the computer actually behaved. I'm waiting to get that cleared up.
I'll try to distill the info that I have so far so that everyone else can catch up and follow along, but it may take me a day or two to get enough time to do it justice.
OP
After receiving correspondence from Sherwood, here is my understanding of how the Wisdom II addresses oxygen exposure concerns.
The computer has to deal with two major concerns: CNS toxicity and whole body (or pulmonary) toxicity.
In the late 1980's and early 1990's R.W. Hamilton described a mathematical method for minimizing the risk of oxygen toxicity among divers who were making repeat dives within a certain time frame using concentrations of oxygen higher than 21%. It involved the calculation of Oxygen Toxicity Units (OTU) based upon the partial pressure exposure to oxygen over time. He then determined the number of units to which a diver could safely be exposed each day for missions that were expected to last a variable number of days. This was called the Repex method (for Repeat exposure).
If you look at the tables which Charlie99 referenced and linked to in this thread, you will see that according to this method, a diver can tolerate 850 units in a one day mission, 700 units per day for a 2 day mission, etc. If you also look at the accompanying graphs, you will see that the curve flattens out at 300 units per day for missions longer than 10 days. In other words, if you limit your OTU exposure to 300 units a day, you can safely dive as many days in a row as you want to. And this is the number that the Wisdom II uses as its 100% Daily oxygen toxicity unit (DOTU) dose limit (user manual, page 78).
One of the assumptions of the Repex method was that divers would avoid the risk of CNS toxicity by limiting their PPO2 to 1.5 bar or less. This was an arbitrary limit based upon research by others that showed a sharp increase in CNS toxicity risk above this level. The NOAA tables, for example, show a maximum daily limit of 720 minutes at 0.6 ata, 300 minutes at 1.0 ata, 120 minutes at 1.5 ata, but only 45 minutes at 1.6 ata.
The Pelagic algorithm (by Pelagic Pressure Systems, Inc.), used by the Sherwood Wisdom II, closely follows the Repex algorithm at PO2 levels 1.5 ata or less. However, in order to stay within NOAA limits, the algorithm adjusts so that above 1.5 ata the number of OTU's per minute rises sharply. For example, at 1.0 ata, one accumlates 1 OTU per minute. At 1.5 ata one accumulates not quite 2 OTU per minute, but at 1.6 ata one accumulates almost 7 OTU per minute. From the user manual, it appears that 300 Single Oxygen Toxicity Units (SOTU) is the per-dive limit (page 78).
Upon reaching 80% (240 units) of either the SOTU or DOTU limit, a cautionary High O2 mode is entered. An alarm beeps, the red light flashes, and High O2 scrolls across the top of the display. If 100% (300 units) is reached, all segments of the O2 bar graph display flash along with the red light and audible alarm.
Note: Whereas the computer will lock you out if you have a major violation with respect to nitrogen loading, the computer does NOT shut down if you exceed oxygen exposure limits. It will just keep telling you to go up.
It is also helpful to be aware that the SOTU limit starts over with each dive. In other words, the computer will allow you to make two back-to-back high PPO2 exposures with only a 10 minute surface interval. (The 10 minutes needed to determine that you have not temporarily surfaced, but indeed have started a new dive.)
In contrast, the DOTU begins to recover when the PPO2 drops below 0.5 ata (such as during a surface interval), using a relaxation half life of 90 minutes.
Now, this is where I still have a question for the Sherwood rep, because our computers did not seem to reflect this credit. After 9 hours (6 half lives) one's oxygen exposure should be back near zero. And yet, after an 11 hour surface interval, our O2 bar graphs still showed near-max oxygen loading.
So I am wondering why the bar graph did not seem to show any credit for our overnight hiatus.
Hope that helps...
The computer has to deal with two major concerns: CNS toxicity and whole body (or pulmonary) toxicity.
In the late 1980's and early 1990's R.W. Hamilton described a mathematical method for minimizing the risk of oxygen toxicity among divers who were making repeat dives within a certain time frame using concentrations of oxygen higher than 21%. It involved the calculation of Oxygen Toxicity Units (OTU) based upon the partial pressure exposure to oxygen over time. He then determined the number of units to which a diver could safely be exposed each day for missions that were expected to last a variable number of days. This was called the Repex method (for Repeat exposure).
If you look at the tables which Charlie99 referenced and linked to in this thread, you will see that according to this method, a diver can tolerate 850 units in a one day mission, 700 units per day for a 2 day mission, etc. If you also look at the accompanying graphs, you will see that the curve flattens out at 300 units per day for missions longer than 10 days. In other words, if you limit your OTU exposure to 300 units a day, you can safely dive as many days in a row as you want to. And this is the number that the Wisdom II uses as its 100% Daily oxygen toxicity unit (DOTU) dose limit (user manual, page 78).
One of the assumptions of the Repex method was that divers would avoid the risk of CNS toxicity by limiting their PPO2 to 1.5 bar or less. This was an arbitrary limit based upon research by others that showed a sharp increase in CNS toxicity risk above this level. The NOAA tables, for example, show a maximum daily limit of 720 minutes at 0.6 ata, 300 minutes at 1.0 ata, 120 minutes at 1.5 ata, but only 45 minutes at 1.6 ata.
The Pelagic algorithm (by Pelagic Pressure Systems, Inc.), used by the Sherwood Wisdom II, closely follows the Repex algorithm at PO2 levels 1.5 ata or less. However, in order to stay within NOAA limits, the algorithm adjusts so that above 1.5 ata the number of OTU's per minute rises sharply. For example, at 1.0 ata, one accumlates 1 OTU per minute. At 1.5 ata one accumulates not quite 2 OTU per minute, but at 1.6 ata one accumulates almost 7 OTU per minute. From the user manual, it appears that 300 Single Oxygen Toxicity Units (SOTU) is the per-dive limit (page 78).
Upon reaching 80% (240 units) of either the SOTU or DOTU limit, a cautionary High O2 mode is entered. An alarm beeps, the red light flashes, and High O2 scrolls across the top of the display. If 100% (300 units) is reached, all segments of the O2 bar graph display flash along with the red light and audible alarm.
Note: Whereas the computer will lock you out if you have a major violation with respect to nitrogen loading, the computer does NOT shut down if you exceed oxygen exposure limits. It will just keep telling you to go up.
It is also helpful to be aware that the SOTU limit starts over with each dive. In other words, the computer will allow you to make two back-to-back high PPO2 exposures with only a 10 minute surface interval. (The 10 minutes needed to determine that you have not temporarily surfaced, but indeed have started a new dive.)
In contrast, the DOTU begins to recover when the PPO2 drops below 0.5 ata (such as during a surface interval), using a relaxation half life of 90 minutes.
Now, this is where I still have a question for the Sherwood rep, because our computers did not seem to reflect this credit. After 9 hours (6 half lives) one's oxygen exposure should be back near zero. And yet, after an 11 hour surface interval, our O2 bar graphs still showed near-max oxygen loading.
So I am wondering why the bar graph did not seem to show any credit for our overnight hiatus.
Hope that helps...
Last edited:
wve,
very good summary.
We are adding Nitrox capabilities to our dive simulator and I was researching the subject when I found this well done post.
Regards,
AM
very good summary.
We are adding Nitrox capabilities to our dive simulator and I was researching the subject when I found this well done post.
Regards,
AM
Sherwood SCUBA rep.
Contributor
Let me get with the engineers and discuss and I will see what I can come up with.
It may not be until next week. Everyone is traveling this week for a sales meeting.
It may not be until next week. Everyone is traveling this week for a sales meeting.
OP
No, that question was never answered.
We have not observed any surface credit for oxygen toxicity units. That is, the OTU's do not decrease after a reasonable number of hours.
It appears to us that rather than using a half-life algorithm, the computer simply drops off dives after 24 hours.
Here is an example. Let's say you rack up a cumulative O2 sat of, say, 65% after 3 dives on day 1 of diving. After an overnight interval of more than 12 hours, your O2 sat will still be 65%.
You will not see any credit given on day 2 until 24 hours after the first dive on day 1 -- at which time the accumulation from dive 1 on day 1 will be subtracted. After 1 dive on day 2, the O2 sat will now be equal to the accumulation from dives 2 and 3 on day 1 plus the accumulation from dive 1 on day 2.
At least that is how it seems to work.
We still love the Wisdom II computer. I just wish there was a way to extract a little more technical information from it in order to better understand how it is calculating these types of things.
OK... I just bought a Wisdom 2 but would still like to know more about this subject if we can get an answer to it.
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