RonR
Contributor
Ron,
I need some help understanding the OTU calcs and what it means for repetitive diving. I just got back from a live aboard and did 27 dives (5 a day). The OTU read 875 for the final dive with a CNS of 18%. Is the OTU cumulative over the 27 dives or in a 24 hr period? What is the alert level that I should begin to worry about O2 tox? My LDS was no help at so I'm reaching out here.
Don
An OTU's is defined as the equivalent O2 exposure to breathing 100% O2 at 1 ATM for one minute. The safe limit is considered to be 24 hours of exposure (1440 minutes, or 1440 OTU's).
For a recreational open circuit computer like the Cobalt, the OTU number is just a reference. You are unlikely to run into OTU limits on open circuit scuba- for a discussion of why, see this link: First Responder Concerns About Oxygen Toxicity
The OTU clock in the Cobalt resets to zero after 24 hours- this is a convention in the algorithm we are using, which displays the most conservative value, with no "off-gassing" allowance like you have for nitrogen. When you do a lot of dives repetitively, it doesn't have time to reset, and the values accumulate. If someone were taken in for hyperbaric treatment after a week of diving like yours, the OTU's might be something the physicians would want to know, so they could watch for signs of pulmonary toxicity. But as a practical matter for the diver, it's not something you need to be concerned about unless you are well into the tech diving realm. For commercial saturation divers, it's a big deal. For scuba, it's not.
CNS toxicity, on the other hand, is something you should be concerned about when diving repetitively with EAN. The Cobalt will alert if the value exceeds 80%. Anywhere near this and you should be taking an "air break".
Ron