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Discontinuous errors in O2 CNS% calculation

nradov
over 3 years ago

I did a dive with my Descent Mk2 (firmware version 8.10) in Multi-Gas mode yesterday and during decompression I suddenly got an alert like this: "CNS toxicity at 150%. End your dive now." That can't be right, there's no way I was over the CNS toxicity limit at that time. We had done about 42 minutes bottom time at an average depth of 145 ft (44 m) on 18/45 trimix, switched to 50% nitrox at 70 ft (21 m) for 4 minute stops every 10 ft (3 m), and then switched to 100% oxygen at 20 ft (6 m) for the final stop. The alert popped up after a few minutes on oxygen.

The FIT File Viewer shows that at timestamp 4/24/2021 11:39:06 AM the "cns_load(percent)" field jumped from 66 up to 150. It had been counting up smoothly until that point but then suddenly jumped. It kept increasing from there until at 11:53:35 AM it reached 226, and then suddenly dropped back down to 23! I'll open a support ticket with Garmin since this seems like a defect. Has anyone else seen similar problems?

Here's a link to the dive activity. I made it public if anyone wants to take a look.

connect.garmin.com/.../6665617152

  • 0 over 1 year ago in reply to nradov

    i did a tech dive (only 2 min and brieth O2 to be safe at 6 )  recently and my CNS on shearwater is 26 but on garmin i got 60% ! this is still very wrong here is the dive

    https://connect.garmin.com/modern/activity/embed/10649531456

  • 0 over 1 year ago in reply to crazylolo

    It has become clear that the CNS% oxygen toxicity calculations implemented by most dive computers are basically useless for mixed-gas technical diving. Everyone is using an exposure limit table published years ago in the NOAA Diving Manual, but the problem is the maximum durations are based on very limited experimental data and only go up to 1.6 ATA. With minor fluctuations in wave action or atmospheric pressure it's easy to get above 1.6 ATA briefly and go to 1.61, 1.62, or whatever for a few seconds even if you have good buoyancy control and hold very close to 20 ft / 6 m. At that point you're technically in uncharted territory. So, in a misguided attempt to be conservative the dive computer developers have used some questionable mathematical tricks to extrapolate the shape of the curve for partial pressures above 1.60. Shearwater does it one way, Garmin does it another way, but we don't have experimental data to prove that one approach is more accurate than the other. This ends up triggering a bunch of false positive alerts which don't represent any real-world risk of hyperoxia. In fact, those spurious alerts like "CNS toxicity at 101%. End your dive now." are actually dangerous because they could encourage divers to skip mandatory decompression.

    Although Garmin fixed the immediate defect, what we really need is the ability to selectively disable certain dive alerts.

  • 0 over 1 year ago in reply to nradov

    Yes I was holding 6m steady  was very calm sea . night wreck dive the problem is also the po2 at 6m on both computer got 1.6 on the shearwater and 1.66 on the Garmin ! I have double check the depth and show it to my buddy as well . If any alarm arise they should still leave the depth display all the time .totally agree with you about cns didn't know that above 1.6 was uncharted ... thanks