Discrepancy between my computers during the dive

[pisauron]

Hi @pisauron

I dive a Shearwater computer and an Oceanic computer. Both computers show the SI for the dive in the on-board log and in the downloaded log. I would imagine your Garmin and Ratio computer do too.

This is from my Shearwater computer download to the Shearwater cloud. It might also be interesting to compare the CNS O2 exposures for your two computers to see if the discrepancy persists. My Shearwater gives the CNS start and CNS end in both the on-board log and in the download. My Oceanic computer does not have this data. We already discussed that it would be interesting to compare end surface GFs if they were available. I don't use Subsurface, somebody else can help you with that.

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If you look at the logs you will see the CNS changes. Dives start with CNS lower than the previous dive-end CNS.

 

[harvalen]

If you look at the logs you will see the CNS changes. Dives start with CNS lower than the previous dive-end CNS.

If you look closer. For example the two dives on the 28th. In the Ratio logs the first dive starts with CNS 6% and ends with CNS 17%, the second dive starts with CNS 16% and ends with 29%. In the Garmin logs the first dive starts clean with CNS 0% and ends with CNS 11%, the second dive then starts at CNS 5% and ends with CNS 20%. So at least form a CNS perspective they are treating the surface interval very different, while the increase during the dive is pretty close.

 

[pisauron]

If you look closer. For example the two dives on the 28th. In the Ratio logs the first dive starts with CNS 6% and ends with CNS 17%, the second dive starts with CNS 16% and ends with 29%. In the Garmin logs the first dive starts clean with CNS 0% and ends with CNS 11%, the second dive then starts at CNS 5% and ends with CNS 20%. So at least form a CNS perspective they are treating the surface interval very different, while the increase during the dive is pretty close.

Indeed. Ratio response was:

In addition to the above, in this specific case, we can see that the two computers start the repetitive dives with different CNS values. The CNS value does not influence the decompression calculation (at least it does not on the RATIO, but I believe Garmin does not consider it either), but it might indicate that the two computers off-gas the tissues slightly differently.

 

[harvalen]

In addition to the above, in this specific case, we can see that the two computers start the repetitive dives with different CNS values. The CNS value does not influence the decompression calculation (at least it does not on the RATIO, but I believe Garmin does not consider it either), but it might indicate that the two computers off-gas the tissues slightly differently.

If CNS is supposed to decrease in proportion to off-gasing of the tissues in the RATIO computer, and not some odd 24 hr reset, then it seems to be doing very little off-gasing during that 93 min surface interval. I wonder why Ratio would comment on it, but not be interested enough to dig deeper into the difference.

 

[pisauron]

If CNS is supposed to decrease in proportion to off-gasing of the tissues in the RATIO computer, and not some odd 24 hr reset, then it seems to be doing very little off-gasing during that 93 min surface interval. I wonder why Ratio would comment on it, but not be interested enough to dig deeper into the difference.

Is it possible that the difference in CNS is caused by a "faulty" surface interval timer? This can also explain the NDL discrepancy.
For expamle dive1 ends, the Ratio counts 3 min (instead of an hour) then dive2 starts, CNS is barely changed, and NDL is affected greatly.

 

[harvalen]

Is it possible that the difference in CNS is caused by a "faulty" surface interval timer? This can also explain the NDL discrepancy.
For expamle dive1 ends, the Ratio counts 3 min (instead of an hour) then dive2 starts, CNS is barely changed, and NDL is affected greatly.

It would explain both issues. Simplest explanation I can think of is that someone didn't consider the computer going into deep sleep during the surface interval to save battery and those calculations now happens less often than they expected, resulting in less off-gasing and CNS not decreasing as much.

Another explanation would be that someone listened too much to arguments like in that other thread and added a penalty for short surface intervals.

 

[pisauron]

It would explain both issues. Simplest explanation I can think of is that someone didn't consider the computer going into deep sleep during the surface interval to save battery and those calculations now happens less often than they expected, resulting in less off-gasing and CNS not decreasing as much.

Another explanation would be that someone listened too much to arguments like in that other thread and added a penalty for short surface intervals.

Alright then i will send a question to Ratio.
Although i can't think of a way to confirm this is what happens from looking at the logs.

 

[scubadada]

Hi @pisauron

One easy thing to do would be to record start and end CNS O2 for all of the dives during that last series to compare the decreases during SIs, including overnight. I could not specifically find how Ratio handles O2 elimination in the user's manual. There are 2 general methods, the 90 min half-life and cliff vesting at the end of a 24-hour period. My Shearwater uses the former, my Oceanic the later. I assume Garmin and Ratio both use the 90 min half-life. Each day should start clean, 0%, given the long time overnight.

From @harvalen post:
Garmin 0-11% and then 5-20%
Ratio 6-17% and then 16-29%

Garmin starts clean, decreases about 1 half-life (90 min) between dive 1 and 2. Ratio starts at 6% and shows nearly no decrease between dive 1 and 2.

The differences in NDL are a different matter and beyond an easy calculation for me, perhaps someone else.

 

[pisauron]

Hi @pisauron

One easy thing to do would be to record start and end CNS O2 for all of the dives during that last series to compare the decreases during SIs, including overnight. I could not specifically find how Ratio handles O2 elimination in the user's manual. There are 2 general methods, the 90 min half-life and cliff vesting at the end of a 24-hour period. My Shearwater uses the former, my Oceanic the later. I assume Garmin and Ratio both use the 90 min half-life. Each day should start clean, 0%, given the long time overnight.

From @harvalen post:
Garmin 0-11% and then 5-20%
Ratio 6-17% and then 16-29%

Garmin starts clean, decreases about 1 half-life (90 min) between dive 1 and 2. Ratio starts at 6% and shows nearly no decrease between dive 1 and 2.

The differences in NDL are a different matter and beyond an easy calculation for me, perhaps someone else.

Thank.
Seeing as the Ratio starts a fresh day with 6% CNS isn't that strange since it has been 24h~ between the previous day dives?

 

[scubadada]

Thank.
Seeing as the Ratio starts a fresh day with 6% CNS isn't that strange since it has been 24h~ between the previous day dives?

Yes it is, why I mentioned it.

You could also pick an NDL near the lowest of each dive and compare the 2 computers at that same time point.

Between the CNS O2 and NDL data, you would have quite a bit to help ferret out the problem, share with Ratio.