Discrepancy between my computers during the dive

[scubadada]

Hi @pisauron

Your 2 computers were 12 minutes apart in their NDLs in the graphs you posted.

Discrepancy between my computers during the dive

So i think i figured it out but i am not an expert and not sure. Today i observed the two computers closely during the dives and noticed the Garmin updates info much faster the the Ratio. I stopped moving completely and waited a few second and the Ratio catched up to the garmin and after a...

scubaboard.com

This was in the setting of a few NDL dives. Personally, I don't find Ratio's explanation adequate for 2 computers running the same algorithm with the same settings. I have dived my Teric and Dive Rite Nitek Q together extensively, and NDL never varied more than 1 min. The difference in CNS O2 continues to bother me.

Nice that they answered you.

Let us know about your future experience.

 

[Germie]

It was the answer, I gave more or less too. I don't see any things wrong, but for you maybe it is confusing.

 

[pisauron]

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It was the answer, I gave more or less too. I don't see any things wrong, but for you maybe it is confusing.

How much discrepancy is wrong and a problem then?

 

[tursiops]

No one got bent either right? There are no absolutes on these, its software guess-o-rythems, the best way to avoid this issue is to get two computers from the same company and era, and if you don’t want to veer to far off from your buddies you should probably all use the same computers, sell the Ratio and buy another Garmin or whatever.

It is a little tiresome that when something is not exactly correct, it is deemed a guess. Just sayin'.

 

[dmaziuk]

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How much discrepancy is wrong and a problem then?

I would consider 15 minutes of NDL vs. deco 30 minutes into the dive: way too much.

As their tech pointed out, the computer start the dive with different CNS values. You gotta wonder if they also start with different residual loading. You'd have to look at the logs for prior dives. I don't know if either of them logs O2%, but if one of them was set to a different mix on the preceding dive(s), that would explain both CNS and residual loading. Pay attention to timestamps and see what the computers think the surface intervals were: clock not ticking in one of them would make its SI much shorter: less off-gassing and O2 washout.

 

[pisauron]

I would consider 15 minutes of NDL vs. deco 30 minutes into the dive: way too much.

As their tech pointed out, the computer start the dive with different CNS values. You gotta wonder if they also start with different residual loading. You'd have to look at the logs for prior dives. I don't know if either of them logs O2%, but if one of them was set to a different mix on the preceding dive(s), that would explain both CNS and residual loading. Pay attention to timestamps and see what the computers think the surface intervals were: clock not ticking in one of them would make its SI much shorter: less off-gassing and O2 washout.

I have sent them all the logs from the start of the trip.
All the mixes were the same and all the surface intervals too.

I can attach the logs here? Maybe someome will be smarter and can check them...

 

[joebob24]

I would consider 15 minutes of NDL vs. deco 30 minutes into the dive: way too much.

This may sound like a big difference, but depending on depth there might not be a big difference in tissue loading between the two. Going up or down a a few feet can change the NDL drastically when you are around 50' after being deep for a while. Some of the compartments are off gassing, while others are still on gassing. The difference between needing 2 minutes of deco at 10' and needing 0 minutes of deco are pretty small still.

 

[scubadada]

Hi @pisauron

So, what is the explanation for the CNS O2 difference when the computers had the same mix, dive time, and SI for all the dives? I believe both computers use the NOAA table for exposure. I also assume that both use a 90 min half-life of elimination for the SI. They were 10% different on the same dive that had the significant NDL discrepancy.

Too bad Ratio does not track GF (GF99) as Shearwater does. You would be able to compare the surfacing GFs of your dives.

 

[dmaziuk]

That's all good and well but if your two sensors read differently, they should do that on every dive. My understanding is, that is not the case here.

I am not sure how likely it is that you'd rake up 15 minutes difference in deco from a couple of feet difference in pressure sensor reading, either. Without running the numbers I suspect it's not actually possible (all other things being equal) on a no-stop dive.

 

[joebob24]

That's all good and well but if your two sensors read differently, they should do that on every dive. My understanding is, that is not the case here.

I am not sure how likely it is that you'd rake up 15 minutes difference in deco from a couple of feet difference in pressure sensor reading, either. Without running the numbers I suspect it's not actually possible (all other things being equal) on a no-stop dive.

I don't think anyone said there was 15 minutes of difference in deco time. They said there was 12 minutes of difference in NDL. If you are at a depth where you are on gassing very slowly, you could already be very close hitting deco from a tissue saturation point of veiw, but since you are on gassing so slowly it would still take a while. If the other computer shows a slightly higher tissue saturation level, then it could already be into deco. The actual deco obligation was only 2 minutes, which is pretty normal variation.

I often dive to over 100' for the first part of a dive, then go up to around 50' for the rest of the dive. The NDL might be all the way down to 1 minute when I get to 45', but by the time I hit 40' it might be all the way back up to 30 minutes or more. There are huge swings in NDL around the depth where you are crossing over from on gassing to off gassing and just a few feet of depth makes a huge difference.

The better number to look at would be surface gradient factor if that is available.