Discrepancy between my computers during the 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.

 

[scubadada]

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...

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

Go back to the initial post, after a half hour of diving the Ratio had gone into deco and the Garmin had 15 min of NDL. Later, the OP posted 2 graphs of the same dive that showed a 12 min difference in the NDLs around the same time. Your explanation for this large discrepancy makes absolutely no sense.

It has already been mentioned that Ratio does not track GF, comparing surfacing GFs is not possible.

 

[USdiver1]

Give a man a watch and he will always know the time. Give a man two watches and he will be forever doubting what time it is. - Confucius [ancient Chinese philosopher/ collector of chronographs]

The solution is to ditch one of your computers, or in the alternative get two of the same model from the same maker.

 

[joebob24]

Your explanation for this large discrepancy makes absolutely no sense.

My only real point is that NDL has huge swings when you are crossing over from on gassing to off gassing. I couldn't find a perfect example from one of my dives, but I marked two points on this one to show how much NDL can fluctuate when you are already close to a surface gradient factor close to your GF high. I changed depth less than 5' and doubled my NDL. The closer you get to your GF high, the larger those swings are when you cross over and a tiny change in tissue loading causes a huge shift in where that line happens.

 

[pisauron]

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.

Settings wise everythings was the same in all my dives. This is also confirmed by Ratio checking it. I didn't find anthing unusal.
I think the discrepancy was always there from the start. Probably in the first dive it was very small and non significant. But as the week progressed it grew larger and larger. Then growing large until i finally noticed it and cought it on camera.

For anyone interested i am attaching the logs here:

Garmin VS Ratio – Google Drive

drive.google.com

 

[scubadada]

Give a man a watch and he will always know the time. Give a man two watches and he will be forever doubting what time it is. - Confucius [ancient Chinese philosopher/ collector of chronographs]

The solution is to ditch one of your computers, or in the alternative get two of the same model from the same maker.

I dive 2 computers, one running Buhlmann at 80/95 and one running DSAT. They run differently but are generally quite close. In fact, I have a very good idea of exactly how they run differently on first and repetitive dives. This does not bother me at all. If it is a no stop dive, I simply let the more conservative computer control the dive. If it is a light deco dive, I clear one or both of the computers. I expect the relatively small differences I see running two different decompression algorithms.

If I were diving the same algorithm and the same settings, I would expect the computers to run nearly identically.

 

[BRT]

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 am greatly amused by all this. My sensors do not always read differently. The one that reads deeper will always read deeper, if it is reading deeper. I suspect it may have to do with atmospheric pressure at turn on. Sometimes they read close enough that I have a hard time telling the difference. After all, the resolution is only 1 foot and it is hard to get them exactly together and watch carefully enough to tell which one changes up or down first. Sometimes by 60 feet there is a clear difference of a foot between them. And when that is the case over a couple of long dives I build up a substantial difference in NDL's. If they are reading the same, the NDL numbers are extremely close.

 

[dmaziuk]

Clearly, all y'all need a third $1000 computer for the tie-breaker.

 

[pisauron]

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Clearly, all y'all need a third $1000 computer for the tie-breaker.

That's the plan. But which one though?

Shearwater or OSTC?

 

[Belzelbub]

The solution is to ditch one of your computers, or in the alternative get two of the same model from the same maker.

Same maker shouldn’t be necessary. I dive with two computers both running the same algorithm. One is a Garmin, one is a Shearwater. As stated earlier, they may disagree a bit, but it’s negligible. Certainly not to the extent seen by the OP.

That's the plan. But which one though?

Shearwater or OSTC?

Well. My vote would be for a Shearwater, as mine agrees pretty close with my Garmin. They also came, out of the box, with the same GF presets, and custom GF capability. The Shearwater came with SurfGF out of the box, the Garmin may have added it via a firmware update. Plus, OSTC is not available where I live.

However, there may be a time where you need service. Divetronics is who I would use, and they have a stellar reputation. Not sure who the regional service is for Israel, so it might be worthwhile to check and see if you can find reviews. I’d do the same wit OSTC as well.