Discrepancy between my computers during the dive

[DiveLikeAMuppet]

You could compare the dive logs from all dives on the day (depths, runtimes, gas mix,...) to see if one of the computers logged something unusual?

You could have the computers set up for different salinity levels but that shouldn't trigger such a major difference. Note to self: this would not impact the deco calculation

 

[Rilelen]

A *little* bit of discrepancy is normal - you may have slightly different runtimes and/or depth sensors may record slightly different depths. Today for my instance my buddy noted that his two Shearwaters were showing an 8’ depth discrepancy (we didn’t think to check them against mine until after the dive). That, over time, will yield slightly different NDL/deco obligations.

But that level of difference seems unusual - have you triple-checked that your gas settings on both are the same (same gases active/in use) and that you’re positive you’re running the same GF hi/low on both, with no additional conservatism settings?

 

[pisauron]

You could compare the dive logs from all dives on the day (depths, runtimes, gas mix,...) to see if one of the computers logged something unusual?

You could have the computers set up for different salinity levels but that shouldn't trigger such a major difference.

Thanks.
I will upload the logs later today after our dives.

But perhaps the answer lies in this post sobmitted not so long ago:

Info - Deeply Safe Labs: A website for dive computer testing

Greetings divers, Over the past half year or so, we have been conducting tests on dive computers using a miniature hyperbaric chamber. Some of the results we have found, regarding repetitive diving, we believe could be of concern. This was identified with our first test protocol, two square...

scubaboard.com

 

[DiveLikeAMuppet]

Thanks.
I will upload the logs later today after our dives.

But perhaps the answer lies in this post sobmitted not so long ago:

Info - Deeply Safe Labs: A website for dive computer testing

Greetings divers, Over the past half year or so, we have been conducting tests on dive computers using a miniature hyperbaric chamber. Some of the results we have found, regarding repetitive diving, we believe could be of concern. This was identified with our first test protocol, two square...

scubaboard.com

Probably not, Deeply Safe Labs are just spreading F.U.D. . I think checking your logs would help, to confirm that both of your computers recorded the same thing.

 

[ahereng]

You could have the computers set up for different salinity levels but that shouldn't trigger such a major difference.

Salinity settings does not affect ndl/deco, it just slightly alter what depth is showing on your computer.
Ndl/deco is calculated from the pressure of the depth-sensor.

 

[rongoodman]

But perhaps the answer lies in this post sobmitted not so long ago:

Info - Deeply Safe Labs: A website for dive computer testing

Greetings divers, Over the past half year or so, we have been conducting tests on dive computers using a miniature hyperbaric chamber. Some of the results we have found, regarding repetitive diving, we believe could be of concern. This was identified with our first test protocol, two square...

scubaboard.com

Please don't resurrect that dead horse again.

 

[pisauron]

Please don't resurrect that dead horse again.

I am not smart enough to figure out the reason for the discrepancy. I am just left with guessing.

Is the stuff written in that post not true? I didn't quite get what's it's about.

 

[rongoodman]

The short version is that the author asserts that a currently popular deco model, Buhlmann GF, is insufficiently conservative, while providing no actual evidence or studies to back that assertion up.

 

[Jai Bar]

Probably not, Deeply Safe Labs are just spreading F.U.D. . I think checking your logs would help, to confirm that both of your computers recorded the same thing.

I am not familiar with the log recording of mentioned computers, but quite often the logs are recorded at different time intervals than the actual sampling during the dive.

For example, during the dive the sample interval can be 1 second, but the logs can be saved every 10 or 30 seconds.

A possible explanation could be if either computers are applying some moving window average: artifical simulation can be the same for both computers, but actual dive profile where the hand movements can cause "noise" (e.g. fluctuations of 1 foot in depth reading) which is averaged. Or there is a slight difference in actual pressure measured by the computers- these sensors are not very accurate.

I doubt either manufacturers will disclose any information regarding these differences, other than recommendation to RTFM..

Now, one can add to the confusion by adding a third computer, becaus then there will be three different NDL (or deco times) it becomes a three body problem

When I go diving Safaris I have two identical computers on wrist (because I once had a dead battery, another time a tank was placed on the computer, so backup it is). Don't ask, but after a couple of days the computers disagree one with the other, so there is that too. They also disagree about water temperature so I don't trust them at that neither- I dip my toes in the water to decide which wetsuit to don...

 

[scubadada]

@pisauron has told us the Garmin NDL was 55 min at 3 min into the dive whereas the Ratio NDL was 48 min. At 30 min, the Garmin had an NDL of 15 min while the Ratio had no remaining NDL time. This is a substantial difference, not the small variation you might expect running identical algorithms side by side. He has confirmed that the computers were both set on the correct mix and were set on the same GFs, 70/85.

At first glance, it would seem like the pressure sensors were OK with identical times on the NDL planners.

I have never heard that Garmin and Ratio might handle repetitive dives differently. Has anybody else heard this? There was some discussion of differences in how Buhlmann and DSAT handle 1st dive and repetitive dives differently on the Deeply Safe Labs thread, it is not a simple topic.

Info - Deeply Safe Labs: A website for dive computer testing

They seem to think that microbubbles from previous dives interfere with gas exchange at the alveolar level enough to at least partially invalidate the dissolved gas models, but as far as I can see have referenced no actual research to support that assertion. Dr. Mitchell, OTOH, did summarize a...

scubaboard.com

I am quite interested in hearing an explanation for the OPs observation, I would imagine that many of you are too. I look forward to hearing further follow up.