Data Consistency

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biggpa

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I have the Perdix AI and noticed something strange with the Android app (v 1.0.1)

"Dives List" has 85.3ft while the "dive details" has 87ft max depth - can someone explain why there is a difference in the data between the two screens? Most of the logged dives are different values within ~3ft.

Any ideas?
 
Anyone? Can any confirm they see two different values while using the app?
 
I have the Perdix AI and noticed something strange with the Android app (v 1.0.1)

"Dives List" has 85.3ft while the "dive details" has 87ft max depth - can someone explain why there is a difference in the data between the two screens? Most of the logged dives are different values within ~3ft.

Any ideas?
I see the same thing (iOs app) - I'm assuming it's a bug as this is still In Beta?
 
I have the Perdix AI and noticed something strange with the Android app (v 1.0.1)

"Dives List" has 85.3ft while the "dive details" has 87ft max depth - can someone explain why there is a difference in the data between the two screens? Most of the logged dives are different values within ~3ft.

Any ideas?

Hi Biggpa,

I will forward this information to the app developers so they can addresses it.The cause is the app is using two sources of information for the maximum depth.

The first is the maximum depth as recorded in the log's closing record. This is the same maximum depth you would view on the Perdix while diving. It is the actual maximum depth measured during the dive.

The second way to determined maximum depth is to choose the deepest depth from all the samples in the dive log. However, these samples are only logged every ten seconds and the instantaneous values are recorded. Therefore, it is possible that these samples do not capture the maximum depth of the dive.

We will address this for consistency so only the first method is used.

Best regards,
Tyler Coen
Shearwater Research
 
I have been uploading to Divelog on iOS and just decided to give the SW Cloud a try.

I noticed several variances of depth from what was uploaded to divelog vs the SW cloud.
 
Hi Biggpa,

I will forward this information to the app developers so they can addresses it.The cause is the app is using two sources of information for the maximum depth.

The first is the maximum depth as recorded in the log's closing record. This is the same maximum depth you would view on the Perdix while diving. It is the actual maximum depth measured during the dive.

The second way to determined maximum depth is to choose the deepest depth from all the samples in the dive log. However, these samples are only logged every ten seconds and the instantaneous values are recorded. Therefore, it is possible that these samples do not capture the maximum depth of the dive.

We will address this for consistency so only the first method is used.

Best regards,
Tyler Coen
Shearwater Research
Thanks for the explanation but, just to be clear: Let's say that I am at the deepest part of a dive to that point (e.g., 100 feet) - I should see 100 feet as current depth and max depth on the Perdix display. If I then drop to 105 feet, the Perdix display would now show cuurent/max depth as 105 feet. However, if I did that drop to 105 feet but ascended back to 100 feet within the 10 second sampling rate, the downloaded dive log would show the maximum depth as 100 feet as the quick excursion to 105 feet was "missed" in the sampling? Therefore, the max depth in the log is not necessarily the actual maximum depth reached during the dive?

I'm just curious about how it works and don't consider it a big ssue as I'm not normally making significant descents/ascents within 10 second windows. However, it would be nice if the log captured the deepest instantaneous depth acheived to truly reflect the max depth reached.

Was this changed since July? Also, what do 3rd party logs like Dive Log have available to pull the max depth from - the first or second method that you describe?
 
Also, what do 3rd party logs like Dive Log have available to pull the max depth from - the first or second method that you describe?

We wanted to respond to specifically address how Dive Log on iOS handles the maximum depth for the Shearwater dive computers.

The situation for us is slightly more complicated than Tyler described in his response. The additional complication is that the dive header associated with the downloaded dive reports the depth as an integer value whereas the value that you can get from the profile data includes a fractional component. So, for example if your profile data had accurately captured the deepest moment in the dive (i.e. the deepest part of the dive happened to fall on the sampling interval) the depth might have been 82.3 feet but the diver header value would have been reported as 82 feet (which is also the value that you would see on the display of your dive computer). The profile, in this case, would contain a sample at 82.3 feet. Note that the situation can be even more pronounced in the case where the dive was set to record metric values since 25.3 meters has a greater difference from 25 meters relative to what you would see in feet (you would particularly notice this in Dive Log if you switched the measurement system between meter and feet). Also, if I recall correctly, the value in the dive header is truncated rather than rounded so an actual max depth of 82.9 feet would be reported as 82 feet (my recollection on this point could be wrong though so feel free to correct me).

When downloading the Petrel or Perdix, you first download a summary of each dive (known as the “manifest”) and then (in the case of Dive Log at least) we show you those dive summaries and you choose the dives that you want to download. Dive Log then proceeds to download each dive that you select. As it turns out, this “summary” happens to include the maximum depth with the factional component (so from the example above it would show the maximum depth as 82.3 feet). So we keep that value around until we’ve downloaded the actual dive and store that value as the maximum depth for the dive instead of the depth reported in the dive's header data (82 feet in the example).

The bottom line is that Dive Log on iOS records the actual instantaneous maximum depth (including the factional part of the value) as reported by the dive computer as the Max Depth for the dive. The profile data will include all the samples available which might not include a sample at the instantaneous maximum depth reported by the dive computer. Other variations in the depth that you might see relative to other applications are likely the result of the difference between the factional value vs integer value reported by the dive computer during the download due to the additional accuracy in the dive summary list vs the actual dive header.

One final note, if for some reason we do not have a valid fractional depth from the summary information, we do calculate the maximum depth from the profile data. I have never seen a case where this happened, but we chose to fall back on the profile depths (which are fractional) instead of the integer value in the dive header. Finally, if there is no profile samples either then we report the dive header value (i.e. the integer value).

Please feel free to contact us directly if you need any additional information about how Dive Log handles the data that it reads from the Shearwater (or any other) computers.
 
Does the Android app discard data from the downloaded logs? I notice that the same dive downloaded from the Perdix AI to the Windows desktop app appears to have more data points than when downloaded using the Android Shearwater app.
 
https://www.shearwater.com/products/perdix-ai/

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