Dive Times are slightly off

[Darius]

I went on a dive trip this weekend, and I'm getting some odd behavior related to my dive times.  I have two separate but possibly related issues.

When I look at the dive summary for all dives in Subsurface installed on Windows 10 up through the dives I did last Saturday, the "total duration of selected dives" is 1 minute longer than it is if I just add up all of the numbers together.  There is one 30 minute dive that is adding 31 minutes to the total duration, however the total duration for just that dive shows 30 minutes.  Subsurface Mobile for Android doesn't add that extra minute.

I also did two dives on Sunday, my dive computer shows then as 20 and 21 minutes.  However, Subsurface shows them as 21 and 22 minutes (this is the same everywhere).  In addition, Subsurface on Windows then removes a minute from the first dive (dive computer shows 20 minutes, Subsurface shows 21) when calculating the total duration so the total duration of the windows and android installations end up being the same now because the windows version added a minute to one dive and then subtracted a minute from the other while the android version just added the displayed times together.

I've had an issue with total duration before (on the mobile app) and was told that it was probably a rounding error and then it ended sorting itself out but this seems more complicated and I don't know what to do (if anything).

Thank you,

Darius

[Robert C. Helling]

All times in  Subsurface are seconds internally. They get rounded to minutes only for the final display. Thus it is well possible that the (rounded) sum of all dive times differs from the sum of (rounded) individual dive times. Differences between the desktop and the mobile version could happen as the display code is different and maybe one lacks correct rounding and only chops off fractional minutes at the end. If you want to see what is going on, export the relevant dives as xml (ssrf) files and open them in a text editor. Look for the duration field to see the seconds.

Best

Robert

[Darius]

So I did some digging and the dive time issues do seem to be related to rounding with my dive computer display rounding everything down where Subsurface appears to round up it is 30+ seconds into the minute (which is fine but is causing some discrepancies between my paper logs and Subsurface that I'll have to deal with).  The rounding differences between windows and mobile are a little annoying but mobile agrees with my paper logs (or will once i adjust dive times) so I can just use that to double check my math because adding up the listed dive times makes more sense to me than going back and adding up seconds that I can't see and then rounding again.

It looks like the biggest issue is that my Cressi Goa is just confused.  I downloaded the Cressi log software and it is showing different information from Subsurface in terms of Depth and Time but those pieces of information are also different from what the computer itself is showing me.  The differences are small (1 foot or less than 30 seconds between the two log softwares) but when the companies log software shows different things from their computer I think that all of the blame can fall on the Cressi side (besides the rounding between Subsurface desktop vs mobile for combined dive times).

I appreciate the guidance on where to start looking!

- Darius

[Dirk Hohndel]

The question "how long is a dive" is surprisingly hard to find consistent answers for.

Divers typically come back to the surface, but the dive computer (console or wrist) very commonly stays "under water" for a while longer. Every app and every dive computer needs to figure out how to determine "the dive has ended". We consider anything that's less than 75cm / 2.5ft to be "no longer diving". A dive computer can make different assumptions. We have seen at least one case where a firmware upgrade in a dive computer actually changed that threshold - and so dives with the old firmware looked shorter on that dive computer than on Subsurface (because their old threshold was a rather brutal 1.5m IIRC) and afterwards they looked longer (because they dropped it to something like 50cm).

So... who cares, really? Does it actually matter? Did you see a Manta Ray? Was there an Octopus? Did you go into deco? Those are the things I'd be far more interested in compared to whether your dive was 20 or 21 minutes long (one of the Subsurface developers categorically states that anything under 60 minutes isn't really a dive, anyway)

/D

[Darius]

I know how my equipment displays dives and dive times, the issue was having what I use for my calculations (which is displayed on the computer) be different than what Subsurface shows when I get around to uploading my dives.

As someone who lives and dives at altitude, I care whether of my dive was 20 or 21 minutes long because at the end of the day that can make the difference between driving home safely and going into deco while driving because I didn't wait long enough before heading home.  I need accurate numbers for table calculations to make sure that I can keep diving.

- Darius

[Dirk Hohndel]

With all due respect to your believe in the accuracy of deco calculations and especially of post-dive altitude calculations... those are all utter random made up nonsense.

Several of us have tried to make sense of how the industry arrives at these thresholds and they all make no sense. And trust me. A minute in dive duration doesn't make any difference whatsoever. None. Zero.

If you believe otherwise... well, good for you, but I don't see any point in trying to address the issue that you mention in order to encourage that believe

/D

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[Darius]

I'd rather trust the professionals that I dive with who have created educational materials about diving at altitude and enjoy my safe diving at over 7,000 feet where I live and you can believe whatever you want.  That has nothing to do with my issue of different things on my end showing me different information, which was my issue that has already been explained and resolved as much as it can be.  Just because something is irrelevant to you doesn't mean that it is irrelevant to other people so putting down my concerns for accuracy (regardless of the reason for those concerns) isn't helpful in any way.  I'm sorry that you don't see any point in accurate information (which I admitted wasn't a Subsurface issue) and also having that information consistently displayed between the different versions of Subsurface.

- Darius

[Dirk Hohndel]

Please enjoy using a different dive log program that takes your concerns at the level of seriousness that you deserve.

Good Bye.

/D

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[Darius]

Thankfully I'll continue to use Subsurface as most members of its community are much less judgemental and more helpful.

- Darius

[Edward Haynes]

This total reliance of the deco calculations from one source is disturbing.

Last Sunday I was out, depending of which software or tables for a 35m dive we had 3 min @ 6m or 4 min @ 9m with another 10 @ 6m. There all theoretical.

Edward Haynes

DO Fyne-divers

+44 (0) 7800 590112

BSAC 

Dive with us

On 29 Jun 2021, at 22:46, Darius <tdariu...@gmail.com> wrote:



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[Gmail im Auftrag von Martin Gröger]

@darius

before getting mad on someone please do me the little favour and start to think over your equipment. in my experience the sensores that are used in the cressi divecomputers are not the most accurate. and I don't think that calculating of cressi is state of the art - it ok for the most of the fundives in your holiydays in the caribian sea - but not more.

when diving in a hight of 7000ft (2133m) I would rather use a shearwater or a heinrichs weikamp than a other divecomputer....

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[Willem Ferguson]

Alternatively, if you do technical dives at altitude, plan your dive beforehand and then dive your plan. While I do not agree with everything that GUE promotes, this is a classical case where they are absolutely correct: an over-reliance on a dive computer is misleading and positively dangerous in most cases.

Kind regards,

willem

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[Robert C. Helling]

Darius,

with my training as a physicist, let me say that the accuracy you are looking for is very likely not justified by what you want to do with it. Let me explain.

You could probably use an atomic clock to measure your dive time to nano-second precision. The only problem is: What exactly is the dive time? Does it end when the top of your head comes out of the water? Or the wrist on which you are wearing the computer? Or your last toe when you climb out of the water? What about the last drop of water on your foot when back on shore, does it mean you are still diving? Or do you stop counting when the center of mass of your computer is shallower than say four feet (which is what some computers count)? All those could be valid definitions of "end of dive" and would give you vastly different numbers of nano-seconds of your dive. Or minutes (which is what we are really discussing here). Does your safety of your drive home depend on this definition of end of dive? Obviously not.

Unless you are freediving (where dive time probably is more meaningful in terms of "how long can you hold your breath?") it does not make sense to state it more accurately than minutes (as more fine grained depend too much of definitions beyond your control). Don't lose sleep over it, the additional "precision" is only in your head, it does not have physiological consequences.

Which brings me to the second part: I invite you to research how "the professionals that I dive with who have created educational materials" arrived at the recommendations of those materials. Honestly, it's interesting. I have an entire blog devoted to such question: https://thetheoreticaldiver.org It turns out, this is actually quite hard because what is going on in the diver's body depends on so many more factors than depth and duration of a dive, for example general fitness, how much water you drank before the dive, your gender, how much sleep you got the night before, if there is some inflammation in your body, how much you exercised in the 48h before the dive, you genes, your dive training, your heart rate during the dive, the water temperature, how much you felt relaxed during the dive just to name a few. So what you have to do is to look at many (at least hundreds, better thousands) of dives that were similar enough in as many parameters as possible and then study the outcomes (in this example of travelling at altitude). Then you can try to draw boundaries between what kind of behaviour might be considered safe and what is risky.

But as things depend on so many factors beyond your control, the boundary will be quite blurry. And from all I know, a few minutes of dive time (or decompression time for those matters) will not greatly change your chances of getting symptoms of decompression sickness during your drive home. In particular, the uncertainty in all those models will overweight the difference between the mentioned 20 or 21 minutes of dive time. The distinction between black and white is an illusion, it's at best varying levels of grey.

I think, diving at altitude is not so much researched as the majority of diving happens to take place at sea level. But because of commercial relevance, what has been looked into quite a bit is flying after diving. In commercial planes, at altitude, cabin pressure is typically maintained at the equivalence of 8,000ft of altitude (roughly your number). And even there, the empirical basis (what came out of the experiments) is murky at best. Most "no-fly" recommendations are at best rule of thumb which large safety margins and way beyond what the models suggest that we routinely use very effectively  to calculate decompression stops. If have written about that here: https://thetheoreticaldiver.org/wordpress/index.php/2018/07/23/can-we-calculate-no-fly-times/

Long story short: Your life does not depend on plus or minus one minute of dive time. There is thus no point in measuring it more precisely.

[Darius]

I think there's been a little too much focus on the side-disagreement that I had with Dirk and the mention of altitude in general.  Altitude has nothing to do with the initial problem that I had which was about numbers on my computer being different than the numbers Subsurface displayed and I was looking for an explanation (even without anything to do with altitude I still would have looked for an explanation because I also keep paper logs of my dives and thus want my paper logs to match my digital ones).  Mentioning altitude was an off-the cuff comment that I apologize for, it was obviously a mistake because it distracted everyone from the issue I was having. 

As I mentioned in my second post, I discovered that pretty much all of my issues were with Cressi and that Subsurface was working as intended/great.  My initial problems are as resolved as they can be without the purchase of a new dive computer.

I am sorry for being the cause of this discussion going off-topic and somewhat off the rails.