Multi Deco vs Baltic - Matching and use

[Fires]

Jeremy is right in that the a coefficients between the B and C Mo values are different, C being more conservative than B. The Mo (surfacing) values differ in the 5, 6, 7, 8, 9, 10, 11, 12, 14, and 15th tissue compartments. This suggests that profiles whose leading compartment is one of these will exhibit differences in deco and run times. The greater difference will probably be toward the slower compartments because they have the larger half-times compared with the faster compartments.

In regards to programming the same algorithm there are two other issues I can think of that will effect the outcome: rounding and the depth for descents and ascents. Consider this calculation: 2.388 x 4.268 = 10.1919. Rounding this result to 2 decimal places gives 10.19. If I do the rounding first I get 2.39 x 4.27 = 10.2053, truncating to two places gives 10.20, a difference of 0.01. 0.01 is not much but when working with bigger numbers or longer calculations with intermediate results the differences can be bigger, possibly affecting the profiles by minutes.

How to treat descents and ascents is an issue. In my spreadsheet I use the Schreiner equation to calculate each tissue compartment's inert gas load. The equation needs the inspired inert gas pressure at the current depth to calculate the tissue loading for that depth. But, which depth to use; The starting, median, or final depth? In the ss you can select which depth you want. Different computer programs may use different depths or some sophisticated calculation using the different types of depths.

Thanks, makes sense and I do agree about the rounding. I had the same issue with Subsurface and working out SACs. I eventually found a lonely FAQ or thread that mentioned how subsurface takes into account the something something of the cylinder and thats why it is different..

I have never used Baltic but multideco desktop and mobile does match my shearwater within about a minute.

Thanks for the confirmation, good to know. I had a test on this last Sunday on one of the dives and also established the same. Although not a perfect test as it was "just" into deco.

I spoke to Shearwater as to why MD and the Shearwater do not match exactly. They said in actual application due to ascent times, the numbers come out to be the same

Makes sense. Thanks for the addition. Not too worried about a minute here or there, but just the bigger gaps.

My two bar is this: I also did my 45 course with someone who used a different deco app than what I was using. We both had different deco times and runtime schedule was close but not the same. We both ran the dive, discussed the outcomes of each and made a decision as a team as to what we felt was the best.

We did not just throw a dart at one. Luckily we were both diving on Shearwater computers so we ran the same dives on the computers and with that info, we were able to make a more informed decision as to what we felt was the most appropriate ascent strategy for our team.

I say this because while the apps might be giving you different times, it is all about being a thinking diver and developing a plan using multiple tools that is the best for the team.

Good luck and let us know how the course goes for you.

Hey hey Thanks for the addition. Yeah I agree completely. And we have discussed it at length now also. But its just odd that they are choosing an app that doesn't run C, yet their computers run C. And the argument is the app is nicer and easy to do +3's etc. But I would rather be using one that gives me more accurate (or matching my computers) times.

Thanks Yeah so far so good. Dive 2 done on Sunday and last 2 dives planned for this weekend where we can get the needed depth. Hoping for a great weekend.

I would not lose sleep over a few minutes of differences. You did not say what you used for gradient factors and for example choosing those (which to some degree is guesswork) has a stronger influence on runtime than the differences you report. If I run a similar dive through Subsurface with reasonable GF's I get similar times as well:

View attachment 571951

Please note that even though different implementations each claim to be ZHL-16x, this algorithm is not specified to a precision that does not leave some room for interpretation. I blogged about this here: Why is Bühlmann not like Bühlmann – The Theoretical Diver

Thanks yeah. I am also not too worried about a minute or 2-3 here or there. But when the distance is greater, it can grow. Whats odd is as I mentioned before, that you add a deco gas and it basically reduces that down to 1min.

I also, having played now with multi deco a lot in the last 2 months have found a few bugs I feel. Some seem to not show again others show. Like the error mentioned before I think about the deco time being more than or greater. Yet there are 3 different numbers. And then when you delete all the gases/levels you are not using (even though they are unselected) it works?

Also this last weekend we could not get rid of a 1min stop at the last level by backing off the bottom time. Eventually over 10min of bottom time reduction and still nothing. i.e WELL within NDL limits, but still showed a deco stop. Eventually putting the last stop to 6m solved the added deco, but kept a minute at 6m still. So definitely still buggy software.

I work in that industry, so can understand bugs. But when its "life" "dependent" and for the money you pay for the app, I would maybe expect better? Unless it is just me or a random once off bug which is difficult to solve.

Any who...on to more dives and checking and lets see

 

[rjack321]

Thanks, makes sense and I do agree about the rounding. I had the same issue with Subsurface and working out SACs. I eventually found a lonely FAQ or thread that mentioned how subsurface takes into account the something something of the cylinder and thats why it is different..



Thanks for the confirmation, good to know. I had a test on this last Sunday on one of the dives and also established the same. Although not a perfect test as it was "just" into deco.



Makes sense. Thanks for the addition. Not too worried about a minute here or there, but just the bigger gaps.



Hey hey Thanks for the addition. Yeah I agree completely. And we have discussed it at length now also. But its just odd that they are choosing an app that doesn't run C, yet their computers run C. And the argument is the app is nicer and easy to do +3's etc. But I would rather be using one that gives me more accurate (or matching my computers) times.

Thanks Yeah so far so good. Dive 2 done on Sunday and last 2 dives planned for this weekend where we can get the needed depth. Hoping for a great weekend.



Thanks yeah. I am also not too worried about a minute or 2-3 here or there. But when the distance is greater, it can grow. Whats odd is as I mentioned before, that you add a deco gas and it basically reduces that down to 1min.

I also, having played now with multi deco a lot in the last 2 months have found a few bugs I feel. Some seem to not show again others show. Like the error mentioned before I think about the deco time being more than or greater. Yet there are 3 different numbers. And then when you delete all the gases/levels you are not using (even though they are unselected) it works?

Also this last weekend we could not get rid of a 1min stop at the last level by backing off the bottom time. Eventually over 10min of bottom time reduction and still nothing. i.e WELL within NDL limits, but still showed a deco stop. Eventually putting the last stop to 6m solved the added deco, but kept a minute at 6m still. So definitely still buggy software.

I work in that industry, so can understand bugs. But when its "life" "dependent" and for the money you pay for the app, I would maybe expect better? Unless it is just me or a random once off bug which is difficult to solve.

Any who...on to more dives and checking and lets see

Multideco is quite clear in the documentation that its not intended to be used for no deco dives. That might be part of your issue.

 

[Fires]

Multideco is quite clear in the documentation that its not intended to be used for no deco dives. That might be part of your issue.

Ah really okay? Cool, well that could be it then. I never knew that.

None the less, its odd, that dropping from a 3m stop to a 6m last stop actually reduced the deco time. Don't think I really made that clear before. That was another aspect of the situation.

 

[rjack321]

Ah really okay? Cool, well that could be it then. I never knew that.

None the less, its odd, that dropping from a 3m stop to a 6m last stop actually reduced the deco time. Don't think I really made that clear before. That was another aspect of the situation.

what was your depth, time, and gasses for this hiccup of a deco dive?

 

[Fires]

what was your depth, time, and gasses for this hiccup of a deco dive?

I need to check again, I was also wanting to replicate it this week.

Will try tomorrow and report back. But both me on iOS multi deco, and another buddy on Android Multi deco had the same issue. And then a third person had the 6m stop and it was better.

Again, wasn't anything major but it was just odd. Will check and let you know.

 

[rjack321]

I need to check again, I was also wanting to replicate it this week.

Will try tomorrow and report back. But both me on iOS multi deco, and another buddy on Android Multi deco had the same issue. And then a third person had the 6m stop and it was better.

Again, wasn't anything major but it was just odd. Will check and let you know.

I have both the app and the desktop version, the app has fewer user controllable settings but most of the functionality is there

 

[EFX]

None the less, its odd, that dropping from a 3m stop to a 6m last stop actually reduced the deco time.

That is odd. I ran a few deco dives using my spreadsheet and compared dives with last stops of 3 m and 6 m. The 6 m last stop dives were all much longer in total deco time. This makes sense considering that (1) at 6 m you don't get the benefit of offgassing going from 6 m to 3 m for a 3 m last stop, (2) the difference in pressure between 6 m and 3 m is greater than the difference between 9 m and 6 m, so the rate of off gassing will be higher at the 3 m stop than the 6 m stop which necessitates lower total deco times and, (3) at 6 m it will take longer for the leading compartment to reach a ceiling of 0 m (6 m of water depth vs. 3 m for the 3 m stop).

I wonder if its something in the settings of the software. I have MD. I'll run some simulated dives and post back.

 

[dmaziuk]

That is odd. I ran a few deco dives using my spreadsheet and compared dives with last stops of 3 m and 6 m. The 6 m last stop dives were all much longer in total deco time. This makes sense considering that (1) at 6 m you don't get the benefit of offgassing going from 6 m to 3 m for a 3 m last stop, (2) the difference in pressure between 6 m and 3 m is greater than the difference between 9 m and 6 m, so the rate of off gassing will be higher at the 3 m stop than the 6 m stop which necessitates lower total deco times and, (3) at 6 m it will take longer for the leading compartment to reach a ceiling of 0 m (6 m of water depth vs. 3 m for the 3 m stop).

Not quite: the rate of off-gassing is half the delta-pressure during half-time. It's the delta-pressure that is greater at 3 m than at 6 m, but half-time is the same. But you only need to off-gas to M0. I can see the leading compartment off-gassing sufficiently at either depth in the same deco time; I don't quite get the longer 3 m stop.

Unless there's also a switch in the leading compartment between 6 and 3 m (to a slower TC). I guess it's possible, hypothetically.

 

[Fires]

Okay so I did a quick one, not sure if it was exact with our settings but I seem to have replicated it. Its VERY minor (1min) and maybe because its so close to NDL, thats why there is an issue as mentioned before AND maybe I am being pedantic, but still just wondering and enjoying the discussion

So:

Multi Deco iOS
GF: 45/80 (usually use 70 for deco dives, but we went higher as we were actually planning non deco)
Altitude: 1275 (as well as aclimatized)
Depth: 23 MFW
Time: 19 minutes
Gas: Air
Last stop: 3m

This gives me:

6m - 0:18
3m - 1:00

Same profile above, but with last stop as 6m gives me:

6m - 0:18

So again, its a minute. But I just don't want this to be a bug on higher values. Again, it would make sense that if the last stop is 6m, the deco is longer or equal. I did another test were there was 3minutes at 3m, and it added that to the 6m stop.

So could really just be funny numbers and rounding. But as I say interesting non the less.

Pics attached.

I have tested for higher values like a 30minute bottom time and it comes out correctly, as in 3m stop yields shorter total run time. So I assume this is just because of the low numbers.

And one final interesting one, is that on the same dive above. A bottom time of anything from 8mins to 20 minutes yields exactly the same 0:18 at 6 metres. So I think this has all come down to what was said, which is that Multi deco doesn't work so well with borderline or NDL dives.

 

[Graeme Fraser]

Okay so I did a quick one, not sure if it was exact with our settings but I seem to have replicated it. Its VERY minor (1min) and maybe because its so close to NDL, thats why there is an issue as mentioned before AND maybe I am being pedantic, but still just wondering and enjoying the discussion

So:

Multi Deco iOS
GF: 45/80 (usually use 70 for deco dives, but we went higher as we were actually planning non deco)
Altitude: 1275 (as well as aclimatized)
Depth: 23 MFW
Time: 19 minutes
Gas: Air
Last stop: 3m

This gives me:

6m - 0:18
3m - 1:00

Same profile above, but with last stop as 6m gives me:

6m - 0:18

So again, its a minute. But I just don't want this to be a bug on higher values. Again, it would make sense that if the last stop is 6m, the deco is longer or equal. I did another test were there was 3minutes at 3m, and it added that to the 6m stop.

So could really just be funny numbers and rounding. But as I say interesting non the less.

Pics attached.

I have tested for higher values like a 30minute bottom time and it comes out correctly, as in 3m stop yields shorter total run time. So I assume this is just because of the low numbers.

And one final interesting one, is that on the same dive above. A bottom time of anything from 8mins to 20 minutes yields exactly the same 0:18 at 6 metres. So I think this has all come down to what was said, which is that Multi deco doesn't work so well with borderline or NDL dives.View attachment 573295 View attachment 573296 View attachment 573297

Something that does seem a bit strange is your equivalent air depth (EAD). Your preprogrammed gas being 21%, the EAD should surely be the same as your actual depth.

Just me being curious as, both profiles being the same, it wouldn't account for this anomaly.