Subsurface Plan Variations

[VE7DAC]

I have questions about the provided plan variations in Subsurface when planning a deco dive.
At the top of a generated plan in Subsurface, there's an option to generate plan variations, for an extra minute of bottom time and an extra meter of depth. It's formatted as below:

Stop times: + 1:26 /m + 1:40 /min

The way it's presented implies that if you overstay your bottom time by a minute, you add 1:40 to each deco stop. But that doesn't jive with my understanding of decompression, since an increased bottom time does not affect all the stops in a linear way.

Here's an example:
Buhlmann 50/80, air backgas with 50% deco gas.
10M/min descent to 40M, 20 minute bottom time.

1:52 @ 12M
1:40 @ 9M
3:40 @ 6M
7:30 @ 3M
Total Runtime 43:30

The variations given are as shown above, but if I increase the bottom time by 1 minute my stops are as follows:
1:53 @ 12M (+0:01)
2:40 @ 9M (+1:00)
3:40 @ 6M (+0:00)
8:30 @ 3M (+1:00)
Total Runtime 46:30

And the +1M stops:
1:41 @ 12M (-0:11)
2:40 @ 9M (+1:00)
3:40 @ 6M (+0:00)
8:30 @ 3M (+1:00)
Total Runtime 45:30

My question is really an academic one. I carry two computers and a written dive plan, so planning for no-computer deco is not at the front of my mind. Let alone having both computers fail, losing my teammate, and also exceeding the planned limits.

I'm sure that adding the given numbers would get me to the surface safely. But is that really how they're meant to be used? If you're planning contingency deco, wouldn't it be better to just write down a separate plan in your wetnotes rather than doing mental math with the variation numbers? Is anyone actually using these generated numbers, or are they a holdover from when computers were less reliable/unable to do deco calculations?

 

[elmo]

I have questions about the provided plan variations in Subsurface when planning a deco dive.
At the top of a generated plan in Subsurface, there's an option to generate plan variations, for an extra minute of bottom time and an extra meter of depth. It's formatted as below:

Stop times: + 1:26 /m + 1:40 /min

The way it's presented implies that if you overstay your bottom time by a minute, you add 1:40 to each deco stop. But that doesn't jive with my understanding of decompression, since an increased bottom time does not affect all the stops in a linear way.

It should be interpreted as 1:40 extra decompression per extra minute of bottom time. It's up to your judgement to determine how to spread the 1:40. It's primarily intended as a guide when planning a dive to understand the consequences of staying longer or going deeper. It can be used if you are forced/choose to make changes on the fly, but for divers running deco from a computer it can be considered a sanity check.

 

[atdotde]

Yes, you are supposed to spread the extra time over all stops (or subtract in case you stay shorter or shallower). A good guiding principle is to distribute it in proportion to the stop durations, i.e. if your stop at 6m/20ft is about half of the total deco time, that stop also gets about half of the extra time.

 

[atdotde]

To the question "do people really use those numbers?": There are people doing ratio deco/deco on the fly which is pretty much this but based on a few standard dives at standard depths. The idea in Subsurface is to make this slightly better by basing it on an actual plan. Experiments show that this is actually quite accurate not only for a single minute or meter plus or minus but actually a few (in which case you have to multiply the extra time), that is the extra time is to a good degree linear in depth and bottom time variations. Covering all those cases in extra dive plans would mean carrying quite a number of extra plans rather than just two numbers.

When comparing to plans where you actually entered the different depths/bottom times, please keep in mind that for a displayed plan, stop duration is always rounded up to full minutes (because nobody wants to watch seconds while holding a stop) but for the plan variations, stops are calculated to second precision to pick up the smaller changes more accurately.