Why do people add a few minutes to their last deco stop?

[rob.mwpropane]

I have read on here (and it make sense to me, and what I follow) that it depends on the dive. If you're doing lots of deco an extra few min won't do a whole lot, but if you're doing less deco then it makes sense because the payoff is greater.

90 min of deco with 3 min added nets you 3.3% more deco

30 min of deco with 3 min added nets you 10% more deco

I am thinking I adopted this from @rjack321, maybe he can chime in and clarify better then I can

Regardless, I crawl to the surface after the clock is cleared and that always adds some as well. Like 3 min from 15'

 

[L13]

I have read on here (and it make sense to me, and what I follow) that it depends on the dive. If you're doing lots of deco an extra few min won't do a whole lot, but if you're doing less deco then it makes sense because the payoff is greater.

90 min of deco with 3 min added nets you 3.3% more deco

30 min of deco with 3 min added nets you 10% more deco

I am thinking I adopted this from @rjack321, maybe he can chime in and clarify better then I can

Regardless, I crawl to the surface after the clock is cleared and that always adds some as well. Like 3 min from 15'

You should look at it as a percentage of your last stop time, not total deco. The last stop time is driven by the controlling tissue's saturation when you surface, all your other tissues are lower. It will also be the slowest tissue "significantly" effected by the dive. Especially if you are breathing O2, you will be accelerating the drop in that tissue's saturation as compared to breathing air on the surface.

 

[VikingDives]

My instructor says he'll often add a few minutes to his last deco stop, and I've seen a few people on these forums mention that they do a similar thing.

It doesn't make a lot of sense to me, when I hear that in my brain I hear "I don't think my deco plan / GFHi is conservative enough so I'm going to add extra deco on top of it". Why not just decrease the GFHi instead of arbitrarily adding a few minutes on the end (Note: I'm not talking about doing the slow ascent from last stop to surface, I'm talking specifically about adding extra time to the last stop before starting the ascent to surface).

I will add time to the stop (if I'm not too cold) because I feel better if I do. It is more of a "it works for me" kind of thing...

To your question, there's some minor changes in the profile depending on your GF-hi, that keep you lower for longer. The way I like to do things is to run a 50/80 profile, but stick around my 10 foot stop until my surfGF is 70, then I do 1 foot per minute ascent (part for conservatism, part for just keeping my buoyancy dialed in) which will put me at the surface with a surfGF of about 63.

Ultimately, I will surface with a 80 if I have to. It just works out to be a little faster if I hit the last stop with a 50/80 profile.

For example this is 50/63 profile for 150' on air with nitrox 50 (it's just what I had in multideco, not a recommendation):

Dec to 150ft (5) Air 30ft/min descent.
Level 150ft 25:00 (30) Air 1.11 ppO2, 145ft ead
Asc to 70ft (32) Air -30ft/min ascent.
Stop at 70ft 3:20 (36) Nitrox 50 1.45 ppO2, 28ft ead
Stop at 60ft 1:00 (37) Nitrox 50 1.31 ppO2, 22ft ead
Stop at 50ft 1:00 (38) Nitrox 50 1.16 ppO2, 16ft ead
Stop at 40ft 4:00 (42) Nitrox 50 1.01 ppO2, 9ft ead
Stop at 30ft 7:00 (49) Nitrox 50 0.86 ppO2, 3ft ead
Stop at 20ft 15:00 (64) Nitrox 50 0.72 ppO2, 0ft ead
Stop at 10ft 31:00 (95) Nitrox 50 0.57 ppO2, 0ft ead
Surface (105) Nitrox 50 -1ft/min ascent.

Here's one at 50/80:

Dec to 150ft (5) Air 30ft/min descent.
Level 150ft 25:00 (30) Air 1.11 ppO2, 145ft ead
Asc to 70ft (32) Air -30ft/min ascent.
Stop at 70ft 3:20 (36) Nitrox 50 1.45 ppO2, 28ft ead
Stop at 60ft 1:00 (37) Nitrox 50 1.31 ppO2, 22ft ead
Stop at 50ft 1:00 (38) Nitrox 50 1.16 ppO2, 16ft ead
Stop at 40ft 2:00 (40) Nitrox 50 1.01 ppO2, 9ft ead
Stop at 30ft 5:00 (45) Nitrox 50 0.86 ppO2, 3ft ead
Stop at 20ft 12:00 (57) Nitrox 50 0.72 ppO2, 0ft ead
Stop at 10ft 23:00 (80) Nitrox 50 0.57 ppO2, 0ft ead
Surface (90) Nitrox 50 -1ft/min ascent.

The 40, 30, and 20-foot stops on the 50/63 profile are a little longer, and I don't really want to do that... I'd rather get up to the last stop with the 50/80 plan then assess how I'm feeling. I know that I feel better if I'm getting out of the water under a 70.

I also do really slow ascents on recreational dives and like to do 5-minute safety stops, again, because that's what works for me.

 

[cerich]

My instructor says he'll often add a few minutes to his last deco stop, and I've seen a few people on these forums mention that they do a similar thing.

It doesn't make a lot of sense to me, when I hear that in my brain I hear "I don't think my deco plan / GFHi is conservative enough so I'm going to add extra deco on top of it". Why not just decrease the GFHi instead of arbitrarily adding a few minutes on the end (Note: I'm not talking about doing the slow ascent from last stop to surface, I'm talking specifically about adding extra time to the last stop before starting the ascent to surface).

it isn't your plan, algorithm or computer that may be off, it's your body that is the wild card. Thus adding a few mins when able on last deco stop,

 

[Manatee Diver]

I can and have ascended once my computer has cleared, so I know in general that it is safe to do. But just to keep myself further away from the line, I extend my final stop and do surface deco. Because oxygen is cheap, and getting bent will probably ruin my week.

 

[Lorenzoid]

It doesn't make a lot of sense to me, when I hear that in my brain I hear "I don't think my deco plan / GFHi is conservative enough so I'm going to add extra deco on top of it". Why not just decrease the GFHi instead of arbitrarily adding a few minutes on the end ....

With the Surfacing GF feature in Shearwater computers, padding the last stop and decreasing GFHi are effectively the same thing. Instead of decreasing GFHi before the dive, you can just monitor SurfGF at the last deco stop and surface when it reaches the number you're comfortable with.

 

[Whitrzac]

Why do people do a safety stop?

Same reason...

 

[leadduck]

If your last stop was on O2 at 6m/20ft, then you should ascend very slowly like 1min per 1m/3ft. Maybe that's what he meant with adding a few more minutes.
I set my computer to a not super conservative value like GFHi=80, then add conservatism manually depending on the situation and environment by staying a bit longer on the last stop.

 

[boulderjohn]

Hmmm. If you're using 100% oxygen, then the last deco stop is at 20 fsw, correct? In this case, is there a need to add minutes to the last deco stop? And, moreover, you need to be mindful of your oxygen clock as relates to not only the current dive but to the repetitive dives, too.

If your last deco gas is NOT 100% oxygen, then aren't the slowest tissues still on-gassing N2 when you add minutes to the last deco stop(s)?

rx7diver

Any tissues that are still on-gasing at 20ft have so little N2 in them that it doesn't really matter, but even more so if you are breathing anything richer than air.

Many people (including me) set the last stop at 10 feet, which when using O2 gives you the freedom to be at any depth between 10-20 feet. I'm usually around 15 feet.

If not using O2 and the slowest tissues are still on-gassing, they can only on-gas to ambient pressure. Research with saturation divers has shown that all tissues can safely surface if saturated to pressures equal to 20 feet or shallower, so any on-gassing should never reach dangerous levels.

 

[cybernetic]

Because you can, dive computer anyway just "predict" based on knowledge and theory that we know...
Adding extra stop => add more safety for yourself...