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

[Bigbella]

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

Are you in that much hurry to return to a cattle boat?

 

[harvalen]

Running a higher GF high will let the watch show you an absolute TTS which is helpful for contingency planning.
Do you surface at a GF higher than preferred but still safe if you're starting to go hypothermic etc.

What is "still safe?". If you're cold then you probably want to surface with a lower GF. Running with the lower GF means you can keep an eye on the TTS and check that it doesn't grow too big if you start to get cold. You know that the TTS with higher GF will be shorter.

 

[grantmac]

That's why it's a contingency.
Ultimately YOU choose what is displayed by your computer.
Do you want it to calculate the profile which is as low risk as possible then accept violating that if you need to leave the water earlier than planned?
Or do you want to set a maximum acceptable risk profile which you then manually choose to lower by extending the stop?

I want to know how far I am from being able to surface with the GFs I have decided are acceptable for me.

 

[harvalen]

That's why it's a contingency.
Ultimately YOU choose what is displayed by your computer.
Do you want it to calculate the profile which is as low risk as possible then accept violating that if you need to leave the water earlier than planned?
Or do you want to set a maximum acceptable risk profile which you then manually choose to lower by extending the stop?

I want to know how far I am from being able to surface with the GFs I have decided are acceptable for me.

I guess it's also a question of how we define "acceptable risk". I prefer to calculate a profile that I'm comfortable surfacing after when things go as planned, not needing any extra padding. If I have to end a dive early because I risk hypothermia, then I might surface with a GF that I wouldn't consider "acceptable risk" for any normal dive, but it would likely be better than freezing to death/drowning.

This time a year I really want to know the TTS for my preferred GF, since it can be close to freezing at the surface and down to 10 m, but much warmer at 30 m. So on-gassing at depth is likely faster than off-gassing when ascending.

Edit:
To clarify the last part a bit. If I dive with GFHigh 85 and paying attention to TTS I might end up diving a profile like this:

To get down to SurfGF 70 I would need 10 min at 3 m or 17 min at 6 m, in waters that are likely much colder than they were at 12 m and below. Something that I definitely don't want to be surprised by when I'm already getting cold. If I dive with GFHigh of 70 then I can ensure that any stop time in the cold shallow water is minimized and I'll likely end up diving a much more conservative profile.

 

[Nick_Radov]

There's nothing wrong with ratio deco, provided that for some reason you want to only dive specific gas mixes to specific depths instead of just buying a ~$300 watch.

I think a lot of divers still misunderstand ratio deco. At it's core it doesn't prescribe any particular deco plan and isn't even based on a specific deco algorithm. It's just a recognition that 2D and 3D curves can be approximated pretty well with linear relationships within certain reasonable bounds. By working through the math yourself and memorizing the relationships applicable to your frequently used gas mixes and depths you can anticipate how a minor change in your dive plan will impact the deco profile and gas usage. Feel free to use a dive computer if you like, but a dive computer won't let you update your plan in the middle of a dive and tell you whether you're at risk of hitting minimum gas. I have found the mental planning tools of ratio deco to be tremendously valuable in terms of knowing what's happening and dynamically adapting to changing circumstances without increasing risk.

 

[lermontov]

but a dive computer won't let you update your plan in the middle of a dive and tell you whether you're at risk of hitting minimum gas. I

firstly any experienced diver who's diving trimix will have already done those volume calculations before they got in the water and knows what to allow on a given TTS and secondly if ive got a prearranged surface time I dont exceed that- so any changes will be within the dive plan times and gas volumes originally calculated

 

[rjack321]

firstly any experienced diver who's diving trimix will have already done those volume calculations before they got in the water and knows what to allow on a given TTS and secondly if ive got a prearranged surface time I dont exceed that- so any changes will be within the dive plan times and gas volumes originally calculated

Clearly you've never been on a boat with "FL Man". My general experience is that MANY people dive with way too thin of gas margins. CCR divers are the worst. For some crazy reason 2x lp50s is now the cat's meow for all kinds of previously unheard of cave and trimix dives.

 

[rjack321]

Did the diver ever figure out what the cause was? PFO or something?

That's scary to me.. if it's a cautious diver doing everything right, crawling to the surface and still get a hit.. that's scary.

They're PFO negative.

Somedays you're on that edge of the bell curve and you're going to come up bent even with a GF high significantly less than 100. It's really just like any other sports injury, sometimes it happens. Those are the days you don't want to be blindly following a computer thinking its "impossible" to get bent, even in the water, at a GF high of 75 (or whatever). If it starts to hurt don't shake your head in denial. Stop, think, act - just like any other in water problem.

 

[Nick_Radov]

firstly any experienced diver who's diving trimix will have already done those volume calculations before they got in the water and knows what to allow on a given TTS and secondly if ive got a prearranged surface time I dont exceed that- so any changes will be within the dive plan times and gas volumes originally calculated

Well that's exactly the point. By having a good sense of the ratios, if your depth ends up being a little different than the plan then you'll know how to adjust your bottom time to stay within the total allowed run time.

 

[Tassi Devil Diver]

For me it's about adding minutes to the final ascent rather than the last deco stop.