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

[leadduck]

Even if your computer does not extend the duration of the safety stop, you may of course overstay voluntarily and I found that this makes a difference to me, feeling better after surfacing. So if there's some jellyfish or a school of fish below the boat to watch for 10min, please do so and don't automatically exit just because your safety stop time is over.

 

[MidOH]

Because I want to stay in the water, as long as my air supply allows.

Even if you're at the surface in boring deep water. Sometimes it's amazing what'll pass by at the surface. Schools of needle fish, random bait ball, lovely jellyfish.

I'll even snorkel around the boat for a while. I'm never the first one back on board.

 

[inquis]

@twistypencil - For perspective, it's been often reported that the PADI Recreational Dive Planner (i.e., tables) are approximately a GFHigh of 95 (what Shearwater has labeled "Low"). These were developed and tested specifically for repetitive recreational diving.

Additionally, the first dive on the extremely common Suunto computers (RGBM algorithm) is equivalent to a GFHigh of about 95 or slightly lower (depending on depth).

My point is, there are many, many dives done every day by divers of all ages and fitness levels at ~95. Obviously, it's your call how much additional margin you would like, but hopefully the above helps if you were not previously aware of it.

 

[OMyMyOHellYes]

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).

Gotta pee before getting back on boat.

 

[dmaziuk]

However, I'm wondering why I wouldn't want to just set the highest conservative GFhi value (in rec mode, you can pick 'low', 'med', 'high' or 'custom')? It seems like that would mean the computer would suggest I do more of a "safety stop" and from reading this thread, it seems like more people are doing longer deco because of their SurfGF values suggest it might be better to be more safe, or to remove decompression stress.

ObAllAnalogiesSuck: consider landing a jet. There's a program that reads the length and condition of the runway, airspeed, etc., and tells the pilot if the runway is long enough to land safely on. That's your NDL, controlled by GF High. It's a prediction based on things that have not happened yet. You won't know if you've landed safely until the plane comes to a stop.

SurfGF is more like a big red blinkenlight coming on after the touchdown: "ENGAGE REVERSE THRUST NOW!!!" -- or not coming on, as the case may be. It's much closer to the actual crash and is a far more accurate prediction. (There may still be a patch of ice or a pothole ahead to thwart it.)

(In the case of SurfGF: "arguably more accurate" , but that's a different can of worms.)

 

[scubadada]

Hi @twistypencil

It sounds like you likely have a Shearwater computer. SurfGF has taken the guesswork out of the safety stop. My comments are related to recreational diving.

If you choose to use a conservative GF high of 75, you may find that you may be going into deco while doing the repetitive diving on your liveaboard. You would need to do the deco time to clear it or acknowledge that you skipped deco time. With a Shearwater, you will not be locked out. If you choose a higher GF high, you are still able to control your surfacing GF by monitoring Surf GF.

If you dive all the way up to your no deco time and then make a direct ascent to the surface at 30 ft/min, you will surface with a GF close to your GF high. A slower ascent and/or a safety stop will lower the surfacing GF below your GF high. The SurfGF tells you your surfacing GF if you were instantaneously at the surface. If you use it at your safety stop, your surfacing GF will be a bit lower depending on your final ascent rate. It is a good idea to make your final ascent slowly, I try to use 10 ft/min, as that is where your GF really blossoms. You can easily see that on the Shearwater Cloud graph of your dives (example below)

Regardless of your GF high, you can control your surfacing GF to whatever you want by using SurfGF and adjusting your safety stop, if needed.

 

[Johnoly]

I try to use 10 ft/min, as that is where your GF really blossoms.

I agree with a very slow crawl to the surface from my 15ft safety stop. With all other things being equal, can you give me an estimate of how much more bottom time or NDL time on my 2nd tank a Shearwater will give me by changing the rate of ascent.. For example, what would be the additional time If I do 10ft / min vs 30ft / min ascent rate from 15ft SS. I'm just curious because I can't see that data on my ProPlus 2.

 

[scubadada]

Hi @Johnoly

I doubt doing the final ascent in a minute and a half vs. 30 seconds makes a significant difference in the subsequent dive NDL. The main advantage is to slow the rapid increase in the GF during the end of the dive while surfacing. The surfacing GF will be just a little bit lower.

I don't know how to more specifically address your question, perhaps someone else does.

 

[dmaziuk]

Probably something between zero and zip: the off-gassing that happens in the 1.5 minutes vs. .5 minutes, even if you're looking at the 4-minute TC, can't be that much and is most likely negligible compared to off-gassing that will happen during SS afterwards.

 

[johndiver999]

Yeah I agree. Your Surface Interval will be shorter if you come up slow, but I bet you could offgas from the ladder to your seat breathing nitrox (and make up for it).