Riding GF99 instead of mandatory/safety stops

[OctopusLover]

This is all theoretical, I'm not even a technical diver, but I am a nerd and have been reading the manual of the Shearwater peregrine and if I'm understanding things correctly, then I don't understand why technical divers still use safety stops when these values are now visible on modern dive computers. First, let me explain in my own words what I understand these values to be:

• GF99: the current pressure difference between the most stressed tissue in the model at the current depth, as a percentage of the pressure difference expected to produce DCS symptoms. Basically, how much nitrogen you're currently releasing where 0 is none and 100 is dangerous.
• SurfGF: What GF99 would look like if you were on the surface. >100 means you've exceeded your NDL.
• GFHigh: The target SurfGF you're asking the computer to keep you under when calculating NDL.
• GFLow: The GF99 value that will be targeted to calculate the depth of your next safety stop.

So... if I've decided a GF99 of 40% is an ideal offgasing rate, why would I go to a depth that achieves that value and then remain there as it lowers instead of just constantly ascending to maintain that value all the way to the surface? It feels like the best decompression procedure would be to maintain an ascend rate that keeps GF99 = GLow all the way to the safety stop depth and then wait until SurfGF = GfHigh

 

[Manatee Diver]

What you are describing is continuous decompression vs the staged decompression where you stop every ten feet.

Staged is easier to control as you are stopping and getting stabilized for a few minutes at a time.

 

[tbone1004]

at the highest level it's because the precision is simply not possible given the dynamic environment. It's one thing in a chamber or even a commercial diving lift where a machine can be in control, but even with those, staged decompression is just much more practical. Historically the gauges carried by divers weren't precise enough to even attempt it, but even now with digital gauges it's just highly impractical.
The practicality comes not only from driving yourself bananas trying to control an ascent rate measured in centimeters/minute, but also the fact that it would require so much attention to a depth gauge to ride that line that you couldn't focus on anything else and those ascents are often used for recording data from a working dive, organizing gear to make exit easier, communicating with surface/buddies/support for what you may or may not need on a longer dive, etc etc.

TLDR, theoretically it's better but the cost in terms of practicality certainly do not outweigh the relatively miniscule benefits.

 

[OctopusLover]

Staged is easier to control as you are stopping and getting stabilized for a few minutes at a time.

The practicality comes not only from driving yourself bananas trying to control an ascent rate measured in centimeters/minute,

That makes total sense, but then pretend I'm a technical diver on a safety stop, I'm fully stabilised, bored and my safety stop prescribes several more minutes. I look at my dive computer and see my GF99, which started at 40% when I first got at this depth has now dropped to 30%. Is there any reason why I would remain at this depth instead of ascending?

I guess what I'm asking is, am I correct that NDL / safety stops / deco stops prescriptions are procedures designed to keep GF99/SurfGF within safe limits, but that I could feasibly ignore all these procedures if I were to just constantly monitor GF99/SurfGF and keep them within values I'm comfortable with?

To make a bad analogy, "don't drink and drive" or "wait an hour for every drink you had before you drive" go out the window if I have an actual breathalyser and I can just sit in my car and wait until it tests below the legal limit before I start the engine.

 

[tbone1004]

That makes total sense, but then pretend I'm a technical diver on a safety stop, I'm fully stabilised, bored and my safety stop prescribes several more minutes. I look at my dive computer and see my GF99, which started at 40% when I first got at this depth has now dropped to 30%. Is there any reason why I would remain at this depth instead of ascending?

all depends on where you're diving.

If I was in the ocean with surf? My happy ass is staying as deep as possible as long as possible to stay out of the surge.
In a quarry or lake and there is a thermocline above my head? I'm chasing that thermocline to get warm
On a shore dive with a sloping sandy bottom? I'll kick around to alleviate the boredom and meander my way back to shore making sure to stay out of the surge.

Too many variables to give any hard and fast "what to do" scenarios and is part of the growing process as a diver. Your training gives you the box to operate within and then your experience gives you the ability to safely poke some holes in the box and figure out the grey area.

 

[tursiops]

I don't understand why technical divers still use safety stops

They don't.

Deco stops are mandatory; safety stops are not....but are a good idea for recreational divers. If you've done your deco stops on a deco dive, there are no safety stops afterwards. Unless you just want to stay in the water. Which is fine.

 

[Tracy]

They don't.

Deco stops are mandatory; safety stops are not....but are a good idea for recreational divers. If you've done your deco stops on a deco dive, there are no safety stops afterwards. Unless you just want to stay in the water. Which is fine.

It depends on the diver and your thinking/training. When I clear my last stop, I ascend from 6m at a rate of 1M per minute.
Essentially, I am taking a five minute safety stop after my last stop. It takes a full five minutes to ascend at that rate.

 

[tursiops]

It depends on the diver and your thinking/training. When I clear my last stop, I ascend from 6m at a rate of 1M per minute.
Essentially, I am taking a five minute safety stop after my last stop. It takes a full five minutes to ascend at that rate.

I doubt most would call that a safety stop, A good practice -- and hard to do! -- but not a safety stop.

 

[inquis]

Shake a soda bottle. You have a choice between cracking it open (not allowed to close it) OR repeatedly open then close it (letting things calm down each time). You make the choice.

 

[L13]

• GFLow: The GF99 value that will be targeted to calculate the depth of your next safety stop.

This is wrong. GFLow is the GF99 value that will be targeted to calculate the depth of the first deco stop. The remaining deco stops will be linearly interpolated between GFLow and GFHigh.

So... if I've decided a GF99 of 40% is an ideal offgasing rate, why would I go to a depth that achieves that value and then remain there as it lowers instead of just constantly ascending to maintain that value all the way to the surface? It feels like the best decompression procedure would be to maintain an ascend rate that keeps GF99 = GLow all the way to the safety stop depth and then wait until SurfGF = GfHigh

You could do that, or increase your GF99 as you cross each depth threshold to the next linearly interpolated GF number.

I think that would be optimal (at least from an off gassing perspective), and maybe even practical for the beginning of deco when stops are just a few minutes, but not so much at the end as stops get shallow and long.

Of course @inquis 's idea about letting things calm down between changes may be the dominant factor. Since very little research has been done on continuous GF99 profiles, we don't know if there are currently unknown factors that make it less effective at preventing DCS, even if it would be the most effective from an off gassing perspective.