Now it's time to choose a wrist computer.

[dmaziuk]

A question i am very interested in, or pose as a thought experiment is, who is safer, the diver using a GF high of 95 or 75, if you end the dive with a SurfGF of 55? I think both are equally safe.

Define "safe".
Do they do it with the same total runtime? Same bottom time?

 

[Belzelbub]

A question i am very interested in, or pose as a thought experiment is, who is safer, the diver using a GF high of 95 or 75, if you end the dive with a SurfGF of 55? I think both are equally safe.

All else being equal, yes they are the same. Your body doesn’t care what conservatism your computer is using. It only cares how much N2 is absorbed, and that can be highly variable between individuals, or even within the same individual at different times.

In the real world, relative safety equivalency requires more information. For one, which tissues are controlling that SurfGF of 55. A diver with the fast compartments controlling that number would most likely be “safer” than a diver where the medium compartments are controlling.

 

[Belzelbub]

Can you set subsurface to 85/85 and do it again?

Sure. Here is 85/85. As I expected, since this was a no-stop dive, it looks the same as 40/85.

 

[dmaziuk]

All else being equal, yes they are the same. Your body doesn’t care what conservatism your computer is using.

All else being equal, diver #1 has to off-gas more rapidly on the way up, possibly subjecting their body to more decompression stress.

 

[Belzelbub]

All else being equal, diver #1 has to off-gas more rapidly on the way up, possibly subjecting their body to more decompression stress.

Diver 1 in my example or in drk5036’s? I read his to assume basically the same dive, with the only difference being the settings on the computer.

Not sure I fully follow if referring to my example, I think what you are saying makes sense, just not sure it changes the relative safety. Assuming divers are completely equal, offgassing would occur at basically the same rates. Yes, the fast compartments would likely be more saturated in the first diver, but those compartments on and off-gas quicker by nature, so should not be cause for concern.

 

[dmaziuk]

If both dives are equal, diver #1 (GF95) is not diving GF95 and the question is moot. Conversely, in order for the GF difference to matter, both divers need to ride their NDL down to 0 at the bottom. Edit: or at least, diver #1 needs to stay at the bottom longer than diver #2's NDL @ GF75.

 

[Belzelbub]

If both dives are equal, diver #1 (GF95) is not diving GF95 and the question is moot. Conversely, in order for the GF difference to matter, both divers need to ride their NDL down to 0 at the bottom. Edit: or at least, diver #1 needs to stay at the bottom longer than diver #2's NDL @ GF75.

OK. I think that was his point. Both divers are not diving to their computers settings. They are diving more to GFHi of 55. Therefore, assuming they dive the exact same profile, and on and off gas at the exact same rate, they are both equally safe. The only difference is that the diver with computer set to 75 will show a lower NDL.

 

[scubadada]

This is a dive that I have previously posted DSAT vs GF high 95, examples This dive was on my Teric at 80/95 using 32%. Dive time was 54 min, max depth 107 feet, avg depth 73 feet. It resulted in 1 min of deco at 10 feet. The orange line is the real GF, GF99. The very short deco cleared on ascent at about 35 feet. The GF99 did not start to increase until about 30 feet, 8%. At the safety stop the GF99 went from 45 to 32%. My SurfGF was 80%. My surfacing GF was 79%. The big bump in the GF99 on final ascent is nicely shown here. Yes, it was an overly slow ascent on a down line in very brisk current. Personally, I have learned quite a bit from looking at my Teric log on the Shearwater Cloud. I wish SurfGF could be tracked and displayed.

 

[drk5036]

OK. I think that was his point. Both divers are not diving to their computers settings. They are diving more to GFHi of 55. Therefore, assuming they dive the exact same profile, and on and off gas at the exact same rate, they are both equally safe. The only difference is that the diver with computer set to 75 will show a lower NDL.

Right, that was my point. I set my computer to GF95 if I know I’m going to be doing a multi-level dive with a long tail at minimal depth, because I don’t want to lose out on time at depth by “going into deco” when I know I’m going to be ending the dive shallow. I suppose I could use a lower GF and still just keep my eye on the SurfGF, to the same effect, but my computer showing I have an overhead is non-preferable if it doesn’t realistically describe the dive I’m doing.

I wish I could explain myself more eloquently.

This is a dive that I have previously posted DSAT vs GF high 95, examples This dive was on my Teric at 80/95 using 32%. Dive time was 54 min, max depth 107 feet, avg depth 73 feet. It resulted in 1 min of deco at 10 feet. The orange line is the real GF, GF99. The very short deco cleared on ascent at about 35 feet. The GF99 did not start to increase until about 30 feet, 8%. At the safety stop the GF99 went from 45 to 32%. My SurfGF was 80%. My surfacing GF was 79%. The big bump in the GF99 on final ascent is nicely shown here. Yes, it was an overly slow ascent on a down line in very brisk current. Personally, I have learned quite a bit from looking at my Teric log on the Shearwater Cloud. I wish SurfGF could be tracked and displayed.

View attachment 698307

This is a very good example. I agree also ive learned a lot from keeping an eye on SurfGF. Where I live in northern Japan, dives are usually short (30-40 minutes) with maximum depths of usually 25 meters. When we get to the safety stop, SurfGF will often be at about 55%. And after 3 minutes, it drops to maybe 52%? Basically, a 5 meter safety stop does…nothing. Spending 3 minutes slowly ascending form 3 meters is far more effective in ending the dive with a lower true SurfGF for this kind of profile.

 

[KenGordon]

but my computer showing I have an overhead is non-preferable if it doesn’t realistically describe the dive I’m doing.

But it does represent the dive you have done so far. If you don’t want to surface with a GF > 85 say then you should not let surface GF exceed 85. Set your computer to x/95 and have an emergency before ascending to the off gassing phase and you will exceed your limit.

I guess it depends on what you think of as a limit, for example I aim to surface with GF lower than 80 but will go to 99 in an emergency and higher if I have absolutely no choice.