Feedback on recent two-tank and dive limits

[Blackcrusader]

If someone cannot understand GFs (for the hard of thinking; I wrote ‘cannot’, not ‘does not’), should they even be diving?

After I bought my Perdix in 2018 I downloaded the user manual, updated my firmware then went about finding out about Gradient Factors, what they meant and how to use them. I'm an older guy but not clueless. The Perdix recreational settings I really did not need to spend any time on to understand NDL diving or choosing settings from least conservative to most conservative.

Now to learn about using GF for deco diving I needed to study up on that. I do use the default 45/95 as I am happy with that. Then with later firmware there is Surf FG in the 2019 V65 firmware and the DECO CLEAR message.

All DC's people use they need to RTFM and not just assume what the default settings are. As I wrote in a previous post a diver with a Peregrine who had never checked her settings told her guide that I must have been going into DECO when my Perdix was on different GF to hers. It was fine she had that concern and I addressed that to her by showing her about the settings and also the dive log graph to show I had not been in Deco. She did not know the Shearwate would show a red line for Deco times in the dive log as she had never exceeded NDL.

 

[David Carron]

You make very good point about situational conditions. And on many occasions the dive you execute may turn out to be quite different to the dive you anticipated and planned.

This is how the frogman executes a NDL recreational dive. I use one of my dive computers which has a DSAT algorithm (no conservative settings). I complete the bottom time. Just before or during my ascent I decide which safety stop profile I adopt, based on the dive conditions I just experienced.

I may choose to do 3 min at 15 ft, or perhaps 5 min at 15 ft, or if it has been bit of a tough dive I may break up the safety stop into 2 x stages, that is, 2 min at 15 ft followed by 3 min at 10 ft. (just a few stop options in my bag of tricks). It's all about using the safety stop to keep the DCS risk value to about 3%.

This is a little bit off-topic and since we're talking about safety stops on an NDL dive probably totally negligable, but you mention that you have a few different safety stop profiles and choose based on the dive conditions. Do you have any particular theory to justify the choice?

When you choose, or stage, your safety stop between 15ft and 10ft what guides you to think that the faster off-gassing at 10ft offsets the increased bubble generation at that depth?

 

[rongoodman]

It's all about using the safety stop to keep the DCS risk value to about 3%.
.

How does that work? Please share with the class.

No messing about trying to decide on what GF to select or changing the GF mid ascent because it is not adequate for the actually experienced dive conditions.

When have you seen anyone here suggesting changing the GF mid-ascent on an NDL dive?

 

[Etmar]

I do use the default 45/95 as I am happy with that.

AFAIK, Perdix default GF is 30/70.

 

[rongoodman]

AFAIK, Perdix default GF is 30/70.

I depends on whether you're in Rec or Tec mode. Default for Rec mode is 40/85.

 

[Blackcrusader]

AFAIK, Perdix default GF is 30/70.

Mine was not. So there you go.
Also as written above what mode are you assuming one is usiing their Perdix in?

 

[Belzelbub]

I may choose to do 3 min at 15 ft, or perhaps 5 min at 15 ft, or if it has been bit of a tough dive I may break up the safety stop into 2 x stages, that is, 2 min at 15 ft followed by 3 min at 10 ft. (just a few stop options in my bag of tricks). It's all about using the safety stop to keep the DCS risk value to about 3%.

How does your computer tell you that the DCS risk will be around 3%? Or how do you calculate it. That's a very specific number, and it doesn't really make sense that you can calculate that during a dive.

For me, I have adaptive safety stops enabled and also use SurfGF to provide me information to decide if I need to do an extended safety stop or not.

No messing about trying to decide on what GF to select or changing the GF mid ascent because it is not adequate for the actually experienced dive conditions.

Did anyone actually suggest doing this? Or is this just another of your imaginings. First it was having to do unplanned stops at 21M, and now it's changing GF on the fly. The posters in this thread have been saying to follow the NDL based on how it is set. If NDL is getting low, it's time to head up.

 

[grantmac]

Or you can simply look at SurfGF/GF99 and know in real time whether your stop is
A Necessary
B Effective

A 3% DCS risk sounds worse than you'd get with an M value of 1.0, not sure I'd want to be following OldBentMan's secret recipe.

 

[crofrog]

How does your computer tell you that the DCS risk will be around 3%? Or how do you calculate it. That's a very specific number, and it doesn't really make sense that you can calculate that during a dive.

He can’t. He’s just completely making it up at this point.

 

[crofrog]

A 3% DCS risk sounds worse than you'd get with an M value of 1.0, not sure I'd want to be following OldBentMan's secret recipe.

That is very dependent on the pRT…