Bought a Peregrine

[L13]

... Though the GF Lo only comes into play on deco dives. For No Stop dives, it doesn’t matter...
...
...Yep. I love SurfGF ...

You never know when a problem will make an NDL dive into a deco dive, that is the wrong time to add unnecessary deep stops and deco time.

Totally agree about SurfGF!

 

[Belzelbub]

You never know when a problem will make an NDL dive into a deco dive, that is the wrong time to add unnecessary deep stops and deco time.

True. Good point. It won’t hurt to raise the GFLo on an NDL dive, and could help on an unplanned deco dive.

 

[scubadada]

You never know when a problem will make an NDL dive into a deco dive, that is the wrong time to add unnecessary deep stops and deco time.

Totally agree about SurfGF!

True. Good point. It won’t hurt to raise the GFLo on an NDL dive, and could help on an unplanned deco dive.

Deco should not be an accident. If it is, it should be short.

Run a few profiles, for light deco, the differences between GF lows are trivial.

100 ft for 35 min on 32%, about 5 min over NDL. On 80/95, 4 min at 10 ft. On 45/95, 1 min at 20 ft, 3 min at 10 ft. Both profiles with 4 min of deco.

 

[RIHappyDiver]

Thanks @scubadada! I truly appreciate the additional educating. I’ll be the first to say that my knowledge of algorithms and gradient factors is minimal, at best.

I am planning on using my Geo 2 as a backup, and I think you’ve said in the past that you ran your Teric at 45/95 and achieved a reasonable match with DSAT. Am I correct is assuming that is with the conservative setting turned off on the Oceanic? My plan was to do the same, at least until I became more knowledgeable about GF. What would you recommend as good reading material on the subject?

Erik

 

[scubadada]

I am planning on using my Geo 2 as a backup, and I think you’ve said in the past that you ran your Teric at 45/95 and achieved a reasonable match with DSAT. Am I correct is assuming that is with the conservative setting turned off on the Oceanic? My plan was to do the same, at least until I became more knowledgeable about GF. What would you recommend as good reading material on the subject?

Erik

Hi Erik,

I dived a Geo 2 as backup to my VT3 from 2011 until I switched to the Nitik Q in 2016. The Geo 2 is a fine computer. My son now uses the Geo 2 as his backup.

Yes, a Buhlmann GF high of 95 is a reasonable match for for DSAT. Yes, that is DSAT without conservatism. DSAT tends to be a little more liberal on the 1st dive, especially deeper. Buhlmann tends to be more liberal on repetitive dives, especially, shallower. For no stop dives you simply use the more conservative of the two computers to control the dive, they are generally not much different. Under certain circumstances, the differences can be greater DSAT vs GF high 95, examples

I like Deco for Divers by Mark Powell. I have read it through a couple of times and often refer back to it. Read the short piece on the Shearwater website by Neal Pollock Flexible Control of Decompression Stress - Shearwater Research

 

[L13]

Deco should not be an accident. If it is, it should be short.

Agree!

Run a few profiles, for light deco, the differences between GF lows are trivial
100 ft for 35 min on 32%, about 5 min over NDL. On 80/95, 4 min at 10 ft. On 45/95, 1 min at 20 ft, 3 min at 10 ft. Both profiles with 4 min of deco.

True, for any dives with short deco there will be no extra runtime. However, two stops are more stressful than one, And being 20 ft down is more risky than 10ft down if you suddenly have to abort deco and surface fast due to complications of whatever cause you to go into deco in the first place.

You are right though, the difference really is minimal, especially near NDL.

But mostly it's about not misusing the Buhlmann deco model. Gradient Factors were developed by Baker based on the idea that the Buhlmann model was mostly right, but that the deep stop people were right that deeper stops were needed. This only works out in the underlying Buhlmann model if the a/b factors underweighted the relative sensitivity of the faster tissues. GF low < GF high produces the same effect as modifying the a/b factors in the Buhlmann model to produce lower m-values for faster tissues and generate deeper stops. It is more intuitive than changing a/b factors directly, and has the added advantage of having an intuitive way of adjusting overall conservatism with GF high. But if you accept Bakers modification of the Buhlmann model, then GF low (not GF high) should be used for NDL dives since the faster tissues are the ones driving NDL limits as well (however, no dive computer does this).

The bulk of data that I have found (and the general consensus) seems to indicate that the original deeper stop settings for GF's overstated the sensitivity of faster tissues (in a way that is possibly dangerous for longer decos). At the same time there is some very limited evidence that the Buhlmann model does underestimate the relative sensitivity of faster tissues on the order of 20%, which can be approximated with (GF low)=0.83x(GF high). This, while setting overall conservatism with GF high, is consistent with both Buhlmann's and Baker's models while adapting to the new data.

Obviously, the Buhlmann (and Baker) model isn't a perfect match to actual inert gas effects on diver physiology. There are likely significant improvements that could be made. But arbitrary changing factors without some hypothesis as to why is just abusing the model.

 

[scubadada]

@L13

I dive a GF low of 80. In all of my light deco, I have never had a stop deeper than 10 ft.

For rec divers not venturing into deco, the GF low makes no difference. These divers have no basis for choosing a GF low for the rare occurrence of an accidental foray into deco.

I think you may be information rich and experience poor. Sorry if my analysis is incorrect.

 

[L13]

@L13

I dive a GF low of 80. In all of my light deco, I have never had a stop deeper than 10 ft.

For rec divers not venturing into deco, the GF low makes no difference. These divers have no basis for choosing a GF low for the rare occurrence of an accidental foray into deco.

I think you may be information rich and experience poor. Sorry if my analysis is incorrect.

The example you gave above had stops at 20 ft and 10 ft if you changed GF low to 45%.

I guess you may be right about rec diving and GF low, the difference is small an not worth confusing people over it.

You may even be right about my information/experience ratio as well. You obviously have more dives than me. But I do note that you use a GF low of 80, which aligns well with what I am saying. Why do you use it?

I guess it is just my mindset. It bothers me when people don't use models (and equipment) as designed unless they have a reason (that makes at least some sense). If they have a reason, I want to understand it and adopt it if it is good. If they don't, or if isn't based in facts, I'm irked and respond (possibly inappropriately, when I should just let it go especially if the difference is trivial).

 

[scubadada]

The majority of divers do not have the need to make some of these decisions and do not have the information to make them. It's not necessarily a bad thing, you need to make decisions appropriate to the situation or setting.

 

[Wayward Son]

Finally got to dive it (last year sucked).

8 dives in 8 days totaling 14.2 hours (854 minutes). Shortest was a deeper drift dive at 45 minutes (max depth 91 FSW), longest was 2 hrs 24 minutes

All but 2 were shore dives, max depth of 23 FSW

The deep dive triggered a safety stop & gave me the chance to have to find the ascent rate indicator.

I generally like the Peregrine a lot. The display was clear, crisp & easy to read at depth. Easy to use, find what I wanted at the time.

2 things stood out: It does not turn on when it gets wet. Once dive I booted it up but by the time I made it to the water & got it wet, it had shut itself off & did not automatically turn on. Not a big deal but good to know. I did poke through the settings but if there's a place to change this, I didn't find it.

The other one gripes me a bit. My buddy ran low on gas when I had plenty left but we had planned for this & agreed that we'd ascend together. When my safety stop was done our other 2 buddies were ascending so I stayed at 15 FSW while they completed safety stops.

The Peregrine counted EDT until the end of my safety stop & then stopped counting. I did not drop back below 20 FSW, possibly that might cause it to resume. Don't know. But I was at depth for an additional 5 minutes or so & the EDT did not account for this.

This isn't a deal breaker but my last computer counted all time at depth, including after a safety stop was completed, so it is different for me.