Gradient Factors For a Rookie

[dflaher]

Hi,

I am a recreational diver with close to 300 dives. I recently purchased a Shearwater Peregrine mainly for the reasons listed here.

Now that I have it I am wondering how I should choose which GF to use. The Peregrine comes with three conservatism presets: Low (45/95), Med (40/85), and High (35/75) for GF Hi and Low, as well as custom ones. I have never gone into deco before, based on my previous computer, which ran a "Modified Haldanean Algorithm" with 12 compartments based on DSAT. I am in petty good shape for my age, but I am in my mid-60s (a Beatles song from Sergeant Pepper hits the nail on the head)

Also I have become quite interested in decompression theory lately and am currently reading Powell's Deco for Divers and have read Erik Baker's 2 articles about M-Values and Deep Stops. While I conceptually understand most of it, I still feel like I'm drinking through a fire hose while it slowly sinks into my slow tissue compartments.

Of relevance also is I really have no interest in decompression diving; I am predominately a vacation diver that may do a land based trip and a liveaboard once a year or so. I'm AOW and NX certified.

So...back to my question. @rongoodman states in post #4 of this thread:

I've been diving Shearwaters for years and for recreational dives, just set the conservatism to Medium(45/85) and forget about it.

Is that pretty much it? Or should I do something else? I guess based on how I am feeling before/after a particular dive I could always adjust it up or down?

Thanks. After you have indulged me here I may hit you up for SurfGF and GF99 questions.

 

[brsnow]

Shearwater has some great videos on YouTube that are very interesting on the topic

 

[Belzelbub]

Is that pretty much it? Or should I do something else? I guess based on how I am feeling before/after a particular dive I could always adjust it up or down?

That’s pretty much it. And the adjustment for how you are feeling is pretty spot on. I’d also add that you may adjust to more conservative if your typical buddy uses a more conservative computer, though not completely necessary. If they aren’t also diving a Buhlmann, they won’t exactly match.

Thanks. After you have indulged me here I may hit you up for SurfGF and GF99 questions.

SurfGF is great. I keep SurfGF on my main screen. It is kind of a what if tool as it displays your current tissue loading if you were to be instantly be transported to the surface. I use it to gauge the saftety if I were to need to get to the surface ASAP.

 

[boulderjohn]

First of all, you are doing some reading that is intended for decompression diving, but you are planning to apply it to NDL diving. There is more of a difference than you may think. Furthermore, your reading does not reflect the latest thinking in regard to deep stops. Mark Powell, for example, no longer believes what he wrote in Deco for Divers about deep stops. See this article for more recent information on this.

But as I just said, those points do not apply to NDL diving. The first gradient factor determines the depth of your first stop, but in NDL diving, there is no first stop, so it doesn't matter. For NDL diving, you only need to pay attention to the second GF. I think you will find that a lot of people will choose the middle course (85) because they fear 95 is too close to the edge and 75 cuts their dive too short.

Some people will argue in favor of doing deep stops for NDL dives, but there is no good research to support that. A couple studies nearly 2 decades ago seemed to favor them, but those studies are not highly regarded today. I have studied the issue of deep stops in NDL diving to a fairly large degree, and I can say with great certainty that I have no idea if they do any good or any harm. I don't do them myself.

 

[boulderjohn]

After you have indulged me here I may hit you up for SurfGF and GF99 questions.

I wanted to make this point separately.

I love SurfGF, and it is what I pay attention to the most. I use it instead of the standard safety stop procedure.

I typically have a GF high of 85 on a recreational, NDL dive. When my ascent brings me to typical safety stop depth, my attention goes to SurfGF. I hang at that depth until I see a number I like. It is often 70-75. That means I often do not feel the need for a safety stop. If I reach safety stop depth with a SurfGF of, say, 50, the only reason I am going to stay there is because I have buddies who did the same dive I did but are obediently waiting for their safety stop countdown to reach zero. This past November I did one dive where the SurfGF peaked at 19 during the dive, and I stayed with everyone else on their safety stop only because I didn't want to start some sort of a ruckus by heading up to the boat while they had their eyes glued to their computer's safety stop countdown. (5...4...3....2..1..0! We can ascend! We are safe now!)

 

[EFX]

I'm a few years older than you and dive with 70/90 on my Perdix dive computer. Most of my dives are NDL but occasionally I do some light deco (less than 5 minutes). Generally speaking, NDL diving can tolerate a higher GFHi than deco dives. Everyone is different. There are many more factors that affect how you feel like hydration, amount of sleep, exercise, age, BMI, etc. As a starting point the medium setting is good. Go from there.

 

[dflaher]

The first gradient factor determines the depth of your first stop, but in NDL diving, there is no first stop, so it doesn't matter.

Thanks John. I meant to include in my OP that I'm aware GF Low is irrelevant for no-stop diving.

SurfGF is great. I keep SurfGF on my main screen. It is kind of a what if tool as it displays your current tissue loading if you were to be instantly be transported to the surface.

This idea of instant transportation to the surface has me confused. Does this mean like in a CESA scenario, OR, does the SurfGF assume a slow ascent (10m/min) and safety stop?

Also, I believe the number has some representation of residual nitrogen embedded in it, correct? So the lower the number, the more one is ready for the next dive?

This past November I did one dive where the SurfGF peaked at 19 during the dive, and I stayed with everyone else on their safety stop only because I didn't want to start some sort of a ruckus by heading up to the boat

LOL. I can see me blowing through a safety stop and telling the DM "My SurfGF was within tolerance"

 

[Brett Hatch]

Thanks John. I meant to include in my OP that I'm aware GF Low is irrelevant for no-stop diving.

This idea of instant transportation to the surface has me confused. Does this mean like in a CESA scenario, OR, does the SurfGF assume a slow ascent (10m/min) and safety stop?

Also, I believe the number has some representation of residual nitrogen embedded in it, correct? So the lower the number, the more one is ready for the next dive?


LOL. I can see me blowing through a safety stop and telling the DM "My SurfGF was within tolerance"

"Surfer Girlfriend" is the gradient factor expected if you were to instantaneously arrive at the surface. So any real-life ascent profile will be slower (and safer) than that. Shearwater has a good article on SurfGF on their website, you may want to have a look at it.

If I read John correctly, whenever his SurfGF number is sufficiently low, he does not really worry about his ascent profile, or any additional stops. I think that's a reasonable way to interpret SurfGF, and while I don't dive that way today, I might give it a shot. Like you say, blowing the safety stop is safe in some circumstances, but it may be (probably is) against the policy of your dive operator, so make sure to follow whatever rules they have.

 

[PfcAJ]

First of all, you are doing some reading that is intended for decompression diving, but you are planning to apply it to NDL diving. There is more of a difference than you may think. Furthermore, your reading does not reflect the latest thinking in regard to deep stops. Mark Powell, for example, no longer believes what he wrote in Deco for Divers about deep stops. See this article for more recent information on this.

But as I just said, those points do not apply to NDL diving. The first gradient factor determines the depth of your first stop, but in NDL diving, there is no first stop, so it doesn't matter. For NDL diving, you only need to pay attention to the second GF. I think you will find that a lot of people will choose the middle course (85) because they fear 95 is too close to the edge and 75 cuts their dive too short.

Some people will argue in favor of doing deep stops for NDL dives, but there is no good research to support that. A couple studies nearly 2 decades ago seemed to favor them, but those studies are not highly regarded today. I have studied the issue of deep stops in NDL diving to a fairly large degree, and I can say with great certainty that I have no idea if they do any good one way or the other. I don't do them myself.

I’m not sure I agree with the idea that gf low doesn’t matter.

If you manipulate your GF low to be low enough, you’ll get a stop where you otherwise wouldn’t have one.

 

[scubadada]

Hi @dflaher

Like @EFX most of my dives are no stop, but about 5% are light deco. I dive an Oceanic VT3 running DSAT and a Shearwater Teric at 80/95. I have SurfGF on my home screen and use it to determine my ascent from a no stop or light deco dive. In my last 300 dives, my average surfacing GF has been 54, with a maximum of 80. So, this is like running a GF high of 80, pretty conservative. My set point of a GF high of 95 allows me bottom time without going into deco and is a good match for my Oceanic computer.

A GF high of 95 is a pretty good match for DSAT, though Buhlmann is a bit more conservative on 1st dive and a bit more liberal on repetitive dives. A GF high of 85 is a reasonable match for PZ+, with the same caveat as for DSAT. For no stop dives, I dive the more conservative of the two computers. For light deco, I clear deco for one or both computers.

Yes, your surfacing GF will always be a little lower than SurfGF because you did not make an instantaneous ascent. At least, it will be the final ascent from safety stop or last deco stop. I make final ascent over at least a minute, the GF (GF99) really increases over the final ascent. Take a look at the GF graph of one of your dives on the Shearwater Cloud. Regardless of my Surf GF, I do a 3 min safety stop on no stop dives, partly old habit, what can it hurt, sometimes I dive with others and don't want to set a bad example.

Glad you are interested in what you are doing.