Gradient Factors and Deep Stops

[Miyaru]

Imo you need a bunch of dives to figure out what works. A succesful dive at a certain gradient factor really doesn’t mean anything. Also, an unsuccessful decompression at a certain gf doesn’t mean anything.

Additionally, a particular gradient factor isn’t iso-risk across all decompression times.

The plural of anecdote isn’t data.

I totally agree! It was just a very short description of the concept of personalizing GF.

Classes are not supposed to be doing aggressive dives. Outside of class the team decides what profile to run. And for that matter, how they will dive.

So you're saying 20/85 is not aggressive?

 

[PfcAJ]

I totally agree! It was just a very short description of the concept of personalizing GF.

So you're saying 20/85 is not aggressive?

I think for gue classes 20/85 has a really good record. There’s probably little motivation to move to an “unknown”. On those short BTs I think it doesn’t matter a whole heck of a lot from a practical standpoint. You can fiddle with the GF low setting and see how little impact it has.

On longer and deeper dives the differences are more pronounced. Last dive I was doin (260 for 45min) I ran 65/85 fwiw. On something like that the difference distinct.

 

[KenGordon]

In addition to John's post on a gradient factor of 20/85:

It's a personal setting, one you have to determine for yourself.
Dr. Neil Pollock explains GF in this video and this article.

The goal is:

• to have a short decompression time
• to use as little decompression gas as possible
• to prevent DCS

20/85 is considered rather aggressive when you start to determine your own GF. A safer starting point is 30/70 (now default for Shearwater), increase in steps of maximum 5% after evaluating how you feel after a dive. The slightest itch, pain or other discomfort is likely an indication that you just passed your own safe factor.
What works for somebody else, is not necessarily the best GF setting for you.

I think your goals are in the wrong order. You also left off the one about not drowning.

I am also of the opinion that ‘suck it and see’ is not an applicable technique for determining GF. Should someone who has a rapid ascent, skipping 30 minutes of deco and gets away with it assume that in all future dives that deco would be unnecessary?

 

[Freewillow]

I am not a tech diver, but I go, at times to an air or nitrox deco dive with a maximum of 15 minutes of time to surface.

I am using an OSTC with Buhlmann with GF set on 80/80. On these settings I realise that I have the most "conservative settings" from other divers with Simple Buhlmann algorythms. I am always the first one in deco and have slightly longer TTS.

I have used this setting for over 100 dives. Should I have died, because of my GF Settings.

 

[Miyaru]

I think your goals are in the wrong order. You also left off the one about not drowning.

I am also of the opinion that ‘suck it and see’ is not an applicable technique for determining GF. Should someone who has a rapid ascent, skipping 30 minutes of deco and gets away with it assume that in all future dives that deco would be unnecessary?

The goals I summed up were in a random order. Surfacing without DCS is of course the most important one, which also implies not drowning.

My point is that I think it's better to start from a conservative setting, changing factors gradually. One change at a time, preferably over several dives. Conditions can vary, e.g. water temperature and currents, so yes it takes several dives to be able to compare them.

But feel free to elaborate on your approach in changing GF.

I am not a tech diver, but I go, at times to an air or nitrox deco dive with a maximum of 15 minutes of time to surface.

I am using an OSTC with Buhlmann with GF set on 80/80. On these settings I realise that I have the most "conservative settings" from other divers with Simple Buhlmann algorythms. I am always the first one in deco and have slightly longer TTS.

I have used this setting for over 100 dives. Should I have died, because of my GF Settings.

There are probably divers out there who can dive GF100/100. I've been diving with friends who use a GF high of 80 and they never had the slightest symptom. Myself, I can notice the difference between 75 and 80 after a deep trimix dive, so I stick to 75, which keeps me longer in the water during the last stops.
__________

My point is that a GF setting is personal. What works for you, may not work for someone else. Plain Bühlmann (100/100) didn't work for everybody. If it did, gradient factors would never have been introduced. Everybody who has done a bunch of dives with deco obligations, has personal experience on how they feel after a dive with the settings they chose.
Now try to take that experience out of the equation. A rec diver without personal experience in finishing a deco obligation. The GF low factor won't be much of an issue for the initial (course-)dives, but what will you advise as a good starting point for GF high?

One possible answer is because the course book says so without further discussion.

If you can motivate your GF choice, even better. Many here have done so, based on experience. If you lack the experience, a conservative approach has my preference. Dr. Pollock did some research into this and advised 30/70 as a starting point. DAN and Shearwater follow this advise, GUE sets 20/85 as a starting point.

I could probably surface with a GF high of 85 on a 40m/130ft dive with 15 minutes deco obligation, without any issues. I won't be able to do the same on a 65m/213ft dive with 30 minutes bottom time. I've never seen the inside of a recompression chamber and I intend to keep it that way.

 

[leadduck]

There's a common misunderstanding about the meaning of GF100/100. Plain ZHL16A was not Bühlmann's recommendation for a safe ascent, but it was rather meant to describe the hard limit beyond which DCS is very likely even under benign conditions. Bühlmann ran experiments exceeding the ZHL16A supersaturation limits of slow compartments by only 5% and had half of the participants complain about joint pain. He expected that exceeding the fast compartment limits by the same small amount will cause severe neural DCS. Bühlmann always recommended adding safety margins when cutting tables. The tables created by Hahn (Deco92, Deco2000) are about GF70/90, with application rules for a larger safety margin in the case of cold water or exertion at depth. Diving by tables was conservative anyway due to the worst-case assumption of a square profile at max.depth and rounding to the next larger depth and bottom time.

So contrary to what some believe, GF100/100 has never been popular, and I'd rather assume that diving by Bühlmann tables 30 years ago in most cases was closer to GF60/80. Looks like we've come full circle.

 

[Freewillow]

@Miyaru. You are from Europe, so I may tell you that the french speaking divers consider low GF down to 30 if using Helium in the mix and much higher GF if the leading gas is Nitrogen.

 

[UWSojourner]

Now... ? Today the focus is on "low stress" deco with an abundance of over inflated shallow time. Naturally all that add on safety time, has a great outcome. That is of course, the basis of all deco planning: the speed vs time compromise.

That does not diminish the success of VPM-B and deeper stops in tech diving ...

However, this latest trend of "low stress" deco, is very late to the game, and it remains to be seen if it has any actual value. Right now its a theoretical improvement only - the "go slow" approach, but were we going too fast before hand? I don't think so.

Your post, Ross, is a complete misrepresentation of the implication of the NEDU study. NEDU's study did not say to do "low stress deco". What it implies is that deep stops are not "efficient".

So suppose a diver wants to spend the SAME time decompressing as a profile generated by, let's say, VPM-B+3. The NEDU study leads us to conclude that a BETTER profile using the SAME time can be obtained by spreading the deep stops toward shallower depths. As we've shown over and over, the GF method in almost any configuration is a pretty good way to do that.

The NEDU study did not focus on "low stress", but "lower stress for the same decompression time". And who doesn't want that? If I can spend the same time decompressing, but do it in a way that lowers my decompression stress, then that's just smarter.

 

[scubadada]

There's a common misunderstanding about the meaning of GF100/100. Plain ZHL16A was not Bühlmann's recommendation for a safe ascent, but it was rather meant to describe the hard limit beyond which DCS is very likely even under benign conditions. Bühlmann ran experiments exceeding the ZHL16A supersaturation limits of slow compartments by only 5% and had half of the participants complain about joint pain. He expected that exceeding the fast compartment limits by the same small amount will cause severe neural DCS. Bühlmann always recommended adding safety margins when cutting tables. The tables created by Hahn (Deco92, Deco2000) are about GF70/90, with application rules for a larger safety margin in the case of cold water or exertion at depth. Diving by tables was conservative anyway due to the worst-case assumption of a square profile at max.depth and rounding to the next larger depth and bottom time.

So contrary to what some believe, GF100/100 has never been popular, and I'd rather assume that diving by Bühlmann tables 30 years ago in most cases was closer to GF60/80. Looks like we've come full circle.

Thanks @leadduck

How do your comments relate to Buhlmann ZH-L16B, and C? It's my impression that ZH-L16A was thought to be too liberal in the middle compartments for common use. M-values were reduced in several compartments for ZH-L16B, and it was suggested for table calculations. M-values were further reduced in many compartments for ZH-L16C, and it was suggested for use in dive computers. Seems like most, but not all, dive computers run C. The most popular computer running Buhlmann with GF has a recreational preset with a GF hi of 95.

Thanks in advance for your help in understanding this topic.

 

[KenGordon]

But feel free to elaborate on your approach in changing GF.

Initially I dived the defaults. When that left me still in the water when my buddy on a Zoop was done I moved to a more aggressive setting which still did not bend my Helo2. Then I backed off a bit. I have done and seen done a lot of dives with those profiles. People are usually (but not always) ok.

Part of the training I got is to go through a bunch of plans plus or minus 5 GF. It becomes apparent that not much is to be gained by pushing GF on a regular ascent.

I believe that these numbers put me on a probability to get hurt. Since people get away with skipping deco I conclude that being slightly bent may not happen on the way to properly bent, so I don’t believe I will detect being slightly bent to know when to stop.