Gradient Factors and recreational diving

[gopbroek]

I am looking at transitioning into a computer that uses Gradient Factors. I have superficially read some data on the theory and have a basic idea of theory. I currently use an Oceanic computer set to DSAT and run it pretty close with EAN. Last week in CZM I did 24 dives (26 hours) with most dives beginning at 110+ while hunting Lionfish. I am comfortable with the DSAT algorithm and would like to transition with as little initial change as possible.

My question is that for repetitive recreational diving: what would the tech folks suggest as the most direct swap over setting from DSAT to GF, for repetitive diving?

This may irritate the true tech divers but as the GF model is transitioning into the rec world this question will probably be asked often, going forward, for the comparisons to the current rec algorithms. With Shearwater becoming the top end gold standard and if Deep 6 can get their exploding goats under control they will own the entry level end, it looks like GF will rapidly be gaining share in the recreational world.

 

[boulderjohn]

If you are comfortable with the DSAT algorithm, I do not see any reason to change.

I use a Shearwater computer, and on recreational dives I do not bother switching to recreational mode. I run it at GFs 50/80/ It puts me at NDL pretty quickly for a first dive, not so quickly for a second. I am not sure if that helps you any.

The DSAT algorithm has not been shown to be a problem for recreational dives. If you are comfortable with it, why change? If, on the other hand, you decide to buy a Shearwater computer (or other similar computer) that does use Buhlmann with GFs, then read their literature very carefully and choose the setting that makes the most sense to you, regardless of what you felt about your previous work with DSAT.

 

[BertStevens]

Here is a great article on DGX that helped me understand better.

https://cdn.divegearexpress.com/pub/media/tektipspdf/Gradient_Factors.pdf

 

[stuartv]

My experience has been that GF90/90 or thereabouts is fairly close to DSAT. Possibly GF95/95, depending on the specific dive.

If you get a Shearwater, I would highly recommend you to use it in OC Rec mode and, if you like DSAT, set it for its Low conservatism setting. It will be a little more conservative than DSAT, but not much.

Until such time as you get tech training for deco diving, I would not recommend playing with custom GF settings. DSAT definitely does not have a direct exact translation to specific GF numbers.

Please don't take that as "you're too dumb to understand GF without sitting through a formal class on it." That is not it at all. But, the why of it is a long explanation and I'm beat and going to bed. If you really want my take on it, PM me and I'll get back to you when I can put a few minutes into it.

 

[Jason]

See this previous discussion...

Computers with non-conservative algorithm.

Basically "low" conservatism (GF 45/95) in Shearwater OC rec mode seems to match up pretty well.

 

[DevonDiver]

One problem is that many 'recreational' models change parameters over repetitive dives. RGBM is a good example of that.

I did some dives with my Shearwater and Suunto Vyper on the same arm. I could change the Shearwater GFhi in-water, so I used that ability to match the RGBM bottom times over several deepish rec dives.

On the first dive, it took GF95 to match the bottom time given by RGBM. On the second dive, I had to drop to GF65 to get a matching bottom time.

Setting a high GFhi for recreational dives makes sense for solitary dives, but do consider dialing it down somewhat for repetitive dives.

For recreational dives, the GFlo should be set very high. If you don't go into deco, then GFlo has no impact. However, if you accidentally go into deco... then you'd want a simplistic ascent and shallow stops. Keep it simple and do 'emergency' deco where your gas will last longest.

 

[stuartv]

If you don't go into deco, then GFlo has no impact.

That completely depends on the computer and how the manufacturer implemented the algorithm.

 

[DevonDiver]

GFlo.. Gradient Factor Low...

What are you talking about Stuart?

Which manufacturer?

I believe only Buhlmann ZHL16 uses GF hi/lo. It's an open source algorithm.

GFlo is the user defined setting that dictates the tissue tension (gradient) where the 1st stop will occur... and thereafter defines the gradient line up to GFhi... which is the user defined maximum tissue tension upon surfacing.

On a no-stop dive.... it makes absolutely no difference where you set your first stop. I thought that'd be quite logical... I was wrong, obviously.

The GFhi (user) defines maximum permissible surfacing tissue tension. For recreational diving, it's basically a conservatism setting.... GF70 is basically 30% conservativism down from the M-value of the controlling tissue compartment.

 

[Nasser]

@OP,

The Petrel / Perdix in Rec Mode has three settings with fixed GF settings that cannot be changed like they can in Tec mode.

Low 45/95
Med 40/85
High 35/75

I like using Rec Mode for NDL dives. I just set it and forget it depending on the dive conditions, the layout is clean, and the countdown timer is a nice feature on safety stops. But make sure you also understand the effect of having gasses switched or off in this mode as it can affect your NDL / deco times.

 

[stuartv]

GFlo.. Gradient Factor Low...

What are you talking about Stuart?

Which manufacturer?

I believe only Buhlmann ZHL16 uses GF hi/lo. It's an open source algorithm.

GFlo is the user defined setting that dictates the tissue tension (gradient) where the 1st stop will occur... and thereafter defines the gradient line up to GFhi... which is the user defined maximum tissue tension upon surfacing.

On a no-stop dive.... it makes absolutely no difference where you set your first stop. I thought that'd be quite logical... I was wrong, obviously.

Yep. I thought the same thing until I experimented with some different implementations and actually read through Erik Baker's original Fortran code that implemented Gradient Factors.

There are extensive discussions on this topic, here on SB, if you want to read up on it.

The short answer is: Some implementations (of Buhlmann with Gradient Factors) work as you said - they do not look at GF Lo until it is determined that a direct ascent to the surface would result in exceeding GF Hi. At that point, the diver is "in deco" and GF Lo is used to determine the first stop.

But, other implementations work differently. For those, if a direct ascent would exceed GF Lo, regardless of whether the ascent would exceed GF Hi at the surface, then that implementation says the diver is "in deco" and prescribes one or more stops.

It's a niche problem. For any given dive, there would be a small window of time (if any - it just depends on the specific profile) where a direct ascent would result in exceeding GF Lo along the way but not exceeding GF Hi at the surface.

No matter which way it is implemented, if you have been down long enough that you would exceed GF Hi on a direct ascent, the results should be approximately the same. And if you have not been down long enough to exceed GF Lo or GF Hi at any point during your ascent, then the dive is an NDL dive by all accounts.

As I said, if you want info on which specific implementations work which way, you can search around here for it.