Gradient Factor

Simon Ciantar

Hi guys,

I'm posting this tread here so nobody will be in any doubt that I'm very new to all this, and hopefully everyone will be a bit forgiving! So, what exactly are gradient factors and how are these relevant when planning one's deco profile ?

Simon

decoweenie

You might not like my reply below...

If you are not already decompression-trained diver, then you should seek instructions first.

If you already are, then you should try to read this paper first. It explains about how GF is derived and what to use them for.

Once you have done all of those homework, then read any abbreviated explanation that anyone might add below my post.

__________________
"...after a while you get bored offering advice to a bull that like to keep butting the fence with its head rather than walking through the open gate..."

- Rebreather World PM

nigelh

My take.

__________________
nigelh

Simon Ciantar

Since I haven't filled out my profile yet, I will just note that I am TDI Nitrox , TDI Trimix , TDI CCR Diver,and CMAS two star instructor since 1994 I have been diving for three years with Trimix OC to 60m + dives.

I have always used proplanner to cut my tables and in my training no mention was ever made of gradient factors. Now from the threads here, I get the understanding that this is some sort of variable which you can manually adjust in some types of Deco software, am I right? If so I just would like some info on it. Thanks for link, and for your help.

Simon

SimonK

For enough reading to keep you going for a year or two see: ftp://ftp.decompression.org/pub/

Simon

decoweenie

Simon,

Your profile doesn't state what level of diving you do, so I hope it is understandable for what I said earlier...

You would be surprised, but I personally know one trimix instructor who has no idea about decompression. He can't explain the difference between VPM, RGBM and GF for example.

So having a card says nothing to me, but the least we could do is we shouldn't be giving deco information for someone who doesn't have the "training". Right ?

Anyway, enough links have been given for you to do your own reading. But basically in layman's terms, GF is intended to add some conservatism to the straight Buhlmann algorithm by allowing you to adjust where you want to start your deep stops, and how much you want to stretch your shallow stops.

GF-low dictates the deep stops, the smaller the value (i.e. 10, 20) will start the first stop deeper than what you usually experienced with Proplanner.

GF-high dictates the shallow stops, the smaller the value (i.e. 85, 70) will lengthen the last few stops.

So GF of 10/90 will start the stops quite early, and you will be making 1-min stop every 3m until your last stop (3 or 6m) when you will stay just a bit longer than normal Buhlmann.

By setting the GF to 100/100, you will the straight Buhlmann algorithm with NO conservatism.

The conservatism used in Proplanner is DSF (deep safety factor) which modified your deepest depth by a percentage fraction in the algorithm to load the theoretical tissues a bit more than actual.

__________________
"...after a while you get bored offering advice to a bull that like to keep butting the fence with its head rather than walking through the open gate..."

- Rebreather World PM

RobPot

Two recommendations, if I may:

1. It is always good to get trained properly, because then you will have an instructor you can ask these questions of.

2. Google (or whichever your preferred search engine is) works for most peoples' initial research, and makes it easy to do some homework beforehand.

Just my PO2's worth.

ROB DAVIE

Folks,

I would like to publicly complement our very own Decoweenie (Phi) for that excellent explanation of Gradient Factors. It is clear, concise, and to the point! Well done, sir, well done!

__________________
[SIZE=2]"CC Rebreathers will become a viable part of tech diving [U]WHEN PIGS FLY[/U]!!"--GI3[/SIZE]

UWSojourner

This thread might also be helpful.

http://www.rebreatherworld.com/techn...planation.html

01RMB

Without getting into the detail of decompression theory the following is a simple guide to the effect of gradient factors.

Gradient factors are a way of adapting the Buhlmann model to incorporate deep stops and create more conservative profiles or even be more aggressive than the standard Buhlmann model.

The best way to think about decompression is in terms of two depths. Below the deeper depth you are on gassing so no decompression is taking place. Above the shallower depth you theoretically get bent. Between the two depths you are decompressing. The closer to the shallower depth the stops are done, the faster the decompression.

The traditional decompression thinking is to get to the shallowest depth possible as fast as possible and move closer to the surface without exceeding the minimum depth that would allow a bend to occur.

The Buhlmann decompression model sets the decompression depth by the minimum allowable pressure for all 16 tissue compartments to avoid, or more correctly, to minimise the chances of a bend. Gradient factors are used to adjust this model.

The low gradient factor works by forcing a stop during the ascent when you reach a point that is a factor above the ambient or minimum decompression line. The lower the GF low value the deeper the stops.

The high gradient factor determines the maximum allowable surfacing pressure. The lower the GF high value the longer the decompression. A high GF of 0 means that you theoretically will never surface.

So a decompression profile with GF 15/85 (low/high) starts the decompression at 15% above the minimum off gassing level and finishes the decompression at 85% of the maximum decompression surfacing level. The standard Buhlmann base line and theoretically the shortest and most aggressive deco is GF 100/100.

To illustrate with a simplistic picture for one tissue compartment the following shows the effect of Gradient Factors on decompression.


Some divers are running 100% + profiles like 10/120. Theoretically these profiles exceed the Buhlmann limits but the use of slow ascents and deeper stops make up for the very aggressive 120% high gradient factor.

APD recommend in the Vision manual.

It is essential to apply Gradient factors to modify the ascent depending on gas mix used, bottom time and diving depth.
The following table represents gradient factors in common use for a variety of dives.


It is clear that Bühlmann models work in the air diving range and result in a low DCS incidence. (Note: the word “low” is used as opposed to “zero”!) Between 40m and 100m there are no validated decompression tables for Trimix and the % of dives that result in DCS is unknown.

__________________
Cheers
Rod