Gradiant Factors

[dunny]

Hi all,

What are gradiant factors, I've got a trimix dive coming up next week and i'm just not comfortable using the defaults unless I understand what they mean. They are not covered in the SMG course, something that I find abit silly since nearly all decompression program use them to determine the saftey factor.

So if anyone could give me a brief explanation of what they are or point me to some decent literature that isn't meant for someone with a Physics PHD i'd appreciate it.

Also what are accepted as the "normal" defaults that they should be set too.

I realise that the low % GF affects your deep stops (i.e. starts them earlier, so slow tisssues can off has but fast tissues will still be on gassing, increasing the runtime) and the high % GF pads out your shallower stops. But thats about as far as my understanding goes.

Thanks
Phil

[IainC]

I realise that the low % GF affects your deep stops (i.e. starts them earlier, so slow tisssues can off has but fast tissues will still be on gassing, increasing the runtime) and the high % GF pads out your shallower stops. But thats about as far as my understanding goes.

That's pretty much all you need from a practical point of view..

I'm guessing you're relatively new to this?

Which deco programme(s) did you use on your course? For your early mix dives, using the stuff you learned on with the settings recommended by your instructor / experienced buddies should suffice.

Once you have done a few dives and are happy, then experiment with other software or different settings and compare the results it gives you to the dives you've done already, then you'll get a feel for whether they are agressive/conservative or just different..

Bascially gf's are a fudge on the buhlmann algorithm.

To get an idea of what they are doing turn them off and look at the 'raw' buhlmann plan for a dive, (on my dives I have used this as my "aagh, I want out of the water ASAP! hopefully not bent.." plan), then add some and see how you get deeper and longer stops..

Different people use widely different gfs, but not everyone uses them, VPM dive planners use a completely different model, a proplanner which i have used happily for some time just has a 'safety factor' which is more of an 'assume the dive is deeper and longer than it really is' approach..

HTH

Iain.

[IainC]

More technical explanation..

Buhlmann zhl16 models (estimates/guesses :) ) the degree of saturation of different tissues i nthe body.

When you ascend, it models the amount of supersaturation (call it overpressure) of these tissues. For each tissue there is a maximum safe overpressure.

The depth at which any tissue (the 'leading tissue') reaches this limit is your decompression ceiling - the first deco stop will be here (or a bit deeper as we tend to round up to 3m for simplicity).

At this stop you calculate how long it will be before the ceilng has 'moved up' to the next stop (normally 3m shallower) and as soon as it is safe, move up and repeat until it is safe to surface..

GF's work like this. If buhlmann says the maximum safe overpressure is X, then I will add some extra safety margin and start my decompression stops when the overpressure is, say 50% of X, This is the LOW GF. The lower it is, the deeper stops start..

Then we come to how long the stops are, buhlmann will move you up to the next stop as soon as you can (i.e. when the overpressure at the next level would be <100%). The HIGH GF reduces this, so you wait at a deco stop until the overpressure is only say 75% of X.. The lower the HIGH GF is, the logner each stop will be.

HIGH GF must be > LOW GF or you would never be able to surface!

That's may rough explanation, I'm sure someone will correct me soon..

Remember they are all just tools and estimates and can go wrong.


Iain.

[dunny]

Ok well based on what you've just said would:

Low % GF: 30%
High % GF: 80%

seem reasonable? Maybe the low one needs to increase abit.

On my course we used V-Planner which was just a safety factor 1-5 (I think) but i haven't brought myself to forking out the £45 for a license just yet!

I could use proplanner or decoplanner atm. I went for decoplanner as proplanner takes ages to enter the dive plan!

Cheers
Phil

[Nigel Hewitt]

My take... (http://www.nigelhewitt.co.uk/diving/rant/index.html#gf)

[IainC]

Read Nigel's stuff, he makes sense...

For my limited number of mix dives I have used proplanner with 10% safety and 100% microbubble (becsasue I am too lazy to work out pyle stops by hand) and I've been happy.

If I were to use a gf based dive planner I would probably pick some values that gave me a similar profile when I compared it..

Then I'd have a raw no mirobubble/no safety % plan some where just in case..

Iain.

[Woz]

You can always do Pyle stops on conjunction with your computer (and save yourself the extra money in the DS version). It's dead easy.

Add max depth to "ceiling" depth. Divide by 2. That's your deep stop depth (doesn't have to be dead accurate).
Repeat until the difference between your stop depth and "ceiling" depth is less than 9m then ascend to your ceiling.
Suuntos will penalise you for this making you stop shallow for longer. This can be mitigated a little by blowing off the 3 min safety stop (which is all the DS version does when you set the deep stops).

[IainC]

You can always do Pyle stops on conjunction with your computer (and save yourself the extra money in the DS version). It's dead easy.

My Vytec DS is in gauge mode for this sort of diving anyway ;)
No hassles with 'suunto minutes'

[johnkendall]

Ok well based on what you've just said would:

Low % GF: 30%
High % GF: 80%

seem reasonable? Maybe the low one needs to increase abit.

On my course we used V-Planner which was just a safety factor 1-5 (I think) but i haven't brought myself to forking out the £45 for a license just yet!

I could use proplanner or decoplanner atm. I went for decoplanner as proplanner takes ages to enter the dive plan!

Cheers
Phil

30/80 is a reasonable place to start.

IainC's explanation is not quite accurate.

GF low controls the start of the ascent, while GF high controls the end of the Ascent, so using 30/80 at the start of the ascent you are able to get to 30% of the maximum overpressure, and at the end of the ascent you are able to get to 80% of the maximum overpressure. So in the middle of the ascent you will be around 50% of overpressure.

If you have decoplanner, then read the help file, it includes quite a bit about deco theory and how it works.

HTH

John

[dunny]

Thanks for the replies people. I atleast don't feel like a guinea pig just leaving it to the defaults anymore.

Cheers
Phil

[tony J]

I realise that the low % GF affects your deep stops (i.e. starts them earlier, so slow tisssues can off has but fast tissues will still be on gassing, increasing the runtime) and the high % GF pads out your shallower stops. But thats about as far as my understanding goes.

Thanks
Phil

I would not quite agree with this summary as it may be misread by others.

"I realise that the low % GF affects your deep stops (i.e. starts them earlier, so slow tisssues can off has(sic)"

whether you did a deep stop or not the tissue would off gas, in fact it would off gas faster without the deep stop - and that is what you are trying to reduce high overpressures.


My take is that you do reduce any nitrogen load in any tissue by deep stops.
All you are doing is modifying the rate of off gassing, by reducing the overpressure.
I understand it as to not produce microbubbles early in the ascent (or get them filtered out in the lungs) so that they are not hanging around when you are shallow when they are bigger and harder to remove (the normal reason to pad shallow stops / get skin bends / bend within the tables)

I'm also not convinced they (deep stops) much difference on air dives, but I still run these profiles.
I have never felt better, diving nitrox, doing deep stops etc although others have reported affects.

Tony

[Sanemancured]

And thanks for a link into your wisdom Nigel, appreciated and I'll have a deeper gander later.

[MattS]

I would not quite agree with this summary as it may be misread by others.

"I realise that the low % GF affects your deep stops (i.e. starts them earlier, so slow tisssues can off has(sic)"

whether you did a deep stop or not the tissue would off gas, in fact it would off gas faster without the deep stop - and that is what you are trying to reduce high overpressures.During a deep stop....and any other decompression stop for that matter;
1. Supersaturated tissues off-gas toward saturation at the ambient pressure of the stop depth.
2. Tissues at saturation at the stop depth neither on-gas nor off-gas
3. Tissues below saturation on-gas toward saturation at the stop depth

In a sentence...FASTER tissues off-gas and SLOWER tissues on-gas.

My take is that you do [not] reduce any nitrogen load in any tissue by deep stops.Absolutely right. With a deep stop, the total volume of inert across all-tissues will be greater than the same ascent without the deep stop. The deep stop proposition is that the total volume of inert is not so important as where the inert happens to be in the tissue spectrum.

All you are doing is modifying the rate of off gassing, by reducing the overpressure.That is one explanation and not one I subscribe to particularly. It is perhaps the best explanation of GFs but I don't subscribe to those either. Baker's GFs frig dissolved gas co-efficients to produce deep stops. The profiles that GFs produce are commonly criticised for having deep stops which are too shallow and shallow stops which are too long - leading some divers to increase the hi GF above 100% in an attempt to get a profile which 'looks' right. The compelling explanations for deep stops are all based on gas phase (bubbles) and I struggle to see how fiddling with dissolved gas parameters is particularly useful.

I understand it as to not produce microbubbles early in the ascent (or get them filtered out in the lungs) so that they are not hanging around when you are shallow when they are bigger and harder to remove (the normal reason to pad shallow stops / get skin bends / bend within the tables)Personally I subscribe to the coke bottle demonstration. If you partly unscrew the cap of a coke bottle the pressure releases gradually, similar to making an ascent. You can see the bubbles coming out of solution and forming a froth. Within the froth small bubbles join together to make larger bubbles (coalescence) and the height of the froth layer continues to grow. A deep stop is like screwing the cap back on the coke bottle, bubbles stop coming out of solution so quickly and the froth subsides.

So I prefer Pyle stops to GFs. My initial ascent is made as close to 10m/min as I can get, to produce the significant gradient required to drive dissolved gas out of solution. At 1/2 max depth(ish) I stop for 2 minutes, to allow my blood stream time to catch up. Repeat with the halfing until 12m and continue in 3m steps completeing the required stops. It works for me as they say.

I'm also not convinced they (deep stops) much difference on air dives, but I still run these profiles.
I have never felt better, diving nitrox, doing deep stops etc although others have reported affects.If JP Imbert (http://archive.rubicon-foundation.org/4662) is to be beleived (and I think he should be), deep stops are specifically useful for extended range scuba diving, whether breathing air, nitrox or helium bottom mixes. The hypothesis being based on the extended bottom segment resulting in a high volume of inert in intermediate tissues (toward the faster end of the spectrum). According to the hypothesis, these tissues in particular are prone to bubbling and the resulting 'froth' may overwhelm the lungs causing some smaller bubbles to shunt past the lungs into the arterial blood supply.

I did quite a bit of personal experimentation in the late 90s which started with feeling completely crap after 45+ minute bottom times on 30m+ dives breathing air and decompressing on traditional tables. Deep stops were one of the first things I introduced into these profiles and the difference they made was very noticable. Next came Nitrox, which improved things on shorter dives but the feeling like crap returned around the same time a 3Ltr pony (of 50%) became insufficient to decompress. I moved on to using higher FO2s (75%+) for decompression and feel great even after diving for 1hr at 50m breathing air.