Gradient Factors

[Skeletor]

Now that deco theory has gained a section all of its very own, could someone out there please explain what the hell gradient factors are really all about.

Several people have explained this to me in the past, but all seem to have slightly differeing interpretations.

My understanding is that they are the upper and lower bounds of a decompression profile, and effectively help to determine where a stop is conducted and for how long. That is to say, that bringing the gradient factors together means that you are closer to the ideal, continuous decompression profile, but the stops become more frequent and shorter as a consequence. But the reason I am asking the question is that I have a strong suspision that I might be talking utter crap.

Using a practical example, I currently use DDPlan to cut tables using the default gradient factors of, IAIR, 30 and 70. What practical effect does altering these factors have on the stop times and depths ? And what are the consequences of altering each one individually ?

Any light shedding gratefully recieved.

Cheers,

John

[Mark Chase]

Gradient factors are a way of adapting Bhulman tables to incorporate deep stops and genital profiles or even modifying them to be even more aggressive than standard Bhulman models.

The shortest deco most aggressive deco is theoretically 100/100GF. This will get you shallow fast and will use the pressure imbalance of the expanding gas in your body to force the nitrogen out fast. If you follow the modern thinking this profile will have you bent as a 9 bob note in the water but will fix it on the shallow stops.

Some divers are running 100% + profiles like 50/120. Theoretically these profiles are imposable but the use of slow ascents and deeper stops allow for the very aggressive 120% Hi Gradient Factor. Well most of the time

The best way to think about Gradient Factors is two horizontal parallel lines. The top line is GF High 100%. The Bottom Line is GF low 0.

Below the bottom line you are on gassing so no decompression is taking place. Above the bottom line decompression is happening to some degree. Above the top line 100% GF High you theoretically get bent. The top and bottom lines are depths in relation to your decompression status.

So a 20/80 profile starts the decompression off 20% above the minimum off gassing level and finishes the decompression at 80% of the maximum decompression profile. The lower the Low decompression GF the deeper the deep stops will be. The Higher the High decompression figure the more shallow stops you will have to do.

I am knackered now but Ill stick some more info up tomorrow

ATB

Mark Chase

[Mdemon]

genital profiles

Something to do with fitting P-valves???!

Mark, you're a diamond mate!







"Chasey for Presidant!"

[Mark Chase]

Originally Posted by Mdemon

Something to do with fitting P-valves???!

Mark, you're a diamond mate!



"Chasey for Presidant!"

Oh bugger


See I told you spell check dosent help. I can edit it now or no one will get the joke.

If you want me i will be in the small hole in the corner.

ATB

Mark Chase

[aclivity]

a while ago, in a Thomson Holidays 'Cruise' brochure, there was an article about how to spend your time on board the ship ....

it said "Start your day with some genital stretching... "

ho ho

[Helen F H]

thanks, i need a new keyboard due to the coffee now languishing between the buttons...........

[dave archer]

This is something I've wondered about too and not got round to asking before.

So Gradient Factors are purely determined by depth/time relative to previous depth/time? Or do they take deco/bottom gas mixes into account in some way?

On a (possibly) related note. When switching to a low nitrogen gas to off gas, you're creating a difference (gradient?) between the nitrogen in your tissues and the nitrogen in the gas in your lungs, which helps the nitrogen come out of your tissues quicker. Is it theoretically possible to make this difference too big, and end up bending yourself by encouraging the nitrogen out too quick? E.g. if oxygen wasn't toxic at depth, if you switched straight to 100% oxtgen then the gradient of tissue/lung nitrogen would be infinite and presumably you'd fizz like a very fizzy thing?

I ask because a lot of threads give the impression that using the lowest nitrogen % possible is the best way to off gas.

I'm (obviously!) not deco trained at the moment, but considering it as a next step.

Thanks,
Dave.

[beanie]

Originally Posted by Mark Chase

The shortest deco most aggressive deco is theoretically 100/100GF. This will get you shallow fast and will use the pressure imbalance of the expanding gas in your body to force the nitrogen out fast. If you follow the modern thinking this profile will have you bent as a 9 bob note in the water but will fix it on the shallow stops.

aparantly gf100/100 dosen't include Bhulman's fudge that he realised was needed after he created his original theory thingy so it's less safe than straight Bhulman I'll see if I can dig out the info (came from starfish enterprise team - britanic trips)

[Skeletor]

So would a 50/50 GF effectively give a continuous slow (exponential?) ascent ? Presumably this would, in theory, be the safest deco profile looked at purely in terms of optimum deco and disregarding any concerns over staying at depth for longer ?

Cheers,

John

[Mark Powell]

Originally Posted by dave archer

So Gradient Factors are purely determined by depth/time relative to previous depth/time? Or do they take deco/bottom gas mixes into account in some way?.

The gradient factor is just a level at which you stop and then continue the ascent. As such the actual factors 80/20, 100/100, 50/120 are set at a level at which you are comfortable. The choice of GF is not affected directly by the depth/time or gas. The GF then result in a set of stops which will be affected by depth time and agsses.

The deco/bottom gas mixes are taken into account as the bottom gas will affect the amount of inert gas that has been disolved. Simillary the deco gas will also affect the impact of GF as, once you get to deco gas depth, the deco mix will move you to the point at which you are ready to move up to the next stop.

Originally Posted by dave archer

On a (possibly) related note. When switching to a low nitrogen gas to off gas, you're creating a difference (gradient?) between the nitrogen in your tissues and the nitrogen in the gas in your lungs, which helps the nitrogen come out of your tissues quicker. Is it theoretically possible to make this difference too big, and end up bending yourself by encouraging the nitrogen out too quick? E.g. if oxygen wasn't toxic at depth, if you switched straight to 100% oxtgen then the gradient of tissue/lung nitrogen would be infinite and presumably you'd fizz like a very fizzy thing?.

The ideal situation is to switch to a deco gas which has 0% of the dissolved gas. So, toxicity aside, switching to 100% O2 would be ideal from a deco point of view. You dont fizz as the gas is diffusing out of the tissues rather than forming bubbles. Diffusing at the maxmum rate is perfectly safe so no you cannot make this difference too big.