Gradient factors

[keylimepie]

I know I'll learn more about this when I do fundies in Oct (unless it is tech1 where they teach it), but I just wondered if someone could explain what gradient factors GUE opts for? I have seen 30/90 mentioned here for doing dives on 32%. Is this the std? If so, why this combo? Does it change once you go to tech depths?

Replies in the KISS mode if at all possible. Ta

Dave

[limeyx]

Quote: (Originally Posted by keylimepie)

I know I'll learn more about this when I do fundies in Oct (unless it is tech1 where they teach it), but I just wondered if someone could explain what gradient factors GUE opts for? I have seen 30/90 mentioned here for doing dives on 32%. Is this the std? If so, why this combo? Does it change once you go to tech depths?

Replies in the KISS mode if at all possible. Ta

Dave

I'm sure people with more knowledge will correct/add to this but here's my take:

The Gradient factors are used with the RGBM model to modify the depths at which the deco stops are performed. The two numbers affect the M-values for different compartments and are used to scale down (make deeper) the depth at which each compartment is considered to start offgassing.

The two values are used to allow a different scaling for the faster tissues (deeper stops) compared to the slower tissues (shallower stops)

This has the effect of pushing the deco stops deeper in the water.
for 30/90, the first M-value will be scaled to 30%, the next 30 + (90-30)/number of stops, and so on until the shallowest stop is 90% of normal (i.e. a linear interpolation between the two values)

Adjusting the values can give a more or less conservative profile compared to straight RGBM.

THis was not covered for me in DIR-F, but was in tech1.

For rec profiled, the MDL tables are generated by RGBM with some gradients (usually 30/90 or 30/85) although It has been I think sugested that 20/80 matches the VPM on +2 conservatism a bit better.

So if you had a factor of 100/100 then you would have straight RGBM.

In tech1, you are advised to print/write a set of air tables in wetnotes for planning deco, and that's where the Gradient Factor comes in. We were advised to use 20/85 (or 20/80 -- dont have my notes handy)

However, in practice we end up using ratio deco which I understand corresponds more closely to a 30/90 factor.

Not sure if that's "KISS" or even coherent.

[Gareth Burrows]

OK

Let's imagine you have a graph, like the one below. Its really crap, so bear with me.



The line marked "saturation" - the blue one at the bottom which really should be in the middle of the graph - indicates the point where the ambient pressure meets the tissue pressure. This means you are not ongassing or offgassing, but have reached a point where thepressure of ambient gas (eg Nitrogen) around you has equalised with the pressure of ambient gas in your tissues. This will obviously be a different depth for each tissue compartment but let's just pretend the body has one tissue compartment to make it simple.

Now everything below that line to the right hand side of the graph means you have greater ambient pressure than tissue pressure. IE you are going down. this means you are ongassing. Everywhere above that line means the tissue pressure is greater than the ambient pressure. Hence you are offgassing. This is known as the "deco zone".

Now, obviously, we all need to offgas during the ascent on a dive, and the more into the decozone we go, the faster we offgas. However, there comes a point where you are offgassing too much. This is the point where the tissue pressure is so much greater than the ambient pressure that bubbles begin to form in your tissues - ie you get bent. Now, the point where this happens is a theoretical line called supersaturation, or the "M value".

Now ye old buhlmann tables took you straight up from a deep dive to the M value, and kept you there until offgassing was complete, hence the reason many people refer to them as a "bend and mend model". This had the advantage of getting you shallow fast, and offgassing fast, but really took you to the edge of being bent. This is why buhlmann tables, and buhlmann based computers, such as the suuntos, get you very shallow, very fast.

Now, lets call the point of saturation, where everything is equal as being 0% and the point of supersaturation, or M value, where we are on the point of being bent, as being 100. The fact that buhlmann tables take you straight to the M value means that you are being taken immediately to 100% of the distance between saturation and the M value, hence the reason buhlmann tables are often called 100/100, becuase all of the stops are done at this 100% point.

However. There is a "grey area" around the M value, as it is only a theoretical line, and the problem is the many people feel like shit diving buhlmann tables, and indeed people got bent. So Gradient factors were introduced as a fudge to make the model safer. Gradient factors are represented by a LOW factor figure and a HIGH factor figure.

The LOW figure represents the percentage of the distance from the equilibrium to the M Value at which we will start our stops. Thus 10/90 gradient factors mean we start our stops very deep indeed at only 10% of the distance to the M Value. 30/70 gradient factors mean we start our stops 30% of the way towards the M value, ie a little shallower.

The HIGH gradient factor represents the maximum percentage we are willing to go between the point of saturation and point of supersaturation before hitting the surface. For example, 20/80 means we will never go above 80% of the way towards the M value during our stops. 10/90 means we go a little closer. Obviously, the higher this figure, the greater the difference between tissue and ambient pressure, and the faster you will be offgassing.

What factors you use is purely empircal, at least in the non-DIR world. Whatever makes you feel good are the right factors to use. A very low LOW factors means you will make lots of deep stops, but will not be offgassing fast so will have a longer deco. A very high HIGH figure will mean you are offgass quickly, but are moving towards the M value, a bad place to be. the most popular combinations are things like 20/80 (which I dive) and 10/90, although you do hear mad combinations.

Anyway, that's gradient factors.

Garf

[Moz]

Quote: (Originally Posted by limeyx)

I'm sure people with more knowledge will correct/add to this but here's my take:

The Gradient factors are used with the RGBM model to modify the depths at which the deco stops are performed. The two numbers affect the M-values for different compartments and are used to scale down (make deeper) the depth at which each compartment is considered to start offgassing.

The two values are used to allow a different scaling for the faster tissues (deeper stops) compared to the slower tissues (shallower stops)

This has the effect of pushing the deco stops deeper in the water.
for 30/90, the first M-value will be scaled to 30%, the next 30 + (90-30)/number of stops, and so on until the shallowest stop is 90% of normal (i.e. a linear interpolation between the two values)

Adjusting the values can give a more or less conservative profile compared to straight RGBM.

THis was not covered for me in DIR-F, but was in tech1.

For rec profiled, the MDL tables are generated by RGBM with some gradients (usually 30/90 or 30/85) although It has been I think sugested that 20/80 matches the VPM on +2 conservatism a bit better.

So if you had a factor of 100/100 then you would have straight RGBM.

In tech1, you are advised to print/write a set of air tables in wetnotes for planning deco, and that's where the Gradient Factor comes in. We were advised to use 20/85 (or 20/80 -- dont have my notes handy)

However, in practice we end up using ratio deco which I understand corresponds more closely to a 30/90 factor.

Not sure if that's "KISS" or even coherent.

I was going to say that...............








Errrm no I wasn't

For the record, both myself and Dave deceided to do a Basic Nitrox course from TDI as we both wish to benefit from the safety factor in the rec range (as our fundies course is in Oct as Dave said)

However, the course seemed to be aimed at accelerated deco, and the Buhlmann tables, CNS loading and U.P.T.D's are a bit much to grasp after a 1 day course. To be honest, I don't think the instructor's actually fully understood the tables!

The diving we are doing at the moment I would be happy to use either PADI or BSAC air tables, although we do always practice slow acents!

One problem I have with the Buhlmann table is the no fly time, as I do fly quite a lot. It seems it's OK to do couple of dives, pack the kit away, have a coffee, and check in for the flight!

David

[Lou]

20/80 is what I use too. So far so good, no real post-dive tiredness etc.

Mind you, on a nice day I actually enjoy the 6m stop, so will hang out there for extra time, just chilling.

[limeyx]

Quote: (Originally Posted by Moz)

I was going to say that...............








Errrm no I wasn't

For the record, both myself and Dave deceided to do a Basic Nitrox course from TDI as we both wish to benefit from the safety factor in the rec range (as our fundies course is in Oct as Dave said)

However, the course seemed to be aimed at accelerated deco, and the Buhlmann tables, CNS loading and U.P.T.D's are a bit much to grasp after a 1 day course. To be honest, I don't think the instructor's actually fully understood the tables!

The diving we are doing at the moment I would be happy to use either PADI or BSAC air tables, although we do always practice slow acents!

One problem I have with the Buhlmann table is the no fly time, as I do fly quite a lot. It seems it's OK to do couple of dives, pack the kit away, have a coffee, and check in for the flight!

David

CNS and UPTD are the easiest part of it if you follow the GUE dive planning/standard gases:

CNS % = Bottom Time/2 + Deco Time (on 50%) /2
or for 100 Deco Time * 2

UPTD = Bottom Time * 1.5 + Deco Time * 1.5
(*2 for O2)

Next?

[keylimepie]

Thanks for the responses. I have been doing some reading on the M Value and Buhlmann on places like Rebreather World. I get the gist of the bend and mend model, but your explanations have cleared up some areas for me, so thanks for taking the time to do that.

As David has said, we'll only be doing minimum deco if any on the dives we plan to be doing. But it is interesting to note how the different combinations give you different stop times.

[Alastair]

The only thing I would add to the above is that while GF give a schedule they don't tend to replicate what you'd do on a Tech dive. I.e. 30/90 doesn't provide the deep stops where I would put them using ratio deco. GF also doesn't emphasise the oxygen window with the longer stops at the gas switched.

So GF are potential starting point for a tech dive but I'd modify the schedule to suit what I prefer.

Cheers
Al

[floater]

Quote: (Originally Posted by limeyx)

I'm sure people with more knowledge will correct/add to this but here's my take:

The Gradient factors are used with the RGBM model to modify the depths at which the deco stops are performed. The two numbers affect the M-values for different compartments and are used to scale down (make deeper) the depth at which each compartment is considered to start offgassing.

The two values are used to allow a different scaling for the faster tissues (deeper stops) compared to the slower tissues (shallower stops)

This has the effect of pushing the deco stops deeper in the water.
for 30/90, the first M-value will be scaled to 30%, the next 30 + (90-30)/number of stops, and so on until the shallowest stop is 90% of normal (i.e. a linear interpolation between the two values)

Adjusting the values can give a more or less conservative profile compared to straight RGBM.

THis was not covered for me in DIR-F, but was in tech1.

For rec profiled, the MDL tables are generated by RGBM with some gradients (usually 30/90 or 30/85) although It has been I think sugested that 20/80 matches the VPM on +2 conservatism a bit better.

So if you had a factor of 100/100 then you would have straight RGBM.

In tech1, you are advised to print/write a set of air tables in wetnotes for planning deco, and that's where the Gradient Factor comes in. We were advised to use 20/85 (or 20/80 -- dont have my notes handy)

However, in practice we end up using ratio deco which I understand corresponds more closely to a 30/90 factor.

Not sure if that's "KISS" or even coherent.

I think you mean Buhlmann or Neo-Haldanian instead of RGBM? At least I've only heard of GF's being used with Buhlmann algorithms and when I run GAP-RGBM deco program there is no option to use GF's.

[limeyx]

Quote: (Originally Posted by floater)

I think you mean Buhlmann or Neo-Haldanian instead of RGBM? At least I've only heard of GF's being used with Buhlmann algorithms and when I run GAP-RGBM deco program there is no option to use GF's.

I use deco planner, and that's RGBM right?