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