**Simple Gradient Factor Description**

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. *