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What Are Gradient Factors?

Published on: 20th October, 2016 | Reviews and Training Tips
Photo credit: Dive Rite
Ever heard a tech diver talking about gradient factors? What are they and why are they so important?

When we exceed our no decompression limit (NDL) we need to make decompression stops on our ascent to minimise the risk of decompression sickness (DCS). In simple terms, gradient factors determine the rate of ascent during a decompression dive.

Robert Workman introduced the term “M-value”, that is, the maximum inert gas pressure that a hypothetical tissue compartment can tolerate without DCS. Workman determined M-values using depths (i.e. relative pressures). The theory was subsequently refined by Buhlmann using absolute pressures. The M-value line sets the maximum rate at which divers can ascend to the surface.

Gradient factors (GF) are used to provide a factor of safety to the Bulhmann M-value model. The gradient factor defines the amount of supersaturation of inert gas allowed in a tissue compartment.

GF 0% is ambient pressure

GF 100% is on the M-value line

The use of low and high gradient factors allow a diver to vary the factor of safety during an ascent.

Low GF defines the first decompression stop, and gradually changes to meet the high GF over the course of the ascent. High GF defines the surfacing value.

The example shown in the images is for a Low GF of 30% and a High GH of 80% (i.e. GF 30/80). The image also shows the ascent of the diver to the GF 30/80 line, waiting as decompression commences, then ascending to the line again and stopping during decompression, and so on, until the surface is reached.

References
The original article ‘Gradient Factors’ by Matti Antilla is available here: http://www.diverite.com/articles/gradient-factors/

AUTHOR

Susan Shield

TDI Adv. Nitrox & Deco. Procedures. Chartered Professional Engineer