Reprinted in full from "Decompression Theory Every Diver Should Know (Part 2), Author: Lin Yu-Ping"
In the previous installment, we discussed how Haldane's half-saturation time decompression model opened the door to decompression research in diving — a path that many scientists have since continued to explore. Among the most significant contributions is the Bühlmann decompression algorithm, developed by Swiss physician Dr. Albert A. Bühlmann, who began building on Haldane's foundational work in 1959. The most advanced model Dr. Bühlmann developed — the ZHL-16C — was designed specifically for dive computers. ZHL-16C employs 16 theoretical tissue compartments with half-saturation times ranging from 4 to 635 minutes. It also introduces revised M-values for each compartment, with higher upper limits assigned to fast tissues (fast compartments). After continuous refinement and evolution, it emerged victorious in early 21st-century comparative decompression trials and is now the widely recognized industry standard — including as the decompression model used in Garmin dive computers.
In Haldane's era, computers did not yet exist, so all decompression calculations had to be done by hand — making complex, highly variable calculations essentially impossible. Early diving also involved heavy equipment with surface-supplied air, meaning divers could not ascend or descend on their own; they relied entirely on diving bells and cages. The hoisting speed of the ship's winch at the time was 1 foot per second, which is the origin of the 18 m/min ascent rate limit still referenced today.
Given a fixed dive depth and discrete time intervals (5–10–15–20–25–30… minutes), it was possible to calculate the theoretical tissue compartment saturation at each depth and time increment. Combined with the time to surface (TTS) — derived by dividing depth by a fixed ascent rate — one could also calculate the off-gassing process during ascent and the progressive decline in compartment saturation during surface intervals. This is the origin of the dive planning tables you learned in your Open Water Diver (PADI/SSI cert) course. Table lookups are always limited to a single fixed depth and time, much like wearing an off-the-rack uniform — it will never fit as well as something tailor-made.
NDL (No Decompression Limit)
Dive computers can perform complex, real-time calculations with a sampling rate of just a few seconds per cycle. Garmin dive computers leverage their powerful processing capabilities to update calculations every second, enabling an even more immediate response to depth changes. We have long since left the era of winch-controlled ascents — although dive computers still use a built-in ascent rate of 10 m/min to estimate TTS (Time To Surface). While at a given depth, the dive computer can calculate how much time remains before each of the 16 theoretical tissue compartments reaches its safe upper limit, as well as the projected residual saturation upon surfacing after accounting for off-gassing during the TTS. The compartment with the least time remaining is displayed on the screen as the no-decompression limit (NDL). As you ascend during a dive, the NDL is not a fixed value — it gradually increases. By staying within the NDL shown on your dive computer, you can avoid absorbing excessive nitrogen.
GF (Gradient Factors)
Scientific research aims to identify the boundary of safety (the M-Line), but in actual diving practice there is no need to push right up against that boundary. The ZHL-16C model therefore incorporates a user-adjustable conservatism buffer. Take GF 30/70 as an example: the 30 is GF Low, meaning that at the deepest point of the dive the allowable supersaturation is set to 30% of the M-value; the 70 is GF High, meaning that at the shallowest point (the surface) it is set to 70%. Connecting the two creates a gradient factor line (GF-Line) that gradually increases from depth (30%) toward the surface (70%).
When the GF Low value is set lower, the diver will begin decompression stops at greater depth and will accumulate more stops overall. When the GF High value is set lower, less residual nitrogen remains in the body upon surfacing, which consequently increases total decompression time. The table below shows results calculated using the decompression dive planning function on a Garmin dive computer for a dive on air to 40 m for 30 minutes.
| Depth (m) / GF | 30/70 | 30/80 | 40/80 | 40/90 | 50/95 |
|---|---|---|---|---|---|
| 18 | 1 | 1 | |||
| 15 | 4 | 3 | 3 | 2 | |
| 12 | 6 | 4 | 3 | 3 | 3 |
| 9 | 9 | 7 | 7 | 6 | 5 |
| 6 | 18 | 14 | 14 | 10 | 9 |
| 3 | 35 | 28 | 29 | 23 | 20 |
| Total Time | 73 | 57 | 56 | 44 | 37 |
*With the same GF Low, a higher GF High shortens total decompression time.
*With the same GF High, a lower GF Low means decompression begins at greater depth.
Does a lower GF value always mean more conservative and safer? As far as GF Low is concerned, a lower value means spending more time at depth. This can cause slow tissues to keep absorbing nitrogen and will increase gas consumption. As for GF High, a lower value extends the total dive time, which introduces additional risks from fatigue and hypothermia.
For no-decompression dives, GF Low has no impact since the dive involves no decompression phase at all — but GF High does influence the length of the NDL. The table below shows NDL values calculated using a Garmin dive computer for a dive on air to 25 m:
| GF | 30/70 | 30/80 | 40/80 | 40/90 | 50/95 |
|---|---|---|---|---|---|
| NDL | 12 | 16 | 16 | 20 | 23 |
Thanks to the rapid advances in dive computer technology, diving habits have shifted from the old approach of carefully planning a dive with tables beforehand, to a more passive style of adjusting on the fly based on real-time information from the dive computer. Yet once we understand the origins of the decompression models behind these computers, we realize that they are, at the end of the day, results generated by a mathematical model — one that cannot fully replicate the actual physiological responses of the human body. All we can say is that, based on practical experience, operating within the parameters shown by a dive computer is generally safe for most situations. Certain physiological factors simply cannot be accounted for within existing decompression theory.
Editor-in-charge: Jenny Tsai
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