Dive Computers and Decompression Management

Every diver using a computer is following an algorithm they probably can't explain. This course changes that. It covers what your dive computer is actually computing, why the numbers on the screen mean what they mean, and what to do when they matter.

No manual-reading. No simplified explanations. A working understanding of the physics and the decisions that follow from it.

What This Course Covers

History and the Models Behind Your Computer

Every dive computer runs one of three decompression frameworks. Understanding where they came from changes how you interpret what the display shows you.

The Haldanean dissolved-gas framework underlies most computers in use today: a 16-tissue ZHL model that checks each compartment's nitrogen loading against an M-value limit every 4 seconds. Bühlmann ZHL-16C is the specific variant used in Shearwater, Suunto EON, and most serious computers — the A, B, and C suffixes reflect different M-value coefficients, and C is the most permissive.

RGBM (Wienke) adds explicit bubble nucleation and repetitive dive penalties. It's why RGBM computers don't expose adjustable GF settings. VPM (Yount) uses varying permeability and a critical bubble radius threshold, producing different stop architectures from RGBM despite similar-sounding aims.

The 16-tissue half-time table runs from 4 minutes (blood) to 635 minutes (ligament). Understanding the spread explains why a single deep dive and a three-day liveaboard require different thinking.

Gradient Factors — What You're Actually Adjusting

GF Lo and GF Hi scale M-values, not stops. That distinction matters. The computer interpolates linearly between GF Lo at your deepest stop and GF Hi at the surface — which is why 30/70 produces deeper stops than 40/85, and why the two settings interact rather than operating independently.

The NEDU and Baker evidence on deep stops is discussed with its actual findings: what the data shows, what it doesn't, and why the GF debate in recreational and technical diving is often more tribal than the evidence warrants.

GF99 and Surface GF in Real Time

GF99 is the percentage of M-value being used by the leading compartment right now. A reading of 92% means the leading tissue is at 92% of its theoretical limit. It is the most useful single number on a Shearwater during a decompression dive.

Surface GF is a forward projection: your GF99 at the surface if you follow the current plan. It is the number to check before leaving your final stop. When SurfGF exceeds 100%, the plan needs to change before you move.

This section covers how to use both together to manage stop extensions, and what to do when gas is tight and the numbers are still climbing.

Ascent Rates — The Variable Most Divers Underestimate

Doubling your ascent rate from 9 to 18 m/min increases DCS incidence by a factor of 2–3 in controlled data. The reason is pressure differential per unit time, which is greatest near the surface. The 6 m to surface transition is the most dangerous part of the ascent profile — not the deep section.

Rate-by-zone: 6–9 m/min from depth, 1–3 m/min in the shallow zone. A sustained fast ascent breaks the algorithm's predictions and forces a recomputed ceiling that is longer than the original plan. Shearwater alerts and what they mean are covered in detail.

Reading Your Dive Log

The log your computer stores is not just a record. It's the primary tool for post-dive safety review.

Shearwater logs every 5–10 seconds: tissue loadings per compartment, CNS%, gas switches, ceiling, ascent rate alerts. The LANL database (3,569 mixed-gas technical profiles, 28 DCS cases) is used to show what outcome patterns look like in real profile data. Sayer's "Silent Witness" case work (2008) showed how downloaded profiles from fatal dives revealed omitted stops, fast ascents, and gas management failures that interviews alone had missed.

Subsurface Integration

Subsurface is the open-source tool that ties dive planning, log download, and pattern analysis together. The course covers downloading from Shearwater (USB-C and Bluetooth), Suunto, Garmin, Mares, and Ratio — including what data each manufacturer actually transfers vs. what Shearwater uniquely provides (tissue N₂/He per compartment).

Planning a deco dive in Subsurface: entering depth, bottom time, gas mixes, GF settings, and reading the stop schedule output. Comparing planned vs. actual tissue loading after the dive. Tracking CNS% and OTU across a multi-day series.

Team Diving with Different Computers

Most buddy pairs are running different algorithms. Planning to the most conservative algorithm in the team, and understanding the ratchet effect — why the team ascends at the pace of the most loaded diver, not the fastest computer — is standard practice, but rarely explained clearly. GF reconciliation, mid-series algorithm switching, and what the LANL data shows about ZHL vs. RGBM outcomes across identical profiles are all covered.

Safety — What Actually Goes Wrong

Sayer's case studies (2008) of 87 diving incidents involving computer use identified a short list of recurring failures. All of them are covered here: using an air algorithm with nitrox, two divers sharing one computer across a repetitive series, altitude diving with sea-level settings, and flying after diving (12 hours after a single no-deco dive, 18–24 hours after deco dives, 48 hours after a deep trimix series).

Who This Course Is For

• Any diver who uses a dive computer and wants to understand what it is actually computing
• Divers who push NDLs, dive repetitively on liveaboards, or dive nitrox and want to make genuinely informed decisions
• Anyone using a Shearwater who doesn't yet use GF99 and Surface GF as active decision tools
• Technical divers who set GF values based on what they were told, not because they understand why

This course follows naturally from Introduction to Decompression Theory, which covers the physiology and dissolved-gas fundamentals. Dive Computers picks up where that leaves off: the specific algorithms in consumer hardware and how to use them in practice.

Format

Duration is 12 hours across 1–2 days depending on depth of discussion and whether practical dives are included. Format is classroom theory with Subsurface live demonstrations. An optional practical component adds planned decompression dives using Shearwater with GF99/Surface GF active monitoring. No limit on student numbers for theory; 1:2 maximum for practical.

Locations: Online · Pondicherry · Lakshadweep · Meghalaya · Sri Lanka · Maldives

Fees and Pricing

Pricing is tailored to your course and location. A full breakdown is provided before any commitment is made.

Enquire

Enquire here — Donarun responds personally to every enquiry.

What Comes Next

• Tec Fundamentals — if you want the technical diving certification pathway
• Dive Planning — gas volumes, turnaround pressures, and deco contingency planning
• Introduction to Decompression Theory — if you want the physiology layer before the algorithm layer