Reading Your Dive Computer in Real Time

Part 3 of 3 — Part 1: How Dive Computers Work · Part 2: Decompression Algorithms

From Ceiling to Live Number

A decompression ceiling tells you the minimum depth you can be at right now. It answers one question: can I go shallower? It does not tell you how close you are to the limit, how much headroom remains, or what would happen if you had to bolt to the surface.

GF99 and Surface GF answer those questions. Both are standard on current Shearwater computers; some other manufacturers show equivalent values under different names.

GF99

GF99 is the real-time supersaturation percentage of the most-loaded tissue compartment at your current depth:

GF99 = (P_leading − P_ambient) / (M_value − P_ambient) × 100%

A GF99 of 72 means the leading compartment is at 72% of its theoretical M-value at your current depth. You have 28% headroom before the model's limit. A GF99 of 100 means you are exactly at the M-value. Above 100 means you have exceeded it — you are where the model says bubbles become likely.

During a stop, you watch GF99 drop as the leading compartment off-gases. When it drops to approximately the interpolated GF for that stop depth, the ceiling clears to the next level. This makes the ceiling countdown concrete: instead of waiting for an arbitrary countdown, you are watching the tissue state itself.

A GF99 above 100 mid-dive is not automatically an emergency, but it is a serious flag. It means either the ascent was too fast, a stop was missed, or the dive profile exceeded the model's conservative envelope. Hold the current depth, breathe normally, and let GF99 fall before ascending further.

Surface GF

Surface GF answers a different question: if you surfaced right now, skipping all remaining stops, what would GF99 be at the surface?

This is the number that determines how dangerous an emergency bolt to the surface would be. During a normal dive it counts down from a high value as you complete your decompression. At the last stop, you hold until Surface GF drops to your GF_Hi setting — that is the moment the model says your leading compartment is within acceptable limits for the surface.

The practical use: if Surface GF is well below your GF_Hi during a stop, you have conservatism headroom. If gas is critical and you need to shorten stops, Surface GF tells you quantitatively how close to the limit you are operating. If Surface GF is above GF_Hi and shows no sign of dropping, you are not done decompressing — regardless of what the ceiling counter says.

A deeper treatment of the GF framework — GF Lo, GF Hi, the interpolation formula, and the evidence behind common settings — is in Gradient Factors and Dive Computers.

Ascent Rate: What the Data Shows

The research on ascent rate and DCS incidence is not subtle. Doubling your ascent rate from 9 m/min to 18 m/min approximately doubles to triples DCS incidence. The relationship is not linear — faster ascents produce disproportionately more bubble formation because supersaturation is proportional to how fast ambient pressure drops relative to tissue off-gassing.

Standard rate guidelines:

The final 6 m is the most dangerous phase of any dive — recreational or technical. The pressure drop from 6 m to the surface is 0.6 bar, which represents the largest proportional pressure change in the entire dive. Every tissue compartment that still carries dissolved nitrogen experiences that drop simultaneously. Rushing this interval for a boat pickup or because the stop feels long is where preventable DCS happens.

If you make a fast unplanned ascent from depth, the recomputed schedule from your new position will be deeper and longer than the original — the penalty grows rather than shrinks. There is no benefit to ascending faster than the model permits; the model responds by extending the obligation.

Staggered Safety Stop

A single 5-minute safety stop at 5 m was the recreational standard for decades. Doppler ultrasound bubble studies run by the Australian Army Aviation Centre showed that replacing it with a staggered ascent reduced post-dive bubble grades by approximately 50%.

The staggered pattern: 1 minute at 9 m, 2 minutes at 6 m, 3 minutes at 3 m. Each step allows fast compartments to off-gas at progressively lower pressure before the next transition. The last move — from 3 m to the surface — is done slowly, over at least a minute. Total time is the same as a single 6-minute stop; the distribution across depths makes a measurable difference to bubble load.

For recreational divers on no-decompression dives, the staggered stop is not mandatory but costs nothing in time. For technical divers completing mandatory decompression, the same principle applies to the final stop: ascend out of it slowly, do not fin straight to the surface the moment the ceiling clears.

Subclinical DCS

Not all DCS produces recognisable symptoms. Divers who complete profiles that their computer clears may still circulate microbubbles detectable by Doppler ultrasound — without joint pain, neurological symptoms, or any subjective complaint. These divers typically show elevated endothelial microparticles and platelet activation markers in blood analysis.

The practical implication: the fact that you feel fine after a dive does not mean your tissues were unaffected. Subclinical bubble load correlates significantly with post-dive fatigue — the tiredness after a heavy dive day is not just physical exertion. Multiple consecutive days of heavy repetitive diving accumulate endothelial stress even when no DCS case develops.

The most accessible intervention is extended surface intervals. Three hours between dives allows most fast and medium compartments to clear substantially. Combined with good hydration, it is more protective than any change to GF settings.

What the Algorithm Cannot See

A dive computer models an ideal diver. The algorithm has no way to observe:

Dehydration. Reduced plasma volume increases blood viscosity and slows nitrogen transport out of tissues. Dehydrated divers have measurably worse Doppler bubble grades for the same dive profile compared to well-hydrated ones. Drink water before the dive.

Cold. Cold water vasoconstricts peripheral tissues, reducing blood flow and slowing nitrogen off-gassing during ascent. The same schedule that clears comfortably in 28°C water may be inadequate in 10°C. Technical divers in cold water routinely extend stops beyond computer minimums for this reason.

Patent foramen ovale (PFO). Approximately 25–30% of adults have a PFO — a hole between the heart's two upper chambers that normally closes at birth but remains open. Venous bubbles that would ordinarily be filtered by the lungs can pass directly into the arterial circulation, reaching the brain and spinal cord. A diver with PFO faces a materially higher DCS risk than the algorithm accounts for. PFO testing is available and increasingly common among technical divers.

Exertion at depth. Heavy exercise increases CO₂ production, cardiac output, and nitrogen loading rate. A diver who swam hard against a current at 40 m has loaded nitrogen faster than one who hovered motionless. The computer saw the same depth and time for both.

CO₂ retention. Carbon dioxide accumulation — from high gas density, skip-breathing, overexertion, or poor regulator performance — has been shown to increase DCS risk through micronuclei activation and CO₂-induced vasodilation. The same 2024 Daubresse review that documented the mechanism also attributed a cluster of eight neurological DCS cases to hypercapnia during pre-dive mask wearing. The algorithm saw nothing unusual in any of those tissue compartments.

Age and fitness. Older divers and those with poorer cardiovascular fitness have reduced nitrogen transport efficiency. No consumer dive computer currently adjusts decompression schedules for age.

Shearwater Post-Dive Data

Current Shearwater computers download full tissue compartment data — nitrogen and helium loading for all 16 compartments at every sample point throughout the dive. No other manufacturer currently provides this level of post-dive data.

This enables forensic analysis of any dive: you can reconstruct exactly which compartment was controlling the decompression at each depth, how close GF99 came to your settings, and whether any phase of the dive pushed loading in unexpected directions. For technical diving instructors and expedition divers logging complex profiles, this data is substantively useful — both for verifying the plan and for identifying patterns across multiple dives.

References

• Nishi RY, Brubakk AO, Eftedal OS. Bubble detection. In: Brubakk AO, Neuman TS (eds). Bennett and Elliott's Physiology and Medicine of Diving. Saunders, 2003.
• Daubresse L, Vallée N, Druelle A, Castagna O, Guieu R, Blatteau J-E. Effects of CO₂ on the occurrence of decompression sickness: review of the literature. Diving Hyperbaric Med. 2024;54(2):110–119. DOI: 10.28920/dhm54.2.110-119
• Wienke BR, O'Leary TR. Understanding Modern Dive Computers and Operation. Springer, 2018. DOI: 10.1007/978-3-319-94054-0
• Baker EC. Clearing Up the Confusion About "Deep Stops." 2010. (Published online.)
• Bakovic D et al. Acute effects of self-contained underwater breathing apparatus diving on arterial stiffness and endothelial function in healthy divers. Diving Hyperbaric Med. 2020;50(3):235–243.

Train With Me

Reading your computer in real time — GF99, Surface GF, and stop management — is part of every technical and CCR course I run. Enquire about training →