Study this course as planning discipline plus model vocabulary. Define half-times, compartments, M-values, gradient factors, bubble models, CNS percentages and OTUs precisely; then build complete run-time and gas plans on paper, including MOD checks and written contingency branches, and gate your readiness with a rebuild-it-unaided rubric.
Why planned decompression breaks your no-stop habits
The course treats a ceiling as an obligation fixed before the dive: your schedule, gas volumes and contingencies are decided in advance. Study accordingly, shifting from reaction-based habits to document-driven planning.
In no-stop diving, a computer alarm functions as the safety net and a direct ascent is always available. In planned decompression, surfacing directly is no longer an option, so the written plan itself becomes the safety net. Expect questions that probe why each element of a run-time plan exists - average depth, run time, switch depths - rather than only how to fill in a blank schedule form.
The practical study implication is to build, not reread. Take any sample profile and generate the full run-time table yourself from a blank sheet. When a question mentions ascent rate, stop depth or a gas switch, trace that element's role in the plan instead of trying to recall an isolated number. Everything here is paper practice; in-water execution belongs to your instructor-led training dives.
Half-times, tissue compartments and M-values: what each term controls
Dissolved-gas models track inert gas in hypothetical compartments with distinct half-times, and M-values cap how supersaturated each compartment may become during ascent. Learn what every named parameter actually governs.
A half-time is the time a compartment takes to move halfway toward saturation with a given inert gas. Fast compartments, on the order of five minutes, load and unload quickly; slow ones of an hour or more accumulate over a diving day and across repetitive days. This is why scenarios involving multiple dives over several days appear: they check whether you understand that slow compartments carry residual nitrogen forward, not just last dive's bottom time.
M-values, originally from Workman, define the maximum tolerated tissue-to-ambient pressure gradient for each compartment on ascent; Buehlmann's ZH-L16 is a modern implementation using its own coefficient sets. A frequent confusion is treating M-values as measured physiological limits. They are chosen model parameters, which is why two reputable schedules for the same dive can differ legitimately - each is a conservative model, not a laboratory ceiling.
- Half-time: time for a compartment to move halfway toward saturation at a fixed pressure.
- Tissue compartment: a hypothetical gas-loading unit with its own half-time, not an anatomical structure.
- M-value: the model's maximum permitted tissue-to-ambient gradient for a compartment during ascent.
- Supersaturation: tissue inert-gas pressure exceeding ambient pressure, the driving force for off-gassing.
- Residual nitrogen: leftover loading in slow compartments carried into subsequent dives.
Bubble models versus gradient factors: what each approach assumes
Bubble models such as VPM and RGBM aim to limit bubble nucleation and growth from the deepest point onward; gradient factors scale Buehlmann's M-values down to reshape stops. They differ in what they claim to control.
Dissolved-gas models tolerate substantial supersaturation on initial ascent; bubble models argue surviving micronuclei can grow, so they constrain the ascent from depth. The practical difference is stop structure: bubble-model outputs, and schedules heavily modified by low gradient factors, tend to place deeper early stops and longer shallow ones. In question form, watch for vocabulary - supersaturation and M-values signal dissolved-gas reasoning, while nucleation and critical volume signal bubble-model reasoning.
Gradient factors modify a dissolved-gas model: the low factor sets the fraction of the M-value used at the first stop, the high factor at the surface, and a setting near 99/99 approximates plain Buehlmann. The misuse to recognize is treating a gradient factor pair as a universal safety rating. It is a schedule-shaping choice: adjusting the low factor can restructure the whole profile while barely changing total decompression time.
| Feature | Dissolved-gas (Haldane lineage, e.g. Buehlmann) | Bubble models (VPM, RGBM) |
|---|---|---|
| What is tracked | Inert gas dissolved in hypothetical compartments | Dissolved gas plus bubble nucleation and growth |
| Key named parameter | M-values; Buehlmann a/b coefficients | Critical volume and bubble-radius limits |
| Stop structure tendency | Comparatively shallower early stops | Deeper early stops, longer shallow stops |
| Common adjustment tool | Gradient factors | Model-specific conservatism settings |
| Common misuse | Reading M-values as measured physiology | Treating outputs as proven safer in all contexts |
Oxygen exposure: the CNS clock, OTUs and the MOD check
CNS limits track short-term oxygen toxicity risk as percentages summed per depth-and-gas segment; OTUs track cumulative pulmonary dose over days. Compute segment by segment and check the MOD of every gas before planning switches.
For CNS tracking, each combination of depth and oxygen fraction corresponds to a maximum exposure time, and you sum the percentages consumed across segments. In a small labeled illustration: at 6 m the ambient pressure is 1.6 bar, so breathing EAN50 there gives a PO2 of about 0.8 bar (1.6 x 0.5), and ten minutes at that PO2 consumes a specific slice of the clock that depends on that value. The same ten minutes on EAN50 near 16 m, where ambient pressure reaches about 2.6 bar and PO2 about 1.3 bar, consumes a much larger slice. Because the slice changes with every gas switch, any single-gas shortcut misstates the whole dive. Also track across the day: a surface interval reduces CNS burden but does not instantly zero it.
OTUs serve a different purpose: they accumulate over repeated days of diving and matter for expedition-style schedules, not for deciding a single dive. Separately, the maximum operating depth of each gas follows directly from your chosen PO2 limit: depth = (PO2 divided by oxygen fraction, minus 1) times 10 metres. The classic planning error is selecting a deco gas for its off-gassing benefit and only afterward discovering its MOD is shallower than the intended switch depth.
Gas planning that survives losing the deco cylinder
A decompression plan must account for total run-time consumption, deco-gas volumes, a lost-deco-gas contingency and minimum gas for sharing. Planning only the bottom segment is the error this course targets.
Scenario, simplified for arithmetic: a diver plans 22 minutes at an average 33 m on EAN32, with EAN50 for deco and a surface air consumption rate of 15 L/min. They total the bottom segment, roughly 15 x 4.3 x 22, about 1,400 litres, then add a vague reserve. The mistake: decompression stops consume gas too - about 600 litres of EAN50 over 25 minutes of shallow stops - and if the deco cylinder is lost, every remaining stop must be completed on back gas at greater average depth, adding roughly another 900 to 1,000 litres. These figures are illustrative; use your own measured rates.
The better decision is a line-by-line build: bottom consumption, each stop on its own gas, a written lost-deco-gas branch with recalculated stop times on back gas, and a minimum-gas (rock bottom) reserve to bring two divers up from the deepest point. It matters because gas-math failures in obligation diving surface as missed stops in mid-water, the situation this training exists to prevent. The exam rewards plans where the contingencies are arithmetic, not improvisation.
Run-time drift: amending the schedule when you fall behind
A run-time plan is a commitment, not an estimate. When execution drifts, amend using rules agreed in advance - typically adding time to the shallowest stops - rather than compressing the schedule to restore the total.
Scenario: a diver reaches their planned turn at depth four minutes late after a navigation detour. At 6 m they shorten the final stop from 12 minutes to 8 so total deco time matches the original plan, reasoning the dive computer shows no violation. The problem: the deep portion of the dive already happened and already loaded gas; the shallow stops are precisely where slow compartments off-gas, and shaving them removes the phase the plan was built on.
The better decision is a pre-agreed amendment rule, decided at the planning table: any execution delay adds minutes to the final stop, or to the shallow stops proportionally, while the deeper portions stay as written. State the nuance honestly: models disagree about how deep stops should respond to delays, so the defensible course-level rule is simply never to shorten a written plan to rescue a schedule. Carrying the plan and amendments on a slate makes the rule enforceable at depth.
A three-pass preparation sequence with a self-check rubric
Study in three passes - concepts, construction, contingency - and gate each pass with a rubric. If you cannot rebuild a complete plan unaided on paper, the earlier pass is not finished.
Pass one covers concepts: half-times, compartments, M-values, supersaturation, and the difference between dissolved-gas and bubble models; sketch a compartment-loading graph by hand. Pass two is construction: build two complete plans from blank paper at different depths and gas combinations, including MOD checks and full run-time tables. Pass three is contingency: lost-deco-gas branches, delayed-run-time rules, and CNS and OTU tracking; then explain each plan aloud in plain sentences. Scale the calendar to your week - the order matters more than the duration. All of this is desk work; the in-water skills are assessed by your instructor during course dives.
Capstone exercise: plan a 28 m dive for 20 minutes bottom time on EAN32 with EAN50 deco, choosing your PO2 limits first. Expected observations: at 28 m the ambient pressure is 3.8 bar, so EAN32 yields a PO2 of about 1.2 bar, within a 1.4 bar limit; EAN50's MOD is about 18 m at 1.4 bar or about 22 m at 1.6 bar, so the plan cannot switch to EAN50 at depth and must place the switch shallower. Treat the rubric below as a learning milestone, not a pass prediction.
- MOD is stated for every gas before any switch depth appears in the plan.
- The run-time table includes descent, bottom time, ascent and every stop.
- Deco-gas consumption is calculated separately from back-gas consumption.
- A written contingency branch covers loss of the deco gas.
- CNS percentages are summed per depth-and-gas segment, not averaged.
- Amendment rules for run-time delays are written into the plan itself.
References and further reading
Use these references to explore the concepts and check the latest information from the relevant organizations.