Treat Tec Trimix study as gas-planning fluency, not fact memorization. The core skill is running partial pressure, equivalent narcotic depth, oxygen exposure, and reserve gas calculations on a concrete dive plan, then reasoning through what happens when a gas is lost. Work one hypothetical deep dive on paper end to end, and check every formula by hand against your software.
Why a correct bottom mix can still be dangerous in shallow water
Helium dilutes both nitrogen and oxygen. When the oxygen fraction drops low enough, the mix cannot sustain life near the surface, so every hypoxic bottom gas requires a travel gas and an explicitly calculated minimum operating depth.
Oxygen partial pressure equals fraction times ambient pressure. For a 10% oxygen trimix, PO2 reaches the commonly taught minimum of about 0.16 ATA only at roughly 6 metres: (0.16 / 0.10 - 1) x 10 = 6 m. Shallower than that, the mix is hypoxic. The classic planning mistake is ascending on the back gas after a deco gas problem, then breathing 10% oxygen at 4 or 5 metres while sorting out the switch. The better decision is to plan a dedicated travel gas (air or EAN21) usable from the surface down, or to define a shallow back-gas window only where PO2 stays at or above your training minimum. Hypoxia underwater gives essentially no useful warning before incapacitation, which is why the switch depth must be written on the plan before the dive, not decided during it.
This is why cylinder discipline carries more weight in trimix diving than in open-circuit nitrox. Every gas should be analyzed after filling and again before the dive, and labeled with the fraction plus both its maximum operating depth and its minimum operating depth. MOD works in the same formula in reverse: for a 13% oxygen mix at a 1.4 ATA ceiling, (1.4 / 0.13 - 1) x 10 = about 98 m. Practice computing both directions — from depth to PO2 and from PO2 to depth — until neither direction requires notes. A plan that names every gas but omits its shallow limit is incomplete.
Equivalent narcotic depth: which gases count as narcotic, and why it changes the answer
END math treats nitrogen as narcotic, helium as essentially non-narcotic, and — depending on the convention your materials use — oxygen as either narcotic or inert. Mixing conventions across a plan produces inconsistent, misleading depths.
Worked example: an 18/45 trimix (18% O2, 45% He, 37% N2) at 60 m is 7 ATA. Under the convention that counts oxygen as narcotic, the narcotic fraction is 0.37 + 0.18 = 0.55, so END = 7 x (0.55 / 0.79) = about 4.9 ATA, roughly 39 m of equivalent air depth. Under the convention that counts only nitrogen, END = 7 x (0.37 / 0.79) = about 3.3 ATA, roughly 23 m. A plausible mistake is computing the plan one way, then quoting the other number in the pre-dive brief, which makes the dive look far clearer than your own math said. The better decision is to pick one convention, write it on the plan, and use the same convention for every dive with the team.
It also matters to treat END as a comparison tool, not a biological prediction. Two dives with identical END can feel different because impairment at depth also involves factors END ignores — gas density and work of breathing, carbon dioxide retention, cold, task loading, and high-pressure nervous syndrome on very helium-heavy mixes at depth. Use END to compare candidate mixes against each other and against a comfort level you have verified with your instructor on actual deep training dives. Do not use it to claim a specific dive will feel like a specific shallow dive.
Tracking oxygen exposure across long decompression and multi-dive days
The CNS clock accumulates a percentage of the allowed exposure for each time spent at each oxygen partial pressure. Deep segments, high-PO2 deco gases, and back-to-back dives all add to one running total that you must log.
For each segment of the dive, find the PO2, look up the single-dive exposure limit for that PO2 in the table your course teaches, and add time divided by limit as a percentage. Example drill numbers: 25 minutes of oxygen decompression at a PO2 of 1.6, against a published single-exposure limit of 45 minutes, contributes about 56% of the clock. A plausible mistake is computing each dive's clock in isolation on a two-dive day and ignoring the residual from the morning dive; the better decision is keeping one running log per 24-hour period for the whole team, with a self-imposed cap well below the published limits so an unplanned extension cannot push you into territory your training flagged as unacceptable.
Distinguish the CNS clock from pulmonary oxygen toxicity, measured in OTUs. CNS limits guard against acute, short-term risk — the relevant concern within a single long dive — while OTUs accumulate across days of repeated high-oxygen exposure and matter on expedition-style trips with daily decompression. Knowing which tool answers which question is itself examinable knowledge: a single deep wreck dive is mostly a CNS question, while a week of liveaboard trimix diving is both. Track the one your plan actually stresses.
Rock bottom gas: planning for a teammate's failure at depth
Rock bottom is the gas needed to bring two divers from the deepest point to the first gas switch, both breathing from one supply, with a time and depth margin. It must be computed per depth segment, not guessed as half a tank.
Worked scenario: two divers on a 13/55 back gas at 50 m (6 ATA). One has a catastrophic gas loss at the bottom. Assume a stressed surface consumption rate of 20 litres per minute per diver. Ascent from 50 m to the first deco gas at 21 m at about 9 m/min takes roughly 3.2 minutes, plus a one-minute hold at depth to stabilize — call it 4.5 minutes at an average pressure of about 5.5 ATA. Combined consumption: 2 divers x 20 L/min x 5.5 ATA x 4.5 min = roughly 990 litres, before any margin for the hold deepening or a slower ascent. A plausible mistake is each diver planning 'turn around at half my back gas' with no depth-and-time math behind it; at 50 m there is no direct surfacing, so the reserve must fund a controlled ascent to a deco gas that itself must be redundant.
The better decision is to compute rock bottom for each depth band of the dive — bottom, mid-ascent, shallow — and confirm the donor's remaining supply exceeds the largest figure with margin. Extend the same logic to the deco phase: losing the oxygen or EAN50 cylinder means breathing a shallower gas longer, so the contingency schedule must be written out with its own gas totals. Why it matters: gas math failures do not surface as quiz errors, they surface as a team that reaches the switch with less gas than the ascent actually required. Run the numbers for your team's real consumption rates measured on training dives, not the textbook example values.
Choosing between air, nitrox, and trimix for a specific dive profile
Gas selection starts from depth: depth sets the oxygen ceiling, then the narcosis target, then deco time versus what you are willing to carry. Each added cylinder adds failure modes, so simpler plans that meet constraints are usually better.
A useful study method is to plan the same hypothetical dive three times — once on air or EAN21-style thinking, once on a normoxic-style trimix, once on a deeper-capable hypoxic mix — and watch which constraint binds first each time. On air at depth, the oxygen partial pressure and narcosis both become unacceptable early. On trimix, the binding constraints shift to hypoxic limits in the shallows, helium supply, and deco gas logistics. This comparison shows why the courses progress the way they do: each new gas range introduces a new failure mode you must manage, and your plan is only as good as the constraint you identified correctly.
The trap to avoid is gas selection by habit or by what teammates carry. Instead, write the constraint chain for your dive: maximum depth fixes maximum PO2 per gas; the narcosis target fixes the helium fraction; the deco schedule fixes which switch gases you need and how much of each. If two mixes both satisfy the chain, the one needing fewer cylinders and simpler switching usually wins, because gas switching is a documented point of task loading. Stay within the depth ranges and mixes your current course level authorizes; your instructor's materials, not general dive-club practice, define what is in scope for you.
| Example gas | Typical role in a plan | Key planning constraint | Check before every dive |
|---|---|---|---|
| Air / EAN21 | Travel gas and shallow breathing for hypoxic back gas | Narcosis and PO2 make it unusable as a deep bottom gas | Labeled with MOD where PO2 hits ceiling |
| Hypoxic trimix (e.g. 10/65) | Deep bottom phase on deep dives | PO2 below survivable range in shallow water | Minimum operating depth computed and labeled |
| Normoxic trimix (e.g. 18/45) | Bottom gas for mid-range tec depths | Depth ceiling from PO2 and END targets | END convention noted on the plan |
| EAN50 | Intermediate decompression gas | MOD limits switch depth; contributes CNS load | Analyzed; MOD matches planned switch depth |
| Oxygen 100% | Shallow decompression gas | Strict PO2 ceiling and the largest CNS contribution per minute | Analyzed; deco clock segment logged |
A paper planning drill you can score yourself on
Plan one hypothetical dive — for example 55 m for 20 minutes bottom time — entirely by hand: gases, depths, END, CNS, rock bottom, and deco gas totals. Then run the same inputs through your deco software and compare.
Do the drill with published example exposure limits and your course's PO2 windows, treating the specific numbers here as drill inputs rather than universal rules. Expected observations: your hand calculations should match the software exactly on PO2 windows, minimum and maximum operating depths, and CNS arithmetic, because those are deterministic. Your deco schedule, however, will usually differ somewhat from the software's output, because schedule models make different assumptions about supersaturation and gradient choices. That difference is the point of the drill: investigate it rather than trusting either source blindly. If you cannot explain why the software gives longer shallow stops than your hand plan, you have found the next topic to study, not a software bug.
Score the plan against this rubric, aiming for a perfect sheet before your course's water work: (1) every gas analyzed on paper with fraction, MOD, and minimum depth written down; (2) every switch depth keeps PO2 within the window your materials teach at both the switch point and just above it; (3) END stated with its convention; (4) a CNS total with a stated margin, not just a raw number; (5) rock bottom computed per depth band with the largest figure carried; (6) a written contingency for losing each individual cylinder, with its own gas totals. A plan failing any item is not ready to brief.
- All gases carry three labels: fraction, maximum operating depth, minimum operating depth
- Switch depths verified in both directions against PO2 limits from your course materials
- END convention named on the plan so teammates compute identically
- CNS logged per segment and per 24 hours, with a self-imposed margin
- Rock bottom recomputed for the team's measured consumption rates, not example values
- One written contingency schedule per gas loss, with gas quantities
A preparation sequence and honest readiness checks before your course
Build fluency in layers: re-derive each formula from gas laws first, then redo your previous tec level's knowledge work, then run daily paper dive plans, then practice failure drills in training with your instructor.
A workable sequence over several weeks: first, re-derive the partial pressure, MOD, END, and consumption formulas from the basic gas laws so you can rebuild any of them from scratch rather than recall them. Second, revisit the knowledge development from your completed TecRec prerequisites and confirm you can still do that level's gas math cold. Third, run one full paper dive plan per day using the drill and rubric above, varying depth and bottom time so the constraint that binds changes. Fourth, during training dives with your instructor, rehearse valve failures, gas switches, and buoyancy with additional cylinders in controlled conditions. Fifth, present a complete plan — including contingencies — for review before the course's deep dives.
Readiness checks that are honest rather than optimistic: you can compute MOD and minimum depth in both directions in under a minute without notes; you can state, for every number on your plan, where it came from and what assumption it carries; you can explain why a hypoxic mix needs a travel gas in one sentence a new teammate would understand; you can run a lost-deco-gas contingency on paper and total its gas; and you can say what your END convention is and why. Treat a self-check result as a study milestone, not a prediction of any exam or in-water outcome — it tells you what to rehearse next, and your instructor's evaluation is the real checkpoint.
References and further reading
Use these references to explore the concepts and check the latest information from the relevant organizations.