How to Plan a Trimix Dive
A 6-step workflow connecting every calculator in the right order
1. Know your SAC
Every gas plan starts with your Surface Air Consumption rate. Without an accurate personal SAC, every downstream number — Rock Bottom, turn pressure, cylinder size — is a guess. Use the SAC/RMV calculator with cylinder volume, start/end pressure, average depth, and dive time across many dives to build a rolling baseline. Resting and exercising SAC can differ by a factor of two to three; log calm and stressful dives separately and use a representative average for the planned profile.
→ Know your SAC2. Pick the mix (MOD, EAD, END)
For a 40-80 metre trimix dive, you need a mix where the MOD covers your maximum depth at 1.4 ata, the EAD is shallow enough that your training-agency tables apply, and the END stays under 30 metres for clear cognition. The MOD calculator solves all four (MOD, EAD, END, Best Mix) in one pass. Try common technical blends like 21/35 or 18/45 and see which one fits your depth and decompression strategy best.
→ Pick the mix (MOD, EAD, END)3. Check gas density
Once you have a candidate mix, check its density at the planned depth against the Anthony 2018 / DAN consensus thresholds. Anything under 5.2 g/L is in the recommended zone; 5.2-6.2 g/L is a caution zone; above 6.2 g/L the work of breathing rises sharply and CO₂ retention becomes likely. Adding helium is the only practical way to keep density safe at depth — a 21/35 trimix at 50 m sits around 4.7 g/L while air at the same depth is 7.4 g/L.
→ Check gas density4. Validate gas switches (ICD)
Plan your bottom-to-deco gas switches and verify each one against the Doolette 1:5 isobaric counter-diffusion rule with the ICD calculator. The classic vestibular-DCS trigger is a switch from a high-helium trimix to a low-helium nitrox (e.g. 21/35 → EAN50 at 21 m fails the 1:5 rule). If a switch fails, the calculator suggests the minimum O₂ a helium-free deco gas would need to make the transition safe — or split the switch into smaller increments.
→ Validate gas switches (ICD)5. Generate the dive plan
With your bottom gas, deco gases, and switch depths chosen, run the full plan in the Dive Planner. Configure your gradient factors (a conservative 30/70 or balanced 40/85 is typical for tech), add all your gases at their switch depths, and review the complete schedule: descent, bottom time, ascent rate, deco stops, CNS%, OTU, runtime, and gas consumption per cylinder. The planner uses Bühlmann ZH-L16C with the same equations as your dive computer.
→ Generate the dive plan6. Confirm your reserves (Rock Bottom)
Finally, calculate Rock Bottom for your maximum depth, cylinder, and SAC to verify you can complete an emergency ascent for two divers even after a worst-case event at depth. If Rock Bottom exceeds your planned turn pressure, either size up your cylinder, choose a shallower profile, or revise the dive. Recalculate whenever depth, team size, cylinder, or SAC changes — never reuse yesterday's number on today's dive.
→ Confirm your reserves (Rock Bottom)