Best mix¶

The Best mix calculator derives the optimal gas percentages for a target depth and PPO₂ limit. Two modes: OC (for cylinders you'll breathe directly) and CCR (best diluent for a loop setpoint).

Open via App menu → Tools → Best mix.

Inputs¶

Field	What it controls
Depth (m)	Target depth. Pre-filled with the deepest segment of your active plan, if any.
PPO₂ (ata)	Maximum allowed PPO₂ at the target depth. Default: 1.40 ata.
Trimix (toggle)	Include helium in the mix. When on, also asks for Target EAD.
Target EAD (m)	Equivalent air depth — controls how narcotic the mix is. Typical: 30 m for deep dives.
Limit gas density (toggle)	Cap gas density at 5.2 g/ℓ as a second constraint. May add more helium than EAD alone requires.

Outputs¶

The Best mix and analysis results:

Result	Meaning
Best mix	The recommended O₂/He percentages (e.g. `16/57`).
MOD	Max operating depth at the requested PPO₂.
N₂ fraction (OC)	Balance gas; can show density warning if relevant.

If you've requested trimix, the Best mix shows both numbers (e.g. 16/57 = 16 % O₂, 57 % He, 27 % N₂).

How it works (OC)¶

For nitrox (no helium):

Best O₂ % = PPO₂ ÷ absolute pressure at depth

For example, at 40 m (Pabs ≈ 5 ata) with PPO₂ 1.4 → O₂ = 28 %, so the recommended mix is EAN28.

The MOD is then the depth where this O₂ % hits the PPO₂ limit again — confirming where you can still safely breathe the mix.

For trimix (helium added):

Best O₂ % = PPO₂ ÷ absolute pressure at depth
Best He %, given target EAD, satisfies:
   N₂ fraction at depth = N₂ fraction of air at EAD
   → N₂ = 0.79 × (EAD + 10) / (depth + 10)
   → He = 1 − O₂ − N₂

If Limit gas density is on, an additional constraint:

Loop density at depth ≤ 5.2 g/ℓ

The helium is increased until both EAD and density requirements are satisfied. Whichever requires more helium wins; the EAD ends up shallower than requested.

How it works (CCR — best diluent)¶

CCR mode is more complex because the loop mix at the bottom is set by your setpoint, not by the diluent. So the calculator solves for the diluent — the gas in the offboard cylinder that fills the loop and doubles as open-circuit bailout. Two parts are derived:

Diluent O₂ % — picked by the diluent O₂ strategy you have chosen (see below).
Diluent He % — adjusted so the loop at the bottom setpoint meets your target EAD, and (if Limit gas density is on) the ≤ 5.2 g/ℓ density target. Both strategies feed this same helium logic — only the oxygen fraction differs.

Diluent O₂ strategy¶

How the diluent's oxygen fraction is chosen is controlled under App menu → Settings → CCR, by the Diluent O₂ from target PPO₂ toggle:

Fixed 10 % (default)Target diluent PPO₂

The diluent holds a flat 10 % O₂. It only drops below 10 % when — very deep — that 10 % would push the diluent's PPO₂ above 1.6 ata if breathed as open-circuit bailout, at which point the calculator lowers it automatically.

This is the conventional "standard hypoxic diluent": one familiar number, carrying the richest practical O₂ fraction while still leaving a usable bailout margin at depth.

The diluent O₂ is set so its partial pressure at the target depth equals a fixed target of 1.0 ata — the value that keeps a small open-circuit PPO₂ margin at the bottom and a sensible loop O₂ floor during a diluent flush:

Diluent O₂ % = 1.0 ÷ absolute pressure at depth

For example, at 100 m (P_abs ≈ 11 ata) this gives ≈ 9 % O₂; shallower depths get a correspondingly richer diluent. The result is clamped to 4–40 % O₂.

Default is unchanged

Out of the box the strategy is Fixed 10 %, so existing CCR plans produce the same numbers until you turn the toggle on.

Choosing a strategy¶

Both give a hypoxic diluent on a deep dive — the difference is how the oxygen fraction is chosen, and what that buys you.

Fixed 10 %

Advantages — one standard blend to remember, analyse and label; being lean it has a deep open-circuit MOD (breathable as OC bailout to roughly 150 m), so the same diluent can double as emergency gas across most of the dive; nothing to decide per dive.
Disadvantages — on a shallower "deep" dive it is more hypoxic than it needs to be, so its shallow breathable limit sits deeper and a diluent flush lands the loop at a lower PO₂; it does not adapt to the dive.

Target diluent PPO₂ (1.0 ata)

Advantages — the diluent's PO₂ at the bottom is a known ~1.0 ata, so a diluent flush leaves the loop at a predictable PO₂ (a handy sanity-check against the cells) and never drives it too low; it adapts to depth, giving a richer, more surface-friendly diluent on shallower dives; it matches common field practice.
Disadvantages — the diluent O₂ changes with planned depth, so you may blend, analyse and label a different diluent for each dive; on a shallower dive the richer diluent has a shallower OC MOD, so it covers a narrower band as bailout and you may need extra bailout cylinders.

Rule of thumb

Want one do-everything diluent that also serves as deep bailout? Keep Fixed 10 %. Prefer to tune diluent to each dive — and like a flush to read back a known loop PO₂? Use Target diluent PPO₂.

Hypoxic diluent: fine on the loop, not on open circuit¶

A diluent under about 16 % O₂ is hypoxic — at the surface its PO₂ is below ~0.16 ata. That includes the default 10 % diluent (surface PO₂ ≈ 0.10).

On the loop this is normal and safe. The solenoid holds your setpoint, so the loop PO₂ — what you actually breathe — does not depend on the diluent's oxygen fraction. Deep CCR dives require a hypoxic diluent so the loop (and any diluent flush) does not become hyperoxic at depth.

It only matters if you come off the loop. Breathed on open circuit — a bailout, or a diluent flush in the shallows — a hypoxic diluent is unsafe near the surface: it is breathable only from the depth where its PO₂ reaches ~0.16 ata down to its MOD. The calculator reports that breathable/bailout depth range and flags the diluent as hypoxic, so you can plan a normoxic travel or bailout gas for any open-circuit use in the shallow zone.

What's displayed¶

The result shows the diluent O₂/He (what is actually in the cylinder), plus:

Loop composition at bottom — the mix actually breathed at the setpoint (e.g. "32 % O₂ / 37 % He / 31 % N₂")
Loop density at depth
Breathable / bailout range — the depth band where the diluent is safe as OC bailout (from its shallow hypoxic limit down to its MOD)
Hypoxic-diluent flag when the diluent O₂ falls below 10 %

Reading the warnings¶

Below the result, depth-specific information appears:

PPO₂ at depth — what the raw mix will deliver
MOD — at the configured limit
Gas density — color-coded as green/orange/red

If the density warning is yellow or red and Limit gas density is off, toggle it on and re-derive; the resulting mix will satisfy both constraints (if mathematically possible).

Practical examples¶

Typical 40 m tech dive¶

Field	Value
Depth	40
PPO₂	1.40
Trimix	off

Result: EAN28 (28 % O₂). MOD 40 m. Suitable for relatively narcosis-tolerant dives; consider EAN26 or trimix for less narcosis.

Deep tech (75 m)¶

Field	Value
Depth	75
PPO₂	1.40
Trimix	on
Target EAD	30
Limit gas density	on

Result: 16/58 trimix (16 % O₂, 58 % He). Density and narcosis both within limits.

CCR diluent for 60 m, SP 1.3¶

With the default Fixed 10 % strategy:

Field	Value
Depth	60
Trimix	on
Target EAD	30
Limit gas density	on

Result: 10/45 (10 % O₂, 45 % He). Loop at depth: 19 % O₂ / 36 % He / 45 % N₂. Density 4.8 g/ℓ. The 10 % diluent is hypoxic — fine to breathe on the loop, but as open-circuit bailout it is usable only from roughly 6 m down to its MOD, so carry a normoxic travel gas for any OC use in the shallows.

Deep CCR with a Target-PPO₂ diluent (100 m)¶

With Diluent O₂ from target PPO₂ turned on and a 1.0 ata target:

Field	Value
Depth	100
Trimix	on
Target EAD	30

Result: diluent O₂ ≈ 9 % (1.0 ÷ 11 ata), with helium set to satisfy the EAD/density targets.

When to use Best mix vs MOD/END¶

Best mix — you know the depth, you need the mix
MOD/END — you have a mix, you want to know where to breathe it

The two are inverses of each other. Best mix derives a mix from a depth. MOD/END derives depths/limits from a mix.