Setpoints¶

A setpoint is the loop oxygen partial pressure (PPO₂) the rebreather electronics aim to maintain. AeroPlus Deco uses up to three setpoints plus a switch depth, plus optional per-segment overrides.

The four configuration fields¶

In the Closed Circuit Setup card on the main plan page:

Field	What it controls
Surface SetPoint	Loop PPO₂ when shallower than the switch depth (surface → switch depth on descent; switch depth → surface on ascent). Typical: 0.7 ata
SP switch depth	The depth at which the dive moves from surface SP to bottom SP. Typical: 20 m
Bottom SetPoint	Loop PPO₂ during the bottom phase, used below the switch depth. Typical: 1.3 ata
Deco SetPoint	Loop PPO₂ at all decompression stops. Typical: 1.3 or 1.5 ata

How the setpoints behave during the dive¶

A typical descent → bottom → ascent → deco profile uses them in this order:

Surface (0 m) ─── Surface SP 0.7 ───→ Switch depth (20 m)
                  ↓ algorithm uses 0.7 ata for tissue saturation

Switch depth (20 m) ─── Bottom SP 1.3 ───→ Bottom (e.g. 60 m)
                       ↓ algorithm uses 1.3 ata throughout this phase

Bottom → Ascent → First stop

First deco stop ─── Deco SP 1.3 (or 1.5) ───→ Last stop (6 m)
                                            ↓ algorithm uses deco SP

Last stop ─── Surface SP 0.7 ───→ Surface

The Surface SP is used during the surface phase and final ascent. The Bottom SP covers the deep portion. The Deco SP covers stops.

What loop PPO₂ does to the diver¶

The loop is a mixture of oxygen and diluent inert. At any depth:

Loop O₂ fraction = setpoint ÷ absolute pressure
Loop inert fraction = 1 − O₂ fraction = (1 − SP/Pabs) of whatever inert mix the diluent provides

For example, at 30 m (Pabs ≈ 4 ata):

Setpoint	Loop O₂	Loop inert
0.7 ata	18 %	82 %
1.0 ata	25 %	75 %
1.3 ata	33 %	67 %
1.5 ata	38 %	62 %

Higher setpoint = more O₂ in the loop = less inert gas = faster off-gassing on ascent.

Setpoint and displayed TTS¶

Most rebreather computers run a fixed bottom setpoint (e.g. 1.3) during the dive and only move to the higher deco setpoint (e.g. 1.5) once you begin the decompression. While you are still on the bottom, the computer's TTS is therefore figured at the bottom setpoint — it has not yet accounted for the richer loop you will actually breathe on the stops. A higher PPO₂ on deco means less inert gas and faster off-gassing, so the real decompression is shorter than the bottom-setpoint TTS implies.

The TTS shown on your handset at depth is therefore conservative — your actual time to surface usually drops once you switch to the deco setpoint. AeroPlus Deco mirrors this: in CCR mode it shows the TTS your computer would display at the bottom setpoint and notes that switching to the deco setpoint shortens it to the planned value. Plan your gas and consumables around the longer (bottom-SP) figure, and treat the drop after the switch as margin in your favour — never the other way round.

Typical setpoint choices¶

Recreational CCR¶

Surface 0.7, switch 6–10 m, bottom 1.0, deco 1.0
Conservative; the lower bottom SP keeps CNS low

Sport tech CCR (40–60 m)¶

Surface 0.7, switch 20 m, bottom 1.3, deco 1.3
Standard for most tech curricula

Aggressive tech CCR (60 m+)¶

Surface 0.7, switch 30 m, bottom 1.3, deco 1.5
Slightly deeper switch and higher deco SP for faster off-gassing
CNS accumulates faster — watch the warning

Hypoxic / cave / wreck penetration¶

Sometimes Bottom SP 1.2 to extend scrubber duration and reduce CNS
Always coordinate with team practice

Per-segment setpoint override¶

For multi-level dives or unusual profiles, you can override the bottom SP per segment. In the segment editor (when CCR mode is on), an SP field appears:

Field	Range	What it does
SP	0.40–1.60 ata	Overrides the bottom SP for this segment only

The segment-level SP only applies during that segment. Descents into / ascents out of the segment use the standard setpoint logic.

Use cases:

A "drift down" segment where you want a lower SP to extend scrubber
A short bounce within a long shallower dive where 1.5 makes sense temporarily
Mixed-team dives where some divers run different setpoints than the algorithm assumes

Reading setpoints in the output¶

The runtime schedule's Loop SP column shows the active setpoint at each row:

↘ 75.0   4   +3:03   SP 1.30   1.51
→ 75.0  15   +11     SP 1.30   1.51
↗ 33.0  19   +4:12   SP 1.30   0.76
→ 33.0  20   +2      SP 1.50   0.76

The fourth column is Loop SP (the planned loop PPO₂). The fifth column is Diluent PPO₂ — the raw diluent's PPO₂ at that depth (what you'd see on a diluent flush or OC bailout to diluent). Diluent PPO₂ is colored red if it exceeds 1.6 ata — a hypoxic-incompatible situation requiring travel/bailout gas to descend safely.

Common questions¶

Why is Surface SP 0.7 and not 0.21 (air)?¶

A surface PPO₂ of 0.7 means the loop has more O₂ than ambient air. It maintains a healthy loop oxygenation during descent before reaching the switch depth. Going too low risks hypoxic loop conditions if the diver swims down with the loop closed.

What about pre-breathe?¶

Most rebreather drills include a 3–5 min pre-breathe to verify the loop and scrubber. AeroPlus Deco doesn't model the pre-breathe explicitly — it starts the calculation from the first segment.

Can I set the bottom SP higher than 1.4?¶

Yes, but be aware of CNS. Bottom SP 1.5 for a long dive accumulates CNS very fast. 1.3 is the practical sweet spot for most tech CCR work.