Gradient factors¶

Gradient factors (GFs) are the conservatism controls layered on top of the Bühlmann ZHL-16C decompression algorithm. They're how you tell the planner "be more conservative" or "less conservative" without changing the underlying tissue model.

The two numbers: GF Low and GF High¶

The Bühlmann algorithm tracks 16 hypothetical tissue compartments and computes, at any moment, the maximum tolerated tissue pressure (the M-value) for each compartment at the current depth. The raw model would let you ascend until any compartment hits exactly its M-value — that's 100 % of allowable.

Most divers want a safety margin under that. Gradient factors give you two:

GF High (GF Hi) — the percentage of the M-value tolerated at the surface. A GF Hi of 85 means "at the surface, stop ascending if any tissue is at 85 % of its M-value." Lower = more conservative.
GF Low (GF Lo) — the percentage tolerated at the first decompression stop (the deepest tissue saturation point of the dive). A GF Lo of 30 means "at the first stop, you may be up to 30 % of the way to the M-value of the leading compartment."

Between the first stop and the surface, the allowed percentage interpolates linearly. Lower GF Lo pushes the first stop deeper; higher GF Lo brings it shallower.

How to read "GF X/Y"¶

The conventional notation is GF Low / GF High:

GF 50/75 — modern default; shallow first stop, moderate surfacing margin (the app's starting point)
GF 50/70 — slightly more conservative shallow, common for divers wanting more surfacing margin
GF 45/75 — slightly more conservative deep, same surfacing
GF 30/85 — older "deep-stop era" approach. Still seen on some computers but produces unnecessarily deep first stops by current understanding (see below)
GF 100/100 — raw Bühlmann, no extra conservatism (do not dive this)

Why deep stops fell out of favor¶

Through the early 2000s, common GF Low values were 20–30 — pushing the first stop very deep, based on the bubble-model thinking that deeper stops would help fast tissues off-gas before they could bubble.

The 2011 US Navy NEDU study (Doolette et al.) compared shallow-stop vs deep-stop ascent profiles head-to-head with Doppler monitoring. The result was unambiguous: deep stops produced more decompression sickness, not less. The slow tissues kept on-gassing during the deep stops, and that on-gassing outweighed any fast-tissue benefit.

Subsequent research has confirmed the finding. The modern recommendation is to set GF Low high enough that the first stop is shallow enough for the leading tissue to actually be off-gassing — typically near the depth of the first deco gas switch. That's what the Andy Davis method achieves automatically.

In practice: GF Low values in the 40–55 range are now standard for OC tech diving. Going below GF Low 35 mandates a deep stop without scientific justification.

Defaults in AeroPlus Deco¶

The app ships with GF 50/75 on a fresh plan — a modern starting point in line with current tech-diving practice. You can change it in Settings → Gradient factor or via the Auto-set GF Low toggle.

Auto-set GF Low¶

For OC tech diving, the app can derive GF Lo automatically using the Andy Davis method: set GF Lo such that the first decompression stop coincides with the depth where your richest enabled deco gas becomes breathable. This maximises off-gassing efficiency on the deco gas rather than wasting time on back-gas deep stops.

Toggle it on in Settings → Auto-set GF Low. The label shows the computed value and the resulting first stop depth.

Why not for CCR?

Andy Davis explicitly notes the method doesn't apply to CCR — rebreather divers breathe an optimal loop mix throughout, so there's no off-gassing-jump benefit to time. The toggle is greyed out in CCR mode. See the Andy Davis method page.

Conservatism is personal¶

There's no universally correct GF setting. DCS susceptibility varies between divers and between days. Cold water, hard work, dehydration, age, and patent foramen ovale all push the curve. Experienced tech divers track how they feel after each dive and tune their GFs gradually.

The Andy Davis article puts it simply: "Start with a low GFhi and slowly throttle back your conservatism over many, many repeated dives."

Practical limits in AeroPlus Deco¶

GF Lo minimum: 10 (any lower and the model will mandate impractically deep stops)
GF Hi maximum: 100 (raw Bühlmann — strongly discouraged)
Minimum delta between GF Lo and GF Hi: 10. The app enforces this on Auto-set GF Low because a delta below 10 (e.g. 70/75) is theoretical — no tech diver runs that close in practice.