---
title: "Hybrid models"
description: "How running Bühlmann and Thalmann together sets the deeper-ceiling rule, what family of schedules Frontier produces, and which warnings both modes raise."
---

> Documentation Index
> Fetch the complete documentation index at: https://docs.divelogic.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Hybrid models

import { HybridDivergence } from "@/components/react/PlannerScreens";
import { trimixDeco } from "@/demo/plans";

A hybrid model runs Bühlmann ZHL-16C and Thalmann E-L on the same dive at the
same time and resolves their two disagreeing ceilings into one schedule.
DiveLogic offers two ways of doing that: **Hybrid OC**, which always takes the
deeper ceiling, and **DiveLogic Frontier**, which produces a family of schedules
and shows the trade between them. Both use a Thalmann parameter set, so read
[the Thalmann page](/theory/thalmann) before reading a number off either.

## Hybrid OC: the deeper ceiling

At every step both models work out a ceiling, and the binding one is the deeper
of the two. The ascent then follows the rule the single models use: hold at each
stop until the binding ceiling clears the depth above, then go up. Where the
models agree the result is identical to either alone; where they disagree the
more conservative one wins, and you pay for the disagreement in runtime. The
**Bühlmann weight** control does not change the ceiling, only the weighting of
the two models in the risk figure the mode reports.

A hybrid plan reports how far apart the models were: the **maximum
disagreement** between the two ceilings at any stop, the **mean disagreement**
across the stops weighted by how long each lasted, and the fraction of
decompression time each model was binding. When the maximum passes **6 m** the
plan carries a warning naming the figure and suggesting a more conservative
schedule; that check is on by default and can be turned off in the hybrid
settings. A large disagreement is information about the models rather than about
the water.

The plan draws that gap directly. Where the trace sits away from zero the two
models wanted different ceilings, and the deeper of them is the one you dived.

If no schedule reaches the surface, because there are no usable gases or the
profile is one nothing can clear, the planner **falls back to a plain Bühlmann
ZHL-16C plan** and adds a warning saying so. What you are then looking at is not
a hybrid schedule, and that warning is the only thing that tells you.

## Frontier: a family of schedules

Frontier produces several schedules rather than one, each trading decompression
time against accumulated oversaturation at a different rate, from the shortest
ascent the gases permit to a markedly longer and gentler one. They are sorted by
runtime and named **Fast** (the shortest), **Balanced** (nearest the midpoint),
**Robust** (the one the two models argue about least, which is not the safest)
and **Conservative** (the longest).

**The plan you get is the Balanced schedule.** The others are shown in the
Hybrid Analysis view, so if you want the Conservative one you read it off that
view yourself. Seeing fewer than four is normal, because several trades land on
the same answer and duplicates are dropped. Each search also has a time budget
and returns what it has when it runs out, which is why the analysis view can
look sparse without the plan ceasing to be a hybrid plan.

The risk figures Frontier reports order the schedules against each other. **They
are not a probability of decompression sickness**, and nothing in DiveLogic
produces one.

## What a hybrid does not give you

Combining two models does not average out their errors. Both are dissolved-gas
models fitted to different trial data, and where they share an assumption, no
bubbles, no diver-specific physiology, no thermal or exertion term, the hybrid
shares it too. Taking the deeper ceiling makes the schedule longer than either
model asked for. It does not make it right.

## Next

- [Thalmann E-L](/theory/thalmann) is the model on the other half of every
  hybrid.
- [Reviewing a plan](/planning/reviewing) shows where the Hybrid Analysis view
  sits.

Source: https://docs.divelogic.ai/theory/hybrid/index.mdx