# components/plant design backlog

Findings from reviewing `pot.py`'s water-bath model against real-plant
behavior (sim/real control mismatch reported by user), refreshed against
the current `delay`-line rewrite of `Pot`.

- [x] **Heat-loss term has wrong units.** Fixed by the `Pot` rewrite:
  `process()` now computes `p_loss = self.L * self.M * (self.temp -
  self.theta_amb)` in Watts and applies `self.temp += (self.delay.get()
  - p_loss) / (self.M * self.C) * self.dt` — the same `/(M*C)`
  normalization for both the gain and loss terms, instead of the old
  mismatched `leak = 1/self.M * self.L * (...)/ self.theta_amb`.

- [x] **Thermal lag modeled as single-pole lag, not transport delay.**
  Fixed: `Pot` no longer uses `HeatDiffusion`'s `alpha=dt/Td`
  exponential smoothing (that module was deleted). It now delays the
  input power through `plant/delay.py`'s `Delay` (true dead time)
  before integrating it, matching what this item recommended.

- [x] **`kn` is loaded but never applied.** Resolved differently than
  originally proposed: `kn` was removed from `Pot` entirely rather than
  wired up as plant-side measurement noise. Sensor noise is now
  modeled one layer up, in `TempSensorSim` (`temp_offset`/`variance`,
  wired from `brewpi.py`'s `TempSensorFactory.create(..., temp_offset=
  -0.15, variance=0.01)`), not in the plant model. The stale
  `Model.kn`/`Plant.kn` keys this item flagged have since been deleted
  from `config-sim.json.tpl`/`config.json.sim` too.

- [x] **`theta_amb` is hardcoded, not configured.** Fixed: added a
  top-level `ambient_temperature` key to `config.json`(`.templ`/`.sim`);
  `brewpi.py` reads it once and passes it to both the real `Pot` and
  (via a new `theta_amb` param) `temp_controller_smith.py`'s internal
  models, so the Smith predictor's model matches the real plant's
  assumed ambient instead of silently defaulting to 20. `Pot` also
  dropped its separate `params['theta']` initial-temperature field —
  it now always starts at `theta_amb` (so the same config value governs
  both the assumed ambient *and* the pot's starting temperature; there's
  no longer a way to start a sim at a temperature other than ambient).
  Real ambient temperature still isn't modeled as a *drifting*
  disturbance (it's a static config value, not a live sensor reading,
  though `Pot.set_ambient_temperature()` now exists as a hook for that),
  so robustness to ambient drift remains untested.

- [ ] **No actuator/process realism.** Partially improved (`process()`
  clamps `self.temp` to `min(100, ...)`, won't simulate past boiling)
  and partially regressed: `Pot` also dropped its `gain` (heater
  efficiency) factor entirely — `process()` now applies 100% of
  commanded power with no loss, whereas before `params['gain']` could
  model e.g. an 80% efficient transfer. `Model.gain`/`Plant.gain` were
  dead keys in `config.json.sim` (already absent from `.templ`) before
  that file was removed entirely — see `components/pid/TODO.md`'s
  config-validation item. Still no
  heater power saturation enforced inside `Pot` itself (handled ad hoc
  in demo harnesses), no evaporation/lid heat loss, and no varying
  thermal mass `M` as volume changes (e.g. boil-off, adding
  water/grain) — `M` is a fixed constant for the whole simulation.

- [x] **Debug `print()`s left in `Pot.process()`.** Fixed: the
  unconditional `p_loss`/`theta_amb`/`temp` prints (leftover from the
  heat-loss-units fix) were removed.