# diffuser

> **Reflects code as of `8cc02ee` · regenerated `2026-05-11` via `npm run wiki:all`**
> If this banner is stale, the page may be out of date. Treat as informative, not authoritative.

## 1. What this node is

**diffuser** models an aeration-diffuser zone. Given header pressure, water-column height, alpha factor, element count and airflow, it interpolates a supplier OTR curve, normalises airflow to Nm³/h, and emits oxygen-transfer rate plus a reactor-zone OTR for the downstream parent. Used as a leaf Equipment Module under a `reactor`.

## 2. Position in the platform

```mermaid
flowchart LR
    src[blower / MGC / dashboard]:::unit -->|data.flow| diff[diffuser<br/>Equipment]:::equip
    setters[dashboard setters]:::ctrl -->|set.density / set.water-height /<br/>set.elements / set.alfa-factor /<br/>set.header-pressure| diff
    diff -->|child.register| reactor[reactor<br/>Unit]:::unit
    classDef unit fill:#50a8d9,color:#000
    classDef equip fill:#86bbdd,color:#000
    classDef ctrl fill:#a9daee,color:#000
```

S88 colours: Unit `#50a8d9`, Equipment `#86bbdd`, Control Module `#a9daee`. Source of truth: `.claude/rules/node-red-flow-layout.md`.

## 3. Capability matrix

| Capability | Status | Notes |
|---|---|---|
| Supplier OTR curve interpolation | ✅ | Density-keyed; falls back to single key when only one available. |
| Air-density correction (header + atm) | ✅ | `_calcAirDensityMbar` per ideal-gas mix. |
| Per-element flow tracking | ✅ | `o_flowElement = nFlow / nElements`. |
| Static head loss from water column | ✅ | `_heightToPressureMbar`. |
| Warning / alarm bands on flow-per-element | ✅ | Hysteresis 2 % (warn) / 10 % (alarm). |
| Reactor-zone OTR for parent | ✅ | `oZoneOtr` derived from `diffuser.zoneVolume`. |
| Idle handling at flow ≤ 0 | ✅ | Resets all derived outputs to zero. |
| Typed `MeasurementContainer` emission | ❌ | All output flows via `getOutput()`; no typed series yet. |

## 4. Code map

```mermaid
flowchart TB
    subgraph nodeRED["nodeClass.js — adapter (BaseNodeAdapter)"]
        nc["buildDomainConfig()<br/>static DomainClass, commands"]
    end
    subgraph domain["specificClass.js — orchestrator (BaseDomain)"]
        sc["Diffuser.configure()<br/>loads supplier specs<br/>setters → _recalculate()"]
    end
    subgraph concerns["src/ concern modules"]
        cmds["commands/<br/>topic registry + handlers"]
    end
    nc --> sc
    nc --> cmds
```

| Module | Owns | Read first if you're changing… |
|---|---|---|
| `commands/` | Input-topic registry + per-topic handlers | Topic naming, payload validation. |
| `specificClass.js` (single file) | Setters, OTR/ΔP curve interpolation, alarms, output composition | Anything domain-side. |

The diffuser was a small node so the P6.4 refactor did not split it into per-concern directories. Future work may extract `curves/` and `alarms/` if the file grows past ~250 lines.

## 5. Topic contract

> **Auto-generated** from `src/commands/index.js`. Do NOT hand-edit between the markers. Re-run `npm run wiki:contract`.

<!-- BEGIN AUTOGEN: topic-contract -->

| Canonical topic | Aliases | Payload | Unit | Effect |
|---|---|---|---|---|
| `data.flow` | `air_flow` | `number` | `volumeFlowRate` (default `m3/h`) | Push the measured air flow into the diffuser model. |
| `set.density` | `density` | `number` | — | Update the air density used in OTR / SOTR calculations. |
| `set.water-height` | `height_water` | `number` | — | Update the water column height above the diffusers (m). |
| `set.header-pressure` | `header_pressure` | `number` | — | Update the header (supply) pressure feeding the diffusers (mbar). |
| `set.elements` | `elements` | `number` | — | Update the count of active diffuser elements. |
| `set.alfa-factor` | `alfaFactor` | `number` | — | Update the alfa factor used in oxygen-transfer correction. |

<!-- END AUTOGEN: topic-contract -->

## 6. Child registration

diffuser is a leaf node — it accepts no children. It registers itself with its upstream parent (typically a reactor) at startup via the standard Port-2 handshake.

```mermaid
flowchart LR
    diff[diffuser]:::equip -->|child.register payload=node.id<br/>positionVsParent=atEquipment| reactor[reactor / parent]:::unit
    classDef equip fill:#86bbdd,color:#000
    classDef unit fill:#50a8d9,color:#000
```

| Direction | Counterparty | Side-effect |
|---|---|---|
| outbound at startup | upstream reactor | sends `child.register` on Port 2 with `positionVsParent` default `atEquipment` |
| inbound | — | none accepted |

## 7. Lifecycle — what one event does

```mermaid
sequenceDiagram
    participant src as upstream source
    participant diff as diffuser
    participant curve as supplier specs
    participant out as Port-0

    src->>diff: data.flow (Nm³/h)
    diff->>diff: setFlow → _recalculate
    alt flow ≤ 0
        diff->>diff: idle = true, reset derived outputs
    else flow > 0
        diff->>diff: air-density correction (atm + header)
        diff->>curve: interpolate OTR by density + flow/element
        diff->>curve: interpolate ΔP curve by flow/element
        diff->>diff: kg O₂/h, combined efficiency, slope
        diff->>diff: _checkLimits (warn / alarm bands)
    end
    diff->>diff: notifyOutputChanged()
    diff->>out: msg{topic, payload (delta-compressed)}
```

## 8. Data model — `getOutput()`

What lands on Port 0. Composed in `Diffuser.getOutput()`, then delta-compressed by `outputUtils.formatMsg`.

<!-- BEGIN AUTOGEN: data-model -->

| Key | Type | Unit | Sample |
|---|---|---|---|
| `alarm` | array | — | `[…]` |
| `efficiency` | number | — | `0` |
| `iFlow` | number | — | `0` |
| `iMWater` | number | — | `0` |
| `iPressure` | number | — | `0` |
| `idle` | boolean | — | `true` |
| `nFlow` | number | — | `0` |
| `oFlowElement` | number | — | `0` |
| `oKgo2H` | number | — | `0` |
| `oOtr` | number | — | `0` |
| `oPLoss` | number | — | `0` |
| `oZoneOtr` | number | — | `0` |
| `slope` | number | — | `0` |
| `warning` | array | — | `[…]` |

<!-- END AUTOGEN: data-model -->

`oZoneOtr` is `kg O₂ / m³ / day`; it is `0` when `diffuser.zoneVolume` is unset.

## 9. Configuration — editor form ↔ config keys

```mermaid
flowchart TB
    subgraph editor["Node-RED editor form"]
        f0[Zone number]
        f1[Element count]
        f2[Diffuser density]
        f3[Water height]
        f4[Header pressure]
        f5[Alpha factor]
        f6[Zone volume]
    end
    subgraph config["Domain config slice"]
        c0[diffuser.number]
        c1[diffuser.elements]
        c2[diffuser.density]
        c3[diffuser.waterHeight]
        c4[diffuser.headerPressure]
        c5[diffuser.alfaFactor]
        c6[diffuser.zoneVolume]
        c7["diffuser.localAtmPressure (hidden)"]
        c8["diffuser.waterDensity (hidden)"]
    end
    f0 --> c0
    f1 --> c1
    f2 --> c2
    f3 --> c3
    f4 --> c4
    f5 --> c5
    f6 --> c6
```

| Form field | Config key | Default | Range | Where used |
|---|---|---|---|---|
| Zone number | `diffuser.number` | `1` | int ≥ 1 | node label (`name_N`) |
| Element count | `diffuser.elements` | `1` | int ≥ 1 | per-element flow |
| Diffuser density | `diffuser.density` | `2.4` | > 0 | OTR curve key |
| Water height | `diffuser.waterHeight` | `0` | ≥ 0 (m) | static head + kg O₂/h |
| Header pressure | `diffuser.headerPressure` | `0` | ≥ 0 (mbar) | air density correction |
| Alpha factor | `diffuser.alfaFactor` | `0.7` | 0–1 | oxygen-transfer correction |
| _(hidden default)_ | `diffuser.localAtmPressure` | `1013.25` | > 0 (mbar) | density baseline |
| _(hidden default)_ | `diffuser.waterDensity` | `997` | > 0 (kg/m³) | static head |
| Zone volume | `diffuser.zoneVolume` | `0` | ≥ 0 (m³) | `oZoneOtr` |

## 10. State chart

Skipped — diffuser is stateless. Every input setter recomputes the full output snapshot; there are no transitions to track. The `idle` flag is a derived predicate (`i_flow ≤ 0`), not a state.

## 11. Examples

| Tier | File | What it shows | Mandatory? |
|---|---|---|---|
| Basic | `examples/01-Basic.flow.json` | Inject `data.flow` + dashboard, no parent | ✅ |
| Integration | `examples/02-Integration.flow.json` | diffuser registered under a reactor zone | ✅ |
| Dashboard | `examples/03-Dashboard.flow.json` | Live FlowFuse charts (OTR, kg O₂/h, efficiency) | ⭕ |

Screenshots under `wiki/_partial-screenshots/diffuser/` when produced. Docker compose snippet under `examples/README.md`.

## 12. Debug recipes

| Symptom | First thing to check | Where to look |
|---|---|---|
| `oOtr` stuck at zero | `i_flow` is zero or negative → `idle = true`. | `_recalculate` early-return |
| Warning / alarm always firing | Flow-per-element outside curve `minX` / `maxX` ± hysteresis. | `_checkLimits` |
| `oZoneOtr` is zero despite valid OTR | `diffuser.zoneVolume` is unset or non-positive. | `getReactorOtr` |
| `nFlow` differs from `iFlow` at non-zero flow | Air-density correction — header pressure or atm differ from reference. | `_calcAirDensityMbar` |
| `efficiency` flat at 0 | OTR or ΔP curve span is zero in the operating band. | `_combineEff` |

> Never ship `enableLog: 'debug'` in a demo — fills the container log within seconds and obscures real errors. Use only for live debugging.

## 13. When you would NOT use this node

- Use diffuser for a **fine-bubble aeration zone** with a supplier OTR curve. For coarse-bubble or jet aeration, model OTR externally.
- Don't use diffuser when the upstream blower already publishes oxygen-transfer telemetry — diffuser duplicates the calculation.
- Skip diffuser if you only need flow-per-element warning bands without OTR — a `measurement` node with bands is lighter.

## 14. Known limitations / current issues

| # | Issue | Tracked in |
|---|---|---|
| 1 | **Port-count change (Phase 6):** pre-refactor the diffuser exposed 4 outputs (process, dbase, reactor control with `topic: 'OTR'`, parent registration). The reactor-control message merged into Port 0 as `oZoneOtr`; consumers reading the dedicated control port must migrate to `payload.oZoneOtr`. No alias is provided — the shape differs (single value vs full process payload). | CONTRACT.md `## Port-count change` |
| 2 | Supplier specs are hard-coded inside `_loadSpecs()` (GVA / ELASTOX-R). A configurable supplier registry is pending. | `specificClass.js _loadSpecs` |
| 3 | No typed `MeasurementContainer` emission — `oOtr` / `oZoneOtr` cannot be subscribed via the generic `ChildRouter` handshake. Parents must read Port 0 messages. | CONTRACT.md `## Events emitted` |
| 4 | Warning / alarm thresholds are fixed (2 % / 10 % hysteresis); not yet config-driven. | `configure()` literals |
| 5 | **Node category mismatch:** `diffuser.html` registers under `category: 'wbd typical'` instead of `'EVOLV'`. All other platform nodes target `'EVOLV'` for consistent palette grouping in the editor. | `diffuser.html` line 3 |
| 6 | **S88 colour — resolved:** `diffuser.html` already declares `color: '#86bbdd'` (Equipment Module). The WIKI_TEMPLATE §16 note listing diffuser as having "no colour set" is stale and can be removed in the next template refresh. | `diffuser.html` line 4 |