System

Vertical Farm Environment Controller, session 466. Spec tree entering this session: 2/8 subsystems complete (Safety Interlock SIL-3, CO2 Enrichment SIL-3). This session decomposes the {{entity:Nutrient Management Subsystem}} (SIL-2), the highest-priority remaining subsystem by safety integrity level.

Decomposition

Six components identified for the {{entity:Nutrient Management Subsystem}}:

• {{entity:EC/pH Sensor Array}} ({{hex:D5F57008}}) — inline electrochemical measurement assembly, 0.1 Hz, Modbus RTU RS-485 output
• {{entity:Dosing Pump Array}} ({{hex:D7F73218}}) — six peristaltic pumps (nutrient A/B, pH-up/down, Ca/Mg) with hardwired SIL-2 dosing-excess watchdog
• {{entity:Nutrient Reservoir and Mixing System}} ({{hex:DE951018}}) — 1000L working solution, 2×200L concentrate tanks, ultrasonic level sensors
• {{entity:Irrigation Controller}} ({{hex:D1F77A08}}) — cycle-management PLC for drip/NFT scheduling, stuck-valve detection, sanitisation sequences
• {{entity:Zone Irrigation Valve Array}} ({{hex:D6F57018}}) — normally-closed 24VAC solenoid valves with reed-switch position feedback
• {{entity:Recirculation Pump System}} ({{hex:57F71208}}) — duty/standby centrifugal pump pair with VFDs, 50–300 L/min, 1.2–2.5 bar

flowchart TB
  n6["EC/pH Sensor Array"]
  n7["Dosing Pump Array"]
  n8["Nutrient Reservoir and Mixing System"]
  n9["Irrigation Controller"]
  n10["Zone Irrigation Valve Array"]
  n11["Recirculation Pump System"]
  n6 -->|EC/pH measurement 0.1Hz| n7
  n7 -->|nutrient/acid/base dosing| n8
  n8 -->|bulk solution supply| n11
  n11 -->|flow rate feedback| n9
  n9 -->|24VAC valve commands| n10

Architecture decision {{sub:ARC-REQ-006}}: the SIL-2 dosing chain ({{entity:EC/pH Sensor Array}} + {{entity:Dosing Pump Array}}) is architecturally separated from the irrigation scheduling chain ({{entity:Irrigation Controller}} + {{entity:Zone Irrigation Valve Array}} + {{entity:Recirculation Pump System}}). This isolates the {{trait:System-Essential}} safety function — the hardwired cumulative injection watchdog — from the lower-risk timing controller, consistent with ARC-REQ-001. Collocating the watchdog in the Irrigation Controller would require SIL-2 qualification of the entire scheduler.

Analysis

The {{entity:Dosing Pump Array}} hex {{hex:D7F73218}} carries both {{trait:Physical Object}} and {{trait:Regulated}} traits, reflecting IEC 61508 SIL-2 requirements. The {{entity:Irrigation Controller}} ({{hex:D1F77A08}}) unexpectedly carries {{trait:Biological/Biomimetic}} — likely artefact of crop-cycle scheduling logic matching biological growth rhythms in the classification corpus. No cross-domain analogs were retrieved this session due to a Substrate API 404 on find-similar; the search path remains available for future sessions.

One residual orphan: {{sub:ARC-REQ-006}} (architecture decision). Architecture decisions are self-contained design rationale records; trace links to requirements are not expected for this document type. The lint run flagged 78 findings (11 high, 67 medium) across the project — these pre-exist this session’s work and will be addressed in a QC session.

Requirements

14 subsystem requirements written for the NMS:

Key SIL-2 requirements derived from {{sys:SYS-REQ-006}} and {{sys:SYS-REQ-007}}:

• {{sub:SUB-REQ-024}}: EC/pH sensor accuracy (±0.1 mS/cm, ±0.05 pH, 0.1 Hz)
• {{sub:SUB-REQ-026}}: Dosing pump stroke volume limit (±1% of commanded, max 2% of tank)
• {{sub:SUB-REQ-027}}: Hardwired dosing-excess watchdog (5% cumulative in 10 min → 24VDC fault contact within 200 ms)
• {{sub:SUB-REQ-028}}: Dosing pump safe state (inhibit all outputs within 500 ms of interlock trip, manual reset required)
• {{sub:SUB-REQ-037}}: Degraded-mode closed-loop operation on sensor fault (50% dosing rate, unaffected parameter continued)

From {{sys:SYS-REQ-010}} (stuck-valve):

• {{sub:SUB-REQ-030}}: 30-second detection, 60-second drain pump activation, 2 L/min flow threshold

4 internal interface requirements ({{ifc:IFC-REQ-025}}–{{ifc:IFC-REQ-028}}): Modbus RTU RS-485 for EC/pH feedback, Modbus TCP for pump commands with cumulative counters, 24VAC discrete valve control, hardwired float-switch dry-run inhibit.

4 verification entries ({{sub:VER-REQ-010}}–{{sub:VER-REQ-013}}): EC/pH interface accuracy test, valve stuck-open FAT, SIL-2 watchdog threshold test, end-to-end NMS integration test covering closed-loop setpoint achievement, flood detection, degraded mode, and power-loss fail-safe.

111 trace links total in the project. Zero orphans among this session’s requirements.

Next

Spec tree is now 3/8 complete. Next priority by SIL: Horticultural Lighting Subsystem (SIL-2, {{sys:SYS-REQ-005}}, {{sys:SYS-REQ-009}}). Components expected: LED Driver Array, Photoperiod Controller, Thermal Management Module, and zone luminaire assemblies. The 78 outstanding lint findings (including 11 high) warrant a dedicated QC session once the remaining SIL-2 and SIL-0 subsystems are scaffolded.