Low-Cost Particle (PM) Witness — Design Blueprint: Corrected Indoor PM2.5 Monitor

DESIGN BLUEPRINT — SIMULATED, NOT MEASURED — NOT BUILT. Build-ready design blueprint derived from 'A Corrected Low-Cost Indoor Particle Witness' (DOI: 10.5281/zenodo.24109061). Specifies a corrected low-cost indoor PM2.5 witness: conventional optical particle counter plus a co-located SHT45-class humidity sensor (US$6.50) and firmware correcting hygroscopic growth (kappa-Kohler, assumed kappa 0.30), coincidence loss (sensing volume 0.00020 cm3), and scattering (fixed factor 0.95) interval by interval before 24-hour averaging. Includes bill of materials with source-stated component prices (total US$55.50, US$4.50 below US$60), stated numerical parameters (the source states no physical dimensions, so none are given), assembly, deployment, per-unit reference calibration and readout procedures including the band output near the guideline and refusal above the humidity trust limit, operating envelope with printed failure regimes (fog-level humidity classifies at 49.8% — chance — and must print no classification; smoke under-reads by about a fifth and must not be cleared), maintenance, failure modes, and explicit non-claims. Mean classification accuracy against the WHO 24-hour PM2.5 guideline of 15.0 ug/m3 rises from 81.8% to 89.7% across six non-fog scenarios (SIMULATED, NOT MEASURED).