A Filter That Silently Dies Is Worse Than None: Design of a Passive End-of-Life Witness Driven by Volume, Time and Flow

SIMULATED, NOT MEASURED. A passive end-of-life witness is designed (parts US$1.72) integrating volume, time (dye-wick) and flow clocks, with computed triggers of 6256 L for ceramic and 20800 L for ultrafiltration and a 30-day time limit for charcoal cartridges that a volume counter alone would falsely certify. Design concept only; nothing was built or field-tested.