SIH26118 | Passive Colorimetric H2S Exposure-Dosimeter Wristband with AI-Based Quantitative Reading

H₂S-ECHO is a passive cartridge with no battery that records a worker's exposure to hydrogen sulfide (H₂S) over a shift, and it can be reset and used again. It fills a gap between two tools that already exist. Real time alarms tell you if H₂S is present right now. Normal passive dosimeters tell you the total exposure over the shift. Neither one tells you how the exposure arrived over time. We want to find out if that timing can be kept inside the cartridge and read back later. The cartridge uses a reversible cobalt phthalocyanine (CoPc) sensing layer and three separate gas paths: fast, medium and slow. All three sensing zones use the same chemistry. The paths are different lengths on purpose, so the gas reaches each zone at a different speed and each zone responds on its own time scale. Read together, the three zones give a fingerprint of how the exposure behaved. Our main hypothesis is that two exposures with the same total dose can leave fingerprints that can be told apart. A short exposure at high concentration and a long exposure at low concentration can add up to the same ppm·h, yet the fast, medium and slow zones should respond differently to each. If that holds, the timing information is already written into the cartridge before any digital analysis starts. The cartridge needs no power while it is worn. H₂S comes in through a water-repelling (hydrophobic) gas interface and moves along the transport paths to the sensing zones. After the shift, the cartridge goes through a controlled recovery. Based on its measured state, the system marks it as FIT, RECOVERING, READY, EXPIRED or INDETERMINATE. The smartphone is only the reader. It does no chemical sensing. The cartridge sits in a cradle with controlled lighting and optical reference patches. The app takes the photo, checks the image quality, identifies the cartridge, picks out the S1, S2 and S3 zones, corrects for lighting and builds a digital fingerprint. The software then checks the readout timing, the recovery, the agreement between channels and whether the reading falls inside the calibrated range. Only after that does it give a dose, an exposure pattern and a confidence level. We built the system to check before it trusts. The critical checks cover image quality, cartridge identity, cartridge fitness, recovery state, readout timing, optical references, sentinel condition, channel agreement, calibration range, dose confidence and pattern confidence. If the evidence is too weak, the app does not force out a number. It returns RETAKE REQUIRED, RECOVERING, OUT-OF-RANGE, INDETERMINATE or DO NOT USE. For the worker the whole thing is five steps: scan, wear, work, read, result. Each result is linked to the worker, the shift, the cartridge, the exposure, the readout time, the calibration version and the validity state, so every exposure record can be traced. A central web platform holds worker history, shift records, cartridge life cycle management, exposure analytics, reports and an audit trail. H₂S-ECHO is not meant to replace real time H₂S alarms or emergency monitoring. Alarms protect the worker in the moment. H₂S-ECHO keeps the record of what the shift was like. The project brings together reversible chemical sensing, passive multi-timescale encoding, smartphone optical reading, software inference, independent validity checks, cartridge recovery and digital records in one design that can be deployed in the field. Later work could move towards chemical memory that holds more information, more advanced sensing materials, models that predict reaction and transport, digital twins, richer optical readout, uncertainty modelling, precision manufacturing, independent standards and cartridges for other chemicals. An H₂S exposure is gone the moment it ends. H₂S-ECHO turns it into safety information that stays on record, can be traced and can be analysed.