A Water Witness for Clinics: Design and Accuracy Budget of a Low-Cost Photometer for Free Chlorine and Turbidity at the Point of Care

Design study of a low-cost dual-channel photometer for clinics: free chlorine by DPD colour development at ~515 nm (Beer-Lambert absorption, 525 nm LED and BPW34 photodiode, 10 mm cuvette) and turbidity by 90-degree scatter at 860 nm. The computed free-chlorine detection limit is 0.063 mg/L with standard uncertainty about 0.020 mg/L across the WHO 0.2-0.5 mg/L residual band; the turbidity detection limit is 0.159 NTU. Parts cost is US $37.90 plus $0.08 per test. The device can flag a lost chlorine residual and adjudicate the 5 NTU limit, but cannot certify samples within about 0.04 mg/L of either chlorine band edge or adjudicate the 5 mg/L health-based value without dilution. SIMULATED, NOT MEASURED: no device was built.