Hard Water Eats Detergent: A Dosing Rule for Washing Cloth Nappies by Water Hardness Band

This study examines a practical question in everyday hygiene by explicit arithmetic from stated assumptions. The hardness tax alone was 14.8 g in soft water and 74.2 g in hard water, a difference of 59.4 g before any cleaning began. Total recommended doses were 42.8 g (soft), 72.5 g (moderate), 102.2 g (hard) and 136.9 g (very hard). Using the hard-water dose in soft water left an excess of 59.4 g entering the rinses, at 0.99 g per rinse litre, while using the soft-water dose in hard water left the cleaning component short by 58.1 percent, a deficit no extra agitation time in the model could recover. The proposed arrangement is as follows. The deliverable is a banded dosing card for the laundry wall: find the local hardness, from a water report or a test strip, and dose by the row. The rule deliberately keeps the cleaning component constant across bands, because the soil does not care how hard the water is; only the tax changes. All results are derived from the accompanying simulation script, in which every assumption appears as a named, replaceable value. SIMULATED, NOT MEASURED.