Weighing the Nappy: A Cloth-Nappy Method for Tracking an Infant's Fluid Balance at Home, with Its Evaporation Error Budget

Whether a sick or small infant is passing enough urine is one of the most useful things a caregiver can know at home, and one of the hardest to judge by eye from a cloth nappy, which absorbs urine invisibly. Weighing the nappy on a kitchen scale converts the question into arithmetic — wet mass minus dry mass is urine mass — but the arithmetic carries errors: cloth nappies differ in dry mass from one another, water evaporates from a wet nappy before it is weighed, and a kitchen scale resolves only so finely. This paper models all three. With stated assumptions — dry nappy mass with a standard deviation of 8 g across nappies, urine of 5 mL/kg per void and seven voids per day, evaporation modelled from temperature and humidity, and a 1 g kitchen scale — a naive method that subtracts the average dry mass and weighs at four-hour intervals without correction has a daily bias of −11.2 g and a daily standard deviation of 21.3 g, and in a hot, dry climate at six-hour intervals its bias reaches −67.2 g per day. The paper originates a protocol — weigh each nappy dry before use (paired tare), change and weigh at most every two hours, seal a nappy in a closed bag if weighing must wait, and apply a printed evaporation correction table by interval and climate band — under which the daily standard deviation falls to 4.0 g with a bias of 0.0 g. The smallest change in daily urine output the protocol can reliably detect for a reference 5 kg infant, whose modelled daily output is 175 mL, is 11.2 g, or 6.4% of the daily total, against 59.1 g (33.8%) for the naive method. The paper prints where the method breaks: a single stool of a stated 10 g counted as urine inflates the day by 5.7% of a 5 kg infant's output, and sweat absorbed into the nappy in hot weather is indistinguishable from urine by mass.