This paper describes model testing of bathtub with the purpose of remaining the water temperature in the bathtub constant and reducing water waste. I establish a model and draw a conclusion. The model describes changes in the temperature of the bathtub water. The model has two parts: the one is endothermic process that a constant trickle of hot water reheats the bathing water and the other one is exothermic process. The exothermic process is divided into three parts: on the water surface, on

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... bathtub wall and between bathtub bottom and ground. Finally, I get a formula about the temperature of the water according to the theory of thermology. I find that it is effective for bubble bath additive to help to keep the temperature constant in the extended model I carry out numerical calculation by using Runge-Kutta method and compare analytic and numerical results with reality, using default parameters; I validate that my method is correct and robust.