Abstract: A liquid sensor includes a light receiver configured to receive light. The light receiver is attachable to an outside of an accommodation unit into which a liquid is introduced and which has translucency. The light receiver receives light that has passed through the liquid in the accommodation unit or light reflected by the liquid, and outputs a signal related to a physical quantity of the liquid.

Abstract: A liquid deterioration determination device includes a light receiver and a determination unit. The light receiver receives detection light transmitted through a liquid or detection light reflected by the liquid. The determination unit determines a degree of deterioration of the liquid. The light receiver outputs first and second detection values. The first detection value represents an amount of a light component of a first color included in the detection light transmitted through the liquid or the detection light reflected by the liquid. The second detection value represents an amount of a light component of a second color included in the detection light transmitted through the liquid or the detection light reflected by the liquid. The second color is different from the first color. The determination unit determines the degree of deterioration of the liquid based on a ratio of the second detection value to the first detection value.