A portable sensor for measuring a hydration level of an object in close physical proximity with the portable sensor, comprising: a portable housing having a total volume of less than about 50 cm 3; and a first circuitry disposed in the housing and comprising: a thermal source; a controller electrically coupled to the thermal source; a temperature sensing element; and a processor coupled to the temperature sensing element, a moisture management layer in thermal contact with the thermal source and the temperature sensing element, the moisture management layer, when substantially dry, having a known time variation of temperature in response to the thermal source being energized with the signal having the known function of time, such that when an object is in close physical proximity with the moisture management layer with the moisture management layer disposed between the object and the thermal source and the temperature sensing element, the moisture management layer controls a moisture transfer rate between the object and the moisture management layer, the thermal source is energized by the controller with the signal having the known function of time, the energized thermal source delivers thermal energy to the moisture management layer, the moisture transfer between the object and the moisture management layer affects a time variation of a temperature of the moisture management layer, the temperature sensing element senses the affected time variation of the temperature of the moisture management layer, and the processor determines a hydration level of the object based on a characteristic of the affected time variation of the temperature of the moisture management layer. a portable housing having a total volume of less than about 50 cm 3; and a first circuitry disposed in the housing and comprising: a thermal source; a controller electrically coupled to the thermal source; a temperature sensing element; and a processor coupled to the temperature sensing element, a thermal source; a controller electrically coupled to the thermal source; a temperature sensing element; and a processor coupled to the temperature sensing element, a moisture management layer in thermal contact with the thermal source and the temperature sensing element, the moisture management layer, when substantially dry, having a known time variation of temperature in response to the thermal source being energized with the signal having the known function of time, such that when an object is in close physical proximity with the moisture management layer with the moisture management layer disposed between the object and the thermal source and the temperature sensing element, the moisture management layer controls a moisture transfer rate between the object and the moisture management layer, the thermal source is energized by the controller with the signal having the known function of time, the energized thermal source delivers thermal energy to the moisture management layer, the moisture transfer between the object and the moisture management layer affects a time variation of a temperature of the moisture management layer, the temperature sensing element senses the affected time variation of the temperature of the moisture management layer, and the processor determines a hydration level of the object based on a characteristic of the affected time variation of the temperature of the moisture management layer.