A device for measuring the humidity in the internal volume of a case of a watch, which device includes a watch including a case housing an optical system, the optical system including a transmitter transmitting a light beam, a multi-pass cell through which the light beam is intended to pass, and a receiver intended to receive the light beam once it has passed through the multi-pass cell; a management module external to the case, including a processing unit configured to communicate with the optical system and to process data transmitted thereby in order to determine the humidity present in the internal volume of the case on the basis of the absorption of the light beam by the water vapour that may be present in the internal volume of the case.

A dryness fraction measuring device, including a variability quantity measuring portion that measures the amount of variability in intensity of light that has been transmitted, or amount of light that has been absorbed, by moist steam that is the subject of measurement; and a dryness fraction measuring portion that measures the dryness fraction of the moist steam based on a correlation relationship between the dryness fraction and the amount of variability in the optical intensity or in the amount of light absorption.

A device for measuring the humidity in the internal volume of a case of a watch, which device includes a watch including a case housing an optical system, the optical system including a transmitter transmitting a light beam, a multi-pass cell through which the light beam is intended to pass, and a receiver intended to receive the light beam once it has passed through the multi-pass cell; a management module external to the case, including a processing unit configured to communicate with the optical system and to process data transmitted thereby in order to determine the humidity present in the internal volume of the case on the basis of the absorption of the light beam by the water vapour that may be present in the internal volume of the case.