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In order to improve the predictability of sprinkler response time for a confined ceiling, thermal properties of sprinkler and environment of sprinklers below a confined ceiling during fire are discussed. Significance of the heat loss from the fusible link of a sprinkler by conduction and a method of estimating the response of the link to external radiation are shown. Measurements of temperature, velocity, radiation, and sprinkler response time below a ceiling are made in a confined enclosure. The measured temperature and velocity are found to be close to previous theory for an unconfined ceiling if a correction for the existence of smoke layer is applied. Finally measured sprinkler response time is compared with calculation based on measured temperature, velocity and radiation history. The result shows the significance of radiation from the fire source in the determination of sprinkler response time.