A system for monitoring reactions with a plurality of receptacle vessels that includes: an incubator; a movable receptacle carrier contained within a temperature-controlled chamber of the incubator; one or more fixed fluorometers configured to measure a fluorescent emission and positioned with respect to the receptacle carrier to measure fluorescent emissions from receptacle vessels carried on the receptacle carrier into an operative position with respect to each fluorometer; one or more fluorescent reference standards mounted on the receptacle carrier; and a controller configured to control operation of the receptacle carrier and the one or more fluorometers to determine if a fluorescent emission intensity of one or more of the fluorescent reference standards deviates from an expected fluorescent emission intensity.