An environmental chamber includes an ultrasonic nebulizer assembly for controlling the relative humidity within the chamber. The ultrasonic nebulizer assembly is connected in closed-loop fluid communication with an enclosed chamber of the environmental chamber and includes an ultrasonic nebulizer module to generate water vapor that is introduced into the enclosed chamber. The ultrasonic nebulizer module is constructed to be immersed in water within the ultrasonic nebulizer assembly and is readily replaceable by the user. A nebulizer hour timer is provided to monitor the length of time of the ultrasonic nebulizer is operating to provide a precise indication to the user of how much life is left in the ultrasonic nebulizer before it needs to be replaced.