This test method is based on the loss in weight of a coal sample in an air atmosphere under rigidly controlled conditions of temperature, time, and airflow. a provision for renewing the preheated air in the oven at the rate of one to four times a minute, with the dry air. an exhaust change-over valve allows the output flow meter to be by-passed during operation, preventing the build of condensation in the flow meter. temperature range : above ambient to 150â°c â± 2â°c. timer provided to set the operation time. 230 v, 50 hz. Power supply.