Accuracy of moisture measurement behind the fiber dryer is a crucial size for the entire further production process. Due to the condition of the fiber (low density, glue deposit) an exact and long-term-stable moisture test was so far hardly possible. Through special developments, e.g. the reference resonator, it is now possible to measure directly behind the dryer. There is no necessary for "fiber collecting mechanisms", as the fiber can be directly measured in the product river. If desired, the measuring system is also available as explosion proof versions.