An apparatus to determine or monitor the amount or the distribution of additional material present in a flow of a flowable substance, said flow of said flowable substance comprising a bulk material and additional material, said apparatus comprising
- a first device to determine the volume flow of said flow of said flowable substance;
- a second device to measure a signal caused by said flow of said flowable substance;
- tools to convert the signal measured by said second device in a signal per unit of volume of said flow of said flowable substance.