The present document discloses a device for monitoring feeding of granules, comprising a duct (12, 111) defining a granule flow path； and a sensor (100, 200, 300, 400, 500). The sensor comprises at least one radiation source (101, 201, 301, 401, 501) and at least one radiation sensor (102, 202, 302, 402, 502), arranged to receive radiation from the radiation source. The radiation source and the radiation sensor are adapted to be arranged so that incident radiation from the radiation source to the radiation sensor can be acted upon by the granules. The sensors also comprise a sensor body (103, 203a, 203b, 303a, 303b, 403a, 403b, 503a, 503b), in which said radiation source and/or radiation sensor is arranged. The sensor body is insertable through at least one opening in a wall defining the duct, by a distance corresponding to at least a width of the duct, viewed in a direction of insertion.
