APPARATUS FOR SAMPLING A MATERIAL TRAVELLING PAST A SAMPLING REGION

A sampling system includes a plurality of sensing means (3) mounted on one or more beams (2) which reciprocate, over a moving web (1) of material. The sensing zones of adjacent sensing means (3) trace out scanning paths which periodically overlap in a sampling region. One of the sensing means (primary) scans over a known reference to update its calibration periodically or continuously and this is transferred to the other sensing means (secondary) effectively via the travelling material (due to the overlap). Calibration is performed by signal processing means, storing and comparing monitoring signal levels or corresponding digital values derived at positions of overlap and making computations for the purpose of calibration. With sufficient overlap of the scanning paths any one of the sensors can be made redundant. Alternatives include sensing means which move relative to other sensing means as well as rotary arrangements.