Cooling plate thickness measurement in a blast furnace

A system for measuring a thickness of a cooling plate, said system comprises: a probe holder unit (10) designed to fit inside a coolant channel (18) of the cooling plate (12), the probe holder unit comprising a housing (30) extending along a length axis (L), an ultrasonic probe (32) being arranged in said housing to be able to transmit and receive ultrasonic waves from a sensor side; a rear housing part (36) being moveably arranged at the rear side, transversally to the length axis, and elastically biased away from the sensor side, thereby permit- ting the probe holder unit to adapt to the cooling channel size; a drive chain (14) to assist the progression of said probe holder unit (10) through the length of the coolant channel, said drive chain linked to first connecting means (48) at said first end of said housing. A cable assembly (60) comprising electric wires connects the ultrasound probe. An encoder arrangement (15) is configured to cooperate with the drive chain such as to measure a length of drive chain passing along it, the encoder arrangement comprising a first gear (76) meshing with said drive chain and coupled to an encoder.