ultrasound...

Nowadays, the measurement is done via ultrasound examination, similar to an ultrasound as in pregnancy testing. The time between the signal that is being transmitted and the time that the sensor is back by reflection determines the thickness of the plate. Usually at the place where the expert has put a cross, first the paint is ground away by the yard or the skipper and then the expert puts a lick thixotropic gel on the clean spot, places the sensor off his meter and reads the value.