Abstract: A measuring device scanning from edge to edge of a continuously fed sheet includes two sensing heads spaced apart from the sheet by air cushions whose thickness is not influenced by pressure of supplied air and is subjected to an invariable approaching gravimetric force. The air cushions are preferably formed of air jets from calibrated holes at the inside of both sensing heads, each of said holes being surrounded by annular barriers with circular rims. While the approaching force applied to the upper head is given by the own weight of the latter, the corresponding opposed force on the lower head is provided by a pressure regulator of internal air at the inside of the lower sensing head.