Abstract: The invention relates to a tracing apparatus operating automatically on the basis of digital imputs.A buffer-loop is formed in the paper band between each drive roll and the control drum, and the loops so formed are balanced by a loop corrector/reader device. In one prior art apparatus, this device includes photoelectric cells.The tracing apparatus is characterized in that it includes three suction chambers, one for each buffer-loop and a third central chamber beneath the control drum, whereby two saggings of the paper are obtained on either side of the control drum that are entirely independent of the two buffer-loops.

Abstract: The invention relates to a negative pressure or depression detector characterized in that it includes a floating element which forms at once a detecting element and the movable disc or plate of an air capacitor.The floating element is placed inside a chamber having its lower end vented and its upper end communicating with the depression source, the fixed plate of the capacitor being located at the lower end of the chamber without interfering with the venting.