“Method to determine, without contact, the weight per unit area or thickness of thin materials, especially foils, in which a transmitter sends ultrasonic signals to a receiver located on the opposite side of the material, characterized in that the transmitter sends pulses of sound at time intervals greater than that which the sound signal takes to propagate between transmitter and receiver, in that only signals leaving the receiver with an
amplitude exceeding a prescribed level are processed, and in that the length of the signal analyzed per pulse is less than the time of signal propagation from transmitter to receiver”

Practically the system measures the emission, and the energy that is transmitted. The lacking energy is the one that is absorbed by the material that is being measured and is proportional to the weight of the material. The
system effects periodical standardization for a measurement of the density of the air (V0). The frequency of the periodical standardization can be chosen by the operator depending on the environmental changes of temperature in the area where the system operates. Standardization is automatic and it takes 7-10 seconds. Usually a standardization every 3-4 hours is a good frequency.