A modern digital density meter – based on the U-tube principle – measures the inertial mass of a known sample volume. The sample is filled into a U-shaped tube that is mounted on a countermass. The U-tube is then excited and starts to oscillate. Depending on the filled-in sample, the characteristic frequency of the oscillating tube differs. The change in frequency is measured and the density can be determined. High frequency can thus be related to low density and vice versa.