The sample is filled into a container with oscillation capacity.
The eigenfrequency of this container is influenced by the
sample’s mass. This container with oscillation capacity is a hollow,
U-shaped glass tube (oscillating U-tube) which is electronically
excited into undamped oscillation (at the lowest possible amplitude). The two
branches of the U-shaped oscillator function as its spring elements.
The direction of oscillation is normal to the level of the two
branches. The oscillator’s eigenfrequency is only influenced by the part of the
sample that is actually involved in the oscillation.
The volume involved in the oscillation is limited by the stationary
oscillation knots at the bearing points of the oscillator. If the oscillator is
at least filled up to its bearing points, the same precisely
defined volume always participates in the oscillation, thus the
measured value of the sample’s mass can be used to calculate its
In the digital density meter, the
mechanic oscillation of the U-tube is e.g. electromagnetically
transformed into an alternating voltage of the same frequency.
The period τ
can be measured with high resolution and stands in simple relation to
the density ρ
of the sample in the oscillator:
ρ= A∙ τ² - B
A and B are the respective instrument
constants of each oscillator. Their values are determined
by calibrating with two substances of the precisely known densities ρ1 and ρ2. Modern instruments calculate and
store the constants A and B after the two calibration
measurements, which are mostly performed with air and water.
They employ suitable measures to compensate various parasitic influences on the
measuring result, e.g. the influence of the sample’s viscosity and the
non-linearity caused by the measuring instrument’s finite mass.