Density Meters Based on the Oscillating U-tube Method

Thereby, the liquid to be measured is inserted into a structure which is able top oscillate — a U-shaped tube most of times. The natural frequency of the oscillator, which is changed by the mass of the liquid, is directly related to liquid density. The key benefit is that the frequency represents a value that can easily be digitized that can be measured with almost any desired precision.

With a simple device of this type you can effortlessly measure density with precision of 0.001 g/cm³, whereby temperature precision of about 0.5 K suffices. Yet an increase of precision is made more difficult by diverse interfering influences. As opposed to purely static measurement in a buoyancy balance, the oscillating U-tube is a moving system, for which other laws apply. Hence, measuring density in oscillating U-tube is also influenced by other features of the liquid to be measured such as viscosity, compressibility (sound velocity), specific heat, and thermal conductivity. To recognise these interfering influences, to quantify them, and, ultimately, to correct or compensate them was our tedious task in the past tens of years. Only through this, it was possible to achieve a precision of up to 0.00001 g/cm³.

Moreover, the construction of the oscillator often requires contradicting measures in order to meet the stability criteria and precision requirements. Among these, there are all oscillation-specific technical measures in the measuring cell and its environment in order to obtain a clear and undisturbed frequency that is only correlated to density and temperature. The long-term stability of the measuring frequency is primarily defined by the oscillating material and its preliminary treatment. Only most recent research has recognised that thermoelastic behaviour influences the measuring uncertainty in some oscillating materials.

Backed by the accumulated know-how of almost 50 years, we are now able to estimate the qualities to be expected of new constructions highly accurately by way of formalism proved by experiments.

The devices we have developed have been distributed for laboratory use under the trade name DMA (Dichte Messapparatur), and for process measuring under the abbreviated designations DPR or L-Dens, since 1967.