Oscillating U-tube

A bending transducer is a device for determining the density of liquids and gases. The density is calculated from the measurement of the natural frequency of a bending vibrator filled with the medium. The principle of measurement is thus based on a spring - mass oscillator whose mass is partially formed by the medium to be measured.

The sample is introduced into an oscillating U-shaped glass tube, the two arms form the spring elements of a bending vibrator, similar to a tuning fork. The direction of vibration is normal ( perpendicular ) to the plane of the two legs. The tube is excited by electronic means to an undamped oscillation with minimum amplitude.

The natural frequency of the oscillator is influenced by that part of the sample mass which is actually participating in the oscillation. The participating to the oscillation volume is limited by the quiescent oscillation node at the clamping of the vibrator.

Is the vibrator filled at least to the clamping points with the sample, always take the same well-defined volume participates in the oscillation and the mass of the sample can therefore be assumed proportional to its density.

Overfilling of the vibrator on the clamping points out is irrelevant for the measurement. For this reason, the densities of the media can be measured by the transducer, the flow through the oscillator ( continuous measurement).

Digital oscillating U-tube gauges

The digital density meter of the bending transducer, the mechanical vibration is electromagnetically converted to an AC voltage of the same frequency, for example. The period τ is measurable with high resolution and is available in a simple manner with the density ρ of the in- Schwinger medium in context:

A and B are device constants of the respective vibrator. Their values ​​are determined by calibration at two substances with precisely known densities ρ1 and ρ2. Modern equipment calculated according to the two calibration measurements, which often air and water are used, the constants A and B and store them. They compensated by appropriate measures various parasitic influences on the measurement result, for example, the influence of the viscosity of the sample and the non-linearity caused by the finite mass of the meter.

The first digital density meter for liquids and gases, which determined the density by means of the oscillating U-tube method of Hans Stabinger and Hans Leopold was in 1967 presented by Anton Paar GmbH at Achema.