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Density measurement redefined New Developments in the Field of Density Measurement

| Author / Editor: Barbara Klug-Santner & Siegfried Hold* / Marc Platthaus

Density determination of liquid, gaseous, and paste-like samples is of great importance in many industries. Anton Paar has been dominating the development of density determination for decades.

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Anton Paar has brought density measurement systems onto the market that work with a new measurement method (symbol image).
Anton Paar has brought density measurement systems onto the market that work with a new measurement method (symbol image).
(Source: Anton Paar)

Before digital density meters conquered the market, density was determined with hydrometers and pycnometers — or very rarely using a hydrostatic balance, a means of density determination that most companies could not afford. Apart from the high required sample volume, density measurement was tedious, required extensive cleaning, and there was an
ever-present risk of breaking the glassware.

The measuring cell of Anton Paar’s density meters, a U-shaped glass tube, is electronically excited and the frequency of the cell’s oscillations is measured. The oscillations depend on the filled-in liquid, gaseous, or paste-like sample, and correlate directly with the density of the respective sample. Thus, a fast, repeatable, and precise method was found that gets by with very small sample volumes of only a few milliliters. Today, this way of density measurement has become indispensable in the quality control in countless fields of application.

Gallery with 5 images

Since the introduction of digital density measurement, the achievable accuracy could be increased to the fifth decimal place. Functions such as Filling Check, which is a metrological registration of interfering particles and gas bubbles in the sample, and numerous other helpful developments were introduced to facilitate the daily handling of the instruments. The technology remained largely unchanged over the following years until, finally, this way of density measurement no longer met contemporary requirements. It had reached its limits. The time had come to question this technology and to radically rethink it, improve it, and renew it according to present user demands. The true revolution of digital density measurement started in 2015.

Pulsed excitation for better results

A dynamic group of young and ambitious developers began to rethink this technology from scratch. The team recognized the weak points in the excitation electronics which were used to excite the U-tube and to measure its oscillating frequency. The team eventually created an entirely new measuring method: the patented “Pulsed Excitation Method” (AT 514620 (B1), see Figure 1), which is now employed in the laboratory density meters DMA 501, DMA 1001, and the DMA M series (see Figure 2).

This milestone in the development of density meters represents a fundamental improvement of the entire measuring principle. For decades a so-called forced oscillation was maintained during a measurement as this was the state of the art at the time.

If the measured frequency and the resonance frequency of the U-tube were different, the exciting frequency was readjusted until it was identical to the resonance frequency. As a consequence, the system was never really in equilibrium, but instead in a constant state of alignment. This, in turn, represented an influencing factor that had to be compensated. With the new “Pulsed Excitation Method”, however, the U-tube is excited to oscillate with a series of impulses (as can be seen in Figure 1) until a constant amplitude is achieved. Then the impulses are stopped and the fade-out properties of the U-tube are monitored. During the fade-out period, the amplitude is measured precisely before the next excitation impulse is initiated. Excitation and fade-out are repeated periodically.

The oscillation characteristics of the U-tube are subject to influence from the density, temperature, and viscosity of the filled-in sample. This new method allows much more raw data to be obtained and, as a consequence, leads to an even better way of describing the oscillation properties. The U-tube is no longer influenced in its resonance frequency, the fade-out oscillations are undisturbed. This is the only correct way to determine the density accurately and precisely.

Completely new algorithms were developed to convert the raw data into understandable information that can be interpreted by the operator. These algorithms open up new possibilities in the field of density measurement. They offer a twofold better viscosity correction when measuring highly viscous samples because the viscosity of a sample adds an extra dampening effect to the oscillations of the measuring cell. This influencing factor has to be corrected.

This new method is even capable of determining the viscosity of Newtonian liquids with an accuracy of 5%. Viscosity measurement in the range from 10 mPa.s to 3,000 mPa.s simultaneously with the density allows even better recognition and compensation of the viscosity’s influence on the density result. This is why density meters operating with the “Pulsed Excitation Method” deliver more accurate results. Viscosity correction is from now on also applicable for density meters with a U-tube made from metal as is the case with DMA 4200 M.