Laboratory Products
New approaches in density measurement Dr Barbara Klug-Santner and Siegfried Hold, Anton Paar GmbH,
www.anton-paar.com
As a leader in the development of digital density measurement, Anton Paar knows that new developments in density determination are of great importance in many industries. One major milestone is the patented ‘Pulsed Excitation Method’.
Before digital density meters conquered the market, density was determined with hydrometers and pycnometers. Companies that could afford it would employ the pricier hydrostatic balances. Apart from the high required sample volume, density measurement was tedious and required extensive cleaning. Additionally, there was an ever-present risk of breaking the glassware. A new method was needed.
This was when the measuring cell of Anton Paar’s density meters, a U-shaped glass tube came into the picture. The cell is electronically excited and the frequency of the cell’s oscillation is measured. The oscillation depend on the fi lled-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 millilitres. Today, this way of density measurement has become indispensable in the quality control in countless fi elds of application.
Digital density measurement enabled a measuring accuracy that could be increased to the fi fth decimal place. In order to facilitate daily handling of the instruments numerous helpful developments were introduced. However, the technology itself remained largely unchanged until this way of density measurement no longer met contemporary requirements. The time had come to renew this technology according to present customer demands.
In 2015 Anton Paar set the tone for the future of digital density measurement.
A group of young and ambitious developers began to rethink this technology. The team recognised the weak points in the excitation electronics which were used to excite the U-tube and to measure its oscillating frequency and eventually created an entirely new measuring method: the patented ‘Pulsed Excitation Method’ (AT 514620 (B1), see Figure 1). It is now employed in the laboratory density meters DMA 501, DMA 1001, and the DMA M series (see Figure 2).
This novelty was a milestone in the development of density meters and enabled fundamental improvements of the entire measuring principle. Previously the commonly employed method was a forced oscillation that would be maintained during a measurement. 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. Therefore, the system was never really in equilibrium, but in a constant state of alignment. This, in turn, represented an infl uencing factor that had to be compensated.
Figure 1. Excitation and fade-out with the ‘Pulsed Excitation Method’.
With the new ‘Pulsed Excitation Method’, however, the U-tube is excited to oscillate with a series of impulses (Figure 1) until a constant amplitude is achieved. Then these 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. Both excitation and fade-out are repeated periodically.
The oscillation characteristics of the U-tube are infl uenced by the density, temperature, and viscosity of the fi lled-in sample. Therefore, this new method gathers much more raw data and leads to an even better way of describing the oscillation properties. Since the U-tube is no longer infl uenced in its resonance frequency and the fade-out oscillations are undisturbed this is the only correct way to determine the density accurately and precisely.
In order to convert the obtained raw data into understandable information new algorithms were developed, opening up new possibilities in the fi eld of density measurement. They offer a twofold better viscosity correction when measuring highly viscous samples because the viscosity of a sample adds an extra damping effect to the oscillations of the measuring cell, creating an infl uencing factor that requires correction. Additionally, this new method can even determine the viscosity of Newtonian liquids with an accuracy of 5%. Simultaneous density and viscosity measurement in the range from 10 mPa.s to 3,000 mPa.s allows an improved recognition and compensation of the viscosity’s infl uence on the density result. This is why density meters operating with the ‘Pulsed Excitation Method’ deliver more accurate results. Viscosity correction is now also applicable for density meters with a metallic U-tube like in DMA 4200 M.
Figure 2. Anton Paar’s broad portfolio of digital density meters.
INTERNATIONAL LABMATE - APRIL 2019
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