Laboratory Products
Digital Density Measurement Redefi ned Dr Karin Biebernik, Anton Paar GmbH Anton Paar’s improved digital density meters open the door to new insights into many fi elds of application.
In 1967, digital density meters by Anton Paar revolutionised the formerly complicated and time-consuming ways of density determination. The measuring cell in Anton Paar’s density meters was a U-tube made of glass. This U-tube, the heart of the instrument, was continuously excited and its oscillations were monitored electronically. The higher the density of the sample fi lled into the U-tube, the slower the oscillations. This oscillating U-tube principle led to fast, repeatable, and unbelievably precise density results with surprisingly small sample volumes. Within a short period of time, this new technology conquered the market in many fi elds of application. Nowadays, digital density meters are indispensable devices for the quality control of all kinds of liquid, paste-like, and even gaseous samples.
For many years this patented technology remained unchallenged until Anton Paar’s ingenious research and development team realised that there was still potential for improvement. The U-tube is infl uenced by factors such as glass inhomogeneity, temperature changes, and the magnets attached to the glass cell to initiate the oscillations. The team worked hard and succeeded in eliminating these drawbacks. In 1997, in a fi rst step, the development of an improved measuring cell made the magnets obsolete. In the next step, a reference oscillator was introduced which compensated the infl uence of temperature changes on the glass. Finally, in 2000, elaborate handicraft and machine processing were combined and infl uences on the glass cell were reduced to a minimum. Thus, optimum hardware was accomplished.
The more insight and awareness of the limitations the R&D team gained, the more their ambition was aroused. They saw that further improvement to the electronics was still possible. Until then, the U-tube was electronically excited, and the oscillating frequency measured. If the oscillating frequency was not identical to the U-tube’s resonance frequency, the electronics fi ne-tuned the excitation until the resonance frequency was reached. As such, the system never was in equilibrium but in a continuous state of alignment – another infl uencing factor which had to be compensated.
The breakthrough could be achieved in 2015 with the Pulsed Excitation Method, in short: PEM. The U-tube is excited to oscillate with a series of impulses until a constant amplitude is reached (as can be seen in Figure 1). Then the excitation pulses are stopped. The properties of the U-tube during the fade-out period are monitored, and the amplitude is measured precisely before the next excitation impulse is initiated. Excitation and fade-out are repeated periodically. This way, even more precise results are delivered as the decay of the U-tube is not infl uenced in its resonance frequency movement at all.
Figure 1. Principle of the Pulsed Excitation Method. All for the Best
Thanks to this new Pulsed Excitation Method, considerable improvements of Anton Paar benchtop density meters (Figure 2) could be accomplished in several ways.
Highest precision can be achieved because the improved viscosity correction for highly viscous samples leads to more precise density results. PEM allows continuous monitoring of the correct functioning and immaculate state of the glass measuring cell. With PEM, the viscosity correction also works for density meters with a U-tube made from metal as is the case with DMA 4200 M. Thus, on top of their robust and reliable construction and design, Anton Paar density meters convince with a unique repeatability of better than 1*10-6
g/cm³.
A new insight for Newtonian fl uids is now possible as PEM delivers the viscosity in addition to the density value with an accuracy of 5% in the range from 10 mPa.s to 3,000 mPa.s.
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