The resonator density measurement
In the previous part (1) we learned about the basics of density measurement and the definition of density. This section is dedicated to the vibration method which is also used by density sensors for density measurement. This method also has some advantages and disadvantages, which are explained in detail.
- How does the resonator density measurement work?
- What are the benefits?
- What are critical influencing factors (disadvantages)?
- When is the resonator density Measurement requested?
- Determination of methane number in gas engines to increase efficiency.
How does the resonator density easurement work?
The higher the vibration frequency, the lower the density of the liquid
and compensated for by the measuring instrument.
The only remaining variable that can be influenced is thus the density of the liquid
- Resonator is firmly clamped at both ends.
- Exciter causes the tube to vibrate.
- Vibration sensors detect the vibration frequency.
What are the advantages?
not required; instead, the temperature is measured in situ. In addition, the extraction of an exact volume is not required.
The miniaturization of the technology that is possible means that only small sample quantities are needed for reliable density determination. This feature is particularly relevant for expensive media. The use of small sample quantities also makes it easier to determine the medium temperature for density measurement.
The measurement can be performed in a closed system and thus under pressure. This is particularly relevant for certain media such as alcohol-water mixtures or gases. Without the appropriate pressure, such media would volatilize, leading to incorrect measuring values.
Finally, the measuring results are available in an extremely short measuring time.
Resonator measuring instruments can also be used for a continual measurement with flow directly in the process.
What are critical factors (disadvantages)?
The measuring sensitivity is influenced by the nature of the measuring tube. The smaller the dead weight of the measuring tube, the lower its influence on the
frequency and the higher the measuring sensitivity. Conversely, measuring low medium masses with a simultaneously high dead weight of the measuring tube is problematic.
The mechanical properties of the resonator are changed by pressure and temperature influences. As a result, the frequency also changes. These dependencies can be compensated for by calibrations with different pressure and temperature points.
In addition, the frequency is also dependent on the viscosity of the measuring liquids. This can give rise to further measuring uncertainties.
Air bubbles in the measuring liquid can also lead to measuring errors. This is not the case if degassing is carried out before the measurement.
The measuring result can be falsified by contamination of the resonator. An indication that the resonator is clean is reading the correct air density when empty.
Where is the resonator density measurement
- In the oil and gas industry, e.g. for determining calorific value, energy content or composition.
- At filling stations for settlement of the correct value and determining foreign particles.
- In transport for settlement of the correct value and to check the medium.
- In aircraft refuelling for optimizing the filling quantity to the flight distance.
- On engine test benches to test for constant fuel quality.
Which versions of resonator densimeters are available?
Unit dimensions: approx. 650 x 450 x 150 mm
from Anton Paar. Universal instrument with highest precision (0.000005 g/cm3).
Unit dimensions: 482 x 340 x 231 mm
Unit dimensions: approx. 450 x 200 x 100 mm
Unit dimensions: 66 x 30 x 15 mm