The oscillating U-tube is the heart and core of every digital density meter. Have you ever wondered how the little piece of glass in a U-tube density meter can do so much for you in contrast to a hydrometer or pycnometer? In this article, experts ranging from physicists to glassblower masters grant deep insights into their work and skills – enjoy a close look at the heart of a high-end digital density meter after 50 years of evolution.
Measuring the physical parameter ‘density’ has always been a way to control the progress or the outcome of a production process. The industrial revolution initiated a constant increase in the variety and amount of commercial goods and thus the need for quicker, simpler, and more precise methods of density measurement. Back in 1967, the oscillating U-tube eliminated the drawbacks of hydrometers, pycnometers or even hydrostatic balances to meet those needs. U-tube density meters are not prone to human errors, require only little sample quantities, and are amazingly fast and accurate.
Today, the measurement of density is commonly used in merchandise trade for incoming and outgoing quality control or to describe a new or unknown material. U-tube density meters have replaced traditional glass spindles in many of these fields of application in the last decades.
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Elements of a digital density meter
The heart of a digital density meter is the U-tube made of glass or metal. However, there is further sophisticated technology required to put this sensitive element into operation: Sensors excite and receive signals and a computing system determines the period of oscillation of the U-tube. First, this signal is transferred to computing devices that calculate the density reading out of it. Then this signal is amplified and sent back to an element that keeps the U-tube moving; in other words it adds oscillation energy to the U-tube which is lost due to natural damping effects.
Another core topic for U-tube density meters is temperature. Again two things are important for precise temperature regulation: an element to accurately measure the temperature in the U-tube’s environment as well as a system to accurately control (i.e. heat or cool) this environment.
Characteristics of the U-tube
There are several types of oscillators that are named according to their oscillation direction:
An X-oscillator is a U-tube where the bend of the oscillator is fixed (in contrast to the Y-oscillator). The moving parts are therefore the straight tubes that move towards each other in opposite directions; these are the most sensitive parts of the U-tube. The characteristic frequency of this type of oscillators is much higher and thus the viscosity influence is worse than for the other types. An X-oscillator is not prone to mechanical vibrations and gives good results without the need for a countermass.
Common benchtop density meters make use of the so-called Y-oscillator, where the bend makes an up-and-down movement. This setup is also compact with reasonable dimensions and allows for precise temperature regulation. The most sensitive part here is the bend of the U-tube because the amplitude reaches its peak here. This type of oscillator is very sensitive and allows highly precise measurements. However, harmful vibrations must be eliminated by means of a countermass that is firmly fixed to the U-tube but has a flexible connection to the rest of the density meter.
At Anton Paar, these U-tubes are 95 % handcrafted by glassblower masters.
A W- or double-Y-oscillator is hallmarked by a total of three bends whereby the first and the last bend oscillate towards each other in opposite directions which makes them the most sensitive parts. This type of oscillator can be used in handheld instruments with limited precision; that means a countermass to eliminate harmful vibrations or an active temperature regulation are not required.
This kind of oscillator is manufactured in an automated way. However, the fine-tuning of the finished U-tube is still a matter of decade-long experienced handicraft.
As well as having different shapes and directions of oscillation, U-tubes can also be made of different materials depending on the application they are needed for:
Metal: Hastelloy C276
The most obvious advantage of a measuring cell made of glass is the possibility to see the sample and therefore check for the integrity of the filling process (i.e. the presence of bubbles or particles which would falsify the measured density). Another equally significant advantage is the chemical resistance to almost all substances except hydrofluoric acid (HF) and hydrogen sulfide (H2S). The temperature coefficient as well as the brilliant overall sensitivity thanks to the low specific weight is also at its best with glass oscillators resulting in repeatability statements of 0.000001 g/cm³. However, the thermal hysteresis requires the application of a reference oscillator for compensation purposes.
Durability and fracture strength against mechanical stress are benefits of metal oscillators. Those chemicals that damage glass oscillators after excessive exposure can be measured without a problem. The low thermal hysteresis does not require a reference oscillator. Thanks to its robustness, a metal oscillator withstands pressures up to 1400 bar and provides a temperature range from -10 °C up to +200 °C. On the other side, this great robustness implicates a limited measurement uncertainty of 0.0001 g/cm³.
What makes the difference?
|Borosilicate glass||Hastelloy C276|
|Measurement uncertainty||Up to 0.000005 g/cm³||Up to 0.0001 g/cm³|
|Repeatability||Up to 0.000001 g/cm³||Up to 0.00005 g/cm³|
|Resistance||All except H₂S, HF||H₂S, HCI, NaOH, HF|
|Pressure range||0 bar to 10 bar||0 bar to 1400 bar|
|Temperature range||0 °C to 100 °C||-10 °C to 200 °C|
Crafted with passion: How our U-tubes are produced
The diversity of applications that the different U-tubes are able to cover can be best handled with the above described geometrics and materials. However, even if the principle is always the same, the individual characteristics allow for valuable differentiation and specialization for certain samples. This is especially the case when it comes to the different materials, namely glass and metal. When looking at the production processes of glass and metal U-tubes it becomes obvious that differences become substantial here.
If you would like to know more about how Anton Paar developed from a master locksmith’s workshop in 1922 into a worldwide high-tech company today, have a look at our history “From the workbench to a global company”.
Stronger than steel
Metal U-tubes are used in handheld- and in benchtop U-tube density meters. The metal used for all of them is Hastelloy C276, a high-performance material that withstands the highest strains. Hastelloy is a very stable alloy and features good thermal conduction and only little thermal hysteresis as well as chemical resistance to aggressive and highly concentrated acids and bases. This is how our metal U-tubes are made:
For handheld density meters
The Hastelloy U-tube with small wall thickness is cut into 2 pieces of defined length and soldered to the oscillator joint and the connector. After this the metal U-tube is built into a thicker metal tube, this is connected to the evaluation unit and then the setup is ready for measuring a wide range of spirits.
For benchtop instruments
The Hastelloy tube is cut to the defined length and then bent to the U-shape before it is mounted onto the oscillator block. Once fixed, the inlet and the outlet thread of the measuring cell are connected to the U-tube, and the magnets for excitation are connected. The next step is a pressure test with 1.5 times the pressure that will be applied during the measurement. Before the oscillating block is closed, it is filled with an inert gas. The final inspection of the U-tube includes a leaktightness test and a check of the signals of the U-tube. After this the metal “heart” is built into the benchtop density meters. There you go – ready for high-pressure and high-temperature applications like the measurement of bitumen/asphalt.
Ready for a leap into the past?
Take a look at all the technologies that have gone into improving the U-tube. The journey starts in 1967…
For the excitation of the U-tube
A system of magnets and coils can be used to excite the oscillation of the U-tube in a simple and cheap manner. However, the additional weight of the magnets on the U-tube has a negative influence on the achievable accuracy.
For the pick-up of the oscillation
Contrary to the application of magnets to keep the U-tube oscillating, the same technology can be used to measure the period of oscillation. Whenever the magnet passes the coil, a little current is induced which can be evaluated.
Evaluation of the oscillation period
Processing information about the oscillation of the U-tube (i.e. the determined characteristic frequency) by means of analog technology is robust and affordable. However, the precision is limited and not all the required information about the oscillation signal can be obtained to allow precise viscosity correction for the full measuring range.
For the sake of completeness, temperature regulation using an external water bath needs to be mentioned here. Water baths were connected to older digital density meters, to regulate the surrounding temperature of the U-tube. However, they are outdated and gave way to space- and time-saving Peltier elements.
Light sensor barrier
A very precise way to detect the period of oscillation is simply a light beam which is interrupted by the oscillating U-tube or more precisely by a tiny coating on the U-tube. This method is very precise although it only requires simple and affordable technology.
The application of Peltier technology linked with precise temperature measurement allows quick and precise temperature regulation in modern U-tube density meters. Peltier elements make use of the Peltier effect, which describes a heat flux due to electric current. In action, one side of a Peltier element heats up while the other side cools down depending on the direction of the current flow. That way, this technology can be used for both heating and cooling.
A piezo element is a crystal or ceramic material that changes its dimension upon applied electrical voltage. However, this requires some safety measures in the electronic circuits as this requires high voltages. Nevertheless, it is the currently most precise method to excite the U-tube.
Evaluation of the oscillation period
Digital signal processors (DSP) offer great advantages over analog technology. The quality factor of the U-tube can be determined, which is directly influenced by the sample’s viscosity and can be best explained by a loss of oscillation energy which has to be compensated for accurate density measurements. This is possible thanks to the simultaneous determination of not only the characteristic frequency but also its first harmonic oscillation and results in a full-range compensation of viscosity-induced measurement errors.
Pick-up of the oscillation
Also the usable effect of piezo elements can be inverted. While the excitation is enabled by applying electrical voltage to a piezo element, the detection of the oscillation is possible with a second piezo element that is pressurized by the moving U-tube periodically. This generates electrical voltage representing the period of oscillation very accurately.
The evolution goes on: The future of density measurement
Now we have a very good insight into the history as well as current technologies in digital density measurement. But the future has just begun: even if modern U-tube density meters provide great convenience compared with older equipment or even traditional methods of density measurement there is still much room for improvements. Two big trends that will have an impact are the miniaturization of components and increased efficiency.
This means that future instruments will be even smaller, faster, and of course smarter.
As discussed above, the individual sub-technologies used in U-tube density meters have also evolved over time and they will continue to do so.
They will make their next evolutionary step when something faster, cheaper, or safer than e.g. piezo or Peltier technology, which is state-of-the-art technology in 2017, appears.
For Anton Paar the exciting journey of research and development continues as new applications and customer needs arise. Especially combined with the above discussed R&D-driven technology push, this will lead to the development of density meters that expand the existing portfolio and create new possibilities.
If you have any questions about this topic, please don’t hesitate to contact us.
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