Vibrating-tube densitometer

A number of researchers have developed techniques to determine partial molar volume. Eckert and coworkers (Eckert et al., 1986) provide a very good description of the method for obtaining partial molar volumes. They facilitated this difficult measurement by using a vibrating-tube densitometer (Mettler-Paar DMA 512). The major uncertainties with this technique are associated with the temperature control, which becomes crucial if experiments are performed at infinite dilution near the solvent s critical point. Shim and Johnston (1991) also note that it is possible to determine partial molar volume information using supercritical fluid chromatography if the mobile and stationary phases can be thermodynamically characterized. 

The drop image is recorded with a video camera and digitized in order to calculate y from the Young-Laplace equation [13]. Independent measurements of the densities of the two phases are made wiA a vibrating tube densitometer. 

The vibrating tube densitometer is an instrument designed to precisely measure liquid or gas density. A tube or spool is vibrated mechanically at its natural frequency and sensors measure the frequency of that vibration. The measured frequency will decrease as density increases, and can be calculated after calibrating the instrument with fluids of known density. Other density related variables such as specific gravity, molecular weight, and concentration may also be calculated. It is now very widely used in industry as weU as scientific laboratories. See, e.g., Majer et al. (1991). 

The most widely used method for ionic liquid density measurement is the vibrating-tube densitometer a method which relies on a calibration as a function of temperature and pressure using appropriate reference fluid.For many reported ionic liquids this is not routinely performed and corrections for the case of viscous fluids (i.e., >100 mPas) are often ignored. Despite these factors the densities of ionic liquids measured with vibrating-tube densitometers have a standard uncertainty to within 0.1%. Alternative methods include the calculation of density through speed of sound measurements or piezometric methods. Both approaches are relatively complex technically but present the advantage of providing extra thermodynamic property data. 

Available process measurements include the dissolved solute concentration and CSD information. To estimate the concentration, a PAAR vibrating U-tube densitometer is used to measure frequency as a function of the fluid s density and temperature. This information is used to interpolate a concentration value from an experimentally constructed database. 

In the laboratory, the density of a cement slurry is usually derived from the measured mass or volume of the different ingredients and from their respective densities. When a measurement is performed, it is usually done with pressurized equipment like a pressurized mud balance (Appendix C of reference 6) as cement slurries always contain a small quantity of air bubbles (these are trapped into the slurry and difficult to eliminate because of the fluid yield stress). In the field, measuring slurry density is a key issue as it is much more cumbersome if not impossible (for continuous mix operations) to measure the mass or volume of the different ingredients. Different types of equipments are used like pressurized mud balances, radioactive densitometers, or vibrating tube devices. Once the slurry is mixed at the right density, it is of utmost importance to make sure it is stable. 

Direct Mass Measurement One type of densitometer measures the natural vibration frequency and relates the amplitude to changes in density. The density sensor is a U-shaped tube held stationaiy at its node points and allowed to vibrate at its natural frequency. At the curved end of the U is an electrochemical device that periodically strikes the tube. At the other end of the U, the fluid is continuously passed through the tube. Between strikes, the tube vibrates at its natural frequency. The frequency changes directly in proportion to changes in density. A pickup device at the cui ved end of the U measures the frequency and electronically determines the fluid density. This technique is usefiil because it is not affec ted by the optical properties of the fluid. However, particulate matter in the process fluid can affect the accuracy. 

Monomer conversion in emulsion and solution polymerization can be determined via density of the reaction medium due to the difference in density between monomer and polymer. The availability of on-line digital densitometers manufactured by Anton Paar of Austria and others make this approach amenable to on-line application [23-26]. These instruments are capable of immediate determination of the density of any fluid, and, if equipped with a flow cell, can continuously monitor the density of a process stream. Results are displayed locally and can be transmitted digitally to a data acquisition computer. Density measurement is accomplished by introducing a test fluid into a glass U-shaped sample tube which is rigidly supported at the open ends. The tube is electronically excited to vibrate at its natural frequency. The frequency of oscillation is continuously monitored electronically, and from the change of frequency caused by the test fluid within the tube, the... 

It may also be noted from Figure 3.17 that, in this example, only one densitometer is used and the two streams to be measured (the sample stream 4 from the feed stream 3 and the sample stream 5 from the overflow 6) are switched through it alternately, with a delay in between to allow each sample stream to reach the sensor head of the instrument. A vibrating U-tube density meter is used in the example. Note also that both of the streams taken through the densitometer have to be well de-aerated and this is achieved by venting the lines and routing them in such a way that de-aeration takes place by gravity. 

---