Vibrating tube densimeter

The two principal experimental apparatuses used to determine the density of a liquid are the pycnometer and the vibrating tube densimeter. The pycnometer method involves measuring the mass of a liquid in a vessel of known volume. The volume of the pycnometer, either at the temperature of measurement or at some reference temperature, is determined using a density standard, usually water or mercury. Using considerable care and a precision analytical balance accurate to 10 5 g, it is possible to achieve densities accurate to a few parts in 10s with a pycnometer having a volume of 25 cm3 to 50 cm3. 

The vibrating tube densimeter relies upon the fact that the frequency /of vibration for a U or V shaped tube depends upon the mass of material in the tube ... 

Calibration of the apparatus is necessary usually water and air or nitrogen are the reference materials. Vibrating tube densimeters designed to operate close to atmospheric pressure can achieve repeatability of parts in 10s. If the reciprocal of the frequency is linear in density, accuracies of 1 part in 105 are readily achievable. 

Phase equilibria and pressure-temperature coordinates of critical points in ternary systems were taken with a high-pressure apparatus based on a thermostated view cell equipped with two liquid flow loops which has been described in detail elsewhere [3]. The loops feed a sample valve which takes small amounts of probes for gas-chromatographic analysis. In addition to temperature, pressure and composition data, the densities of the coexisting liquid phases are measured with a vibrating tube densimeter. Critical points were determined by visual oberservation of the critical opalescence. 

A high pressure vibrating tube densimeter (Mettler-Paar DMA 512) was used to measure the density of the AOT/water/supercritical ethane solutions. By recirculating water from a thermostated water bath through the water-jacketed measuring cell, the temperature of the cell could be controlled to 0.01 C. The micelle solutions were prepared by loading measured amounts of AOT and water into a... 

Because the solubility of acrylamide (JEL), water ( ), and the surfactants in ethane or propane is low, the viscosity of the continuous phase was taken to be that of the pure fluid. The viscosity of the various ethane/propane mixtures was calculated using a reduced-density correlation developed by Dean and Stiel (IQ.), which is reported to be accurate to within 2 to 4% for light hydrocarbon mixtures. The density of the ethane/propane mixtures was either calculated via a modified Benedict-Webb-Rubin equation of state (11.) or, in some cases, measured using a Mettler-Paar DMA-512 vibrating tube densimeter. The densimeter was thermostated via a circulating water bath to within 0.01 C, and calibrated using water and propane at the ten ratures of interest. 

Another method of measuring density relies on the change in the resonant frequency of a tube (often U-shaped) when it is filled with a fluid. Vibrating-tube densimeters are commercially available they can be a convenient measuring tool in many circumstances. These instruments must be calibrated (usually with water if liquids are being measured, although for liquids whose density is significantly different from water a calibration fluid with a similar density is preferable) at the temperature and pressure of interest. While the precision of these instruments is often better than... 

Density (p) was measured using an Anton Paar DMA 5000 M vibrating-tube densimeter in the temperature (7) range of (298 to 353) K. The uncertainty is estimated to be 0.05 kg/m. JCinematic viscosity (v) was determined using a certified U-tube capillary viscometer (PSL) with an estimated uncertainty of 0.3%. Dynamic viscosity (u) was calculated from the relation // = vp. 

A method of sharp drop in the amplitude of vibration of the tube in a vibrating tube densimeter (VTFD in Table 1.1) was suggested by Crovetto and Wood (1991) for experimental determination of phase boundary and phase transition from homogeneous fluid to liquid-gas equilibrium. 

Hynek et al (1997a) describe a modified version of the fiow vibrating-tube densimeter with a photoelectric pick-up system and a new concept of an electromagnetic drive system was designed for density difference measurements in the temperature and pressure ranges from 25 to 300 and up to 35 MPa. Similar equipment was used by Hakin et al (2000). 

Excess molar volumes of v of 1,2-ethanediol + water were measured atp = 0.1 MPa with a faithful copy of the vibrating tube densimeter DMA 602 from Anton Paar. All binaiy mixtures were measured at the temperatures (308.2, 313.2, and 318.2) K. Values of V are negative for all the mixtures studied over the whole concentration range and for all temperatnres. Resnlts were correlated by polynomial equations of Redlich and Kister (1948). 

The densities of most polymer dispersions are close to 1 g cm as the corresponding polymers (with the exception of polyvinyl chloride and poly(vinylidene chloride)) have densities in the range 1.0 to 1.2 g cm [2]. Since the densities of the polymer particles almost match the density of the aqueous phase, sedimentation is usually only a problem in emulsion polymers if they contain very coarse particles. Density measurements have been used in the past to follow the course of emulsion polymerization reactions, because the density of the monomer is usually lower than that of the polymer (densitometry [1]). Densities can, for instance, be determined quite simply with a pycnometer (see ISO 2811). Very high precision density measurements ( 5 X 10 g cm" ) are possible with a vibrating-tube densimeter [3]. In this method, the change in the resonant frequency of the tube, which depends on its total mass, is measured when the dispersion is placed in it. It is essential that the sample is wholly free of gas bubbles. 

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