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- The viscosity of five liquid hydrocarbons at pressures up to 250 MPaPublication . Oliveira, Carla Padrel de; Wakeham, William A.This paper reports new measurements of the viscosity of toluene, n-pentane, n-hexane, n-octane, and n-decane at pressures up to 250 MPa in the temperature range 303 to 348 K. The measurements were performed with a vibrating-wire viscometer and with a relative method of evaluation. Calibration of the instrument was carried out with respect to reference values of the viscosity of the same liquids at their saturation vapour pressure. The viscosity measurements have a precision of _0.1% but the accuracy is limited by that of the calibration data to be +0.5%. The experimental data have been represented by polynomial functions of pressure for the purposes of interpolation. The data are also used as the most precise test yet applied to a representation of the viscosity of liquids based upon hard-sphere theory.
- The viscosity of R32 and R125 at saturationPublication . Oliveira, Carla Padrel de; Wakeham, William A.This paper reports new measurements of the viscosity of R32 and R125, in both the liquid and the vapor phase, over the temperature range 220 to 343 K near the saturation line. The measurements in both liquid and vapor phases have been carried out with a vibrating-wire viscometer calibrated with respect to standard reference values of viscosity. It is estimated that the uncertainty of the present viscosity data is one of 0.5-1%, being limited partly by the accuracy of the available density data. The experimental data have been represented by polynomial functions of temperature for the purposes of interpolation.
- Validation of a vibrating-wire viscometer: measurements in the range of 0.5 to 135 mPa'sPublication . Oliveira, Carla Padrel de; Caetano, Fernando J. P.; Fareleira, João M. N. A.; Wakeham, William A.The present article describes a novel vibrating-wire viscometer that has been shown to be able to measure viscosities up to 135 mPaâs, after calibration against water at 20 °C, to provide direct traceability to the primary reference for viscosity. For the purpose of validating the instrument, measurements of the viscosity of some selected fluids, including standard reference liquids with viscosity on the order of 100 mPaâs at 20 °C and 2,2,4-trimethylpentane with a viscosity of about 0.5 mPaâs at the same temperature, were performed. The results obtained show that the instrument is capable of performing viscosity measurements with an estimated overall uncertainty better than (0.8% over the range of (0.5 to 135) mPa's.
- An industrial reference fluid for moderately high viscosityPublication . Oliveira, Carla Padrel de; Caetano, Fernando J. P.; Fareleira, João M. N. A.; Fröba, Andreas P.; Harris, Kenneth R.; Leipertz, Alfred; Trusler, J. P. Martin; Wakeham, William A.In industrial practice, there is a demand for a reference standard for viscosity that is established for a readily available fluid to simplify the calibration of industrial viscometers for moderately high viscosities [(50 to 125) mPa · s]. Diisodecyl phthalate (DIDP) has been suggested as that reference fluid, and a number of studies of its properties have been carried out in several laboratories throughout the world, within the auspices of a project coordinated by the International Association for Transport Properties. That project has now progressed to the point where it is possible to collate the results of studies of the viscosity of the fluid by a number of different techniques, so as to lead to a proposed standard reference value which will be included in the paper. To support this recommended value, the various measurements conducted have been critically reviewed, and the sample purity and other factors affecting the viscosity have been studied. Density and surface tension measurements have also been performed. This paper does not describe the individual viscosity determinations carried out in independent laboratories because these are the subject of individual publications, but it does describe the ancillary studies conducted and their relevance to the viscosity standard. In addition, the paper contains recommended values for the viscosity of liquid DIDP. The samples of DIDP to which the recommended values refer are isomeric mixtures available commercially from certain suppliers, with a minimum purity by gas chromatography of 99.8 %. The recommended values result from a critical examination of all the measurements conducted to date and are supported by careful arguments dealing with the likely effects of the isomeric content of the sample as well as of other impurities. The proposed reference standard is intended particularly to serve an industrial need for a readily available calibration material with a viscosity close to that required in practical situations. To that end, the recommended value has an overall relative uncertainty of approximately 1 %. It is therefore not intended to supersede for the reference value for the viscosity of water at 20 °C, which is known much more accurately, but rather to complement it.
- Viscosity measurements of liquid toluene at low temperatures using a dual vibrating-wire techniquePublication . Oliveira, Carla Padrel de; Caetano, Fernando J. P.; Mata, José L. Correia da; Fareleira, João M. N. A.; Wakeham, William A.A recently developed dual vibrating-wire technique has been used to perform viscosity measurements of liquid toluene in the temperature range 213 K ≤ T ≤ 298 K, and at pressures up to approximately 20 MPa. The results were obtained by operating the vibrating-wire sensor in both forced and free decay modes. The estimated precision of the viscosity measurements, in either mode of operation, is ±0.5%, for temperatures above or equal to 273 K, increasing with decreasing temperature up to ±1% at 213 K. The corresponding overall uncertainty is estimated to be within ±1% and ±1.5%, respectively.
- Viscosity measurements of diisodecyl phthalate using a vibrating wire Instrument operated In free decay mode: comparison with results obtained with the forced mode of operationPublication . Oliveira, Carla Padrel de; Caetano, Fernando J. P.; Fareleira, João M. N. A.; Wakeham, William A.Recently, several authors described measurements of liquid viscosities higher than 100 mPa · s, using the vibrating wire technique in the forced oscillation mode of operation. The use of a vibrating wire instrument to measure increasingly higher viscosities poses experimental difficulties due to the consequent decrease of the quality factor of the resonance. This problem affects both modes of operation: in the steady-state, forced oscillation mode, there is a loss of definition of the resonance curve, and in the transient, free decay mode, there is an increase of the decrement of the sinusoidal time response of the vibrating wire. In the present work, we have measured the viscosity of diisodecyl phthalate (DIDP) at 0.1 MPa with a vibrating wire of nominal radius of 190 μm using the free decay mode of operation. The measurements covered a range of viscosities from (64 to 265) mPa · s. The results agreed with those we have obtained with a vibrating wire instrument operated in the forced mode and with the literature data, within the estimated experimental uncertainty (k ) 2) of ( 2 %. The comparisons indicate that the use of the free decay mode of operation does not necessarily imply a significant increase of uncertainty of the viscosity results at moderately high viscosities.
- Vibrating-wire viscometry on liquid hydrocarbons at high pressurePublication . Oliveira, Carla Padrel de; Assael, Marc J.; Papadaki, Maria; Richardson, Stephen M.; Wakeham, William A.A vibrating-wire viscometer for the measurement of the viscosity of liquids at elevated pressures is described. The instrument is one of a pair, developed collaboratively in different laboratories, that operate in a relative manner according to a complete theory of the instrument. The instrument described here has been used for measurements at pressures up to 80 MPa in toluene, and in n-heptane along two isotherms.
- Vibrating-wire viscometers for liquids at high pressuresPublication . Oliveira, Carla Padrel de; Assael, Marc J.; Papadaki, Maria; Wakeham, William A.The design and operation of two independent vibrating-wire viscometers are described. The instruments are intended for operation in the liquid phase at pressures up to 300 MPa and have been designed specifically for this purpose using the detailed theory of the device. Extensive evidence is adduced to demonstrate that the operation of the viscometers is consistent with the theory. Although the instruments attain a precision in viscosity measurements of __+1% when used in an absolute mode the accuracy that can be achieved is no better than __+3%. However, if the instrument is calibrated for two welldefined instrumental parameters, the uncertainty in the reported viscosity is improved to _+0.5%. The results of measurements of the viscosity of normal heptane in the temperature range 303 to 348 K at pressures up to 250 MPa made with one of the viscometers are reported. The results are shown to be totally consistent with measurements reported earlier using the instrument designed for lower pressures.
- The viscosity of liquid R134aPublication . Oliveira, Carla Padrel de; Wakeham, William A.The paper reports new measurements of the viscosity of liquid R134a over the temperature range 235 to 343 K and pressures up to 50MPa. The measurements have been carried out in a vibrating-wire viscometer calibrated with respect to the viscosity of several liquid hydrocarbons. It is estimated that the uncertainty in the viscosity data reported is +0.6%. The data therefore have a lower uncertainty than that of earlier measurements of the viscosity of this environmentally acceptable regrigerant. The viscosity data have been represented as a function of density by means of a formulation based upon the rigid, hard-sphere theory of dense fluids with a maximum deviation of _+0.3%. This representation allows the present body of experimental data to be extended to regions of thermodynamic state not covered by the measurements.
- Viscosity of new refrigerantsPublication . Oliveira, Carla Padrel de; Wakeham, William A.Measurements of the viscosity of several environmentally-acceptable refrigerants that have been made in recent years are reviewed. In particular, a detailed survey of the results for R134a in the liquid phase is presented because, for this fluid, the greatest number of independent experimental results is available. It is shown that there are significant discrepancies between the results of various authors and that they lie outside the bounds of the mutual uncertainty claimed. The nature of the discrepancies between any two sets of results is shown to be systematic whether they have been performed in similar or different types of instrument. For R32 and R125 in the liquid phase, where there are very many fewer experimental results available, the situation is essentially the same. It is argued that these discrepancies arise from a lack of proper precautions in the performance of the measurements either because of difficulties associated with the purity of the samples employed or because, in some instruments, there has been insufficient attention devoted to discrepancies among the results of research workers with established records of high-quality measurements are damaging to the external perception of the science of viscometry and that remedial action to remove the difficulties is essential and urgent. Specific recommendations for such action are made.