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- 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.
- Viscosity of Compressed Liquid 1,1,1-Trifluoroethane (HFC-143a) and Pentafluoroethane (HFC-125)Publication . Avelino, Helena Maria da Nóbrega Teixeira; Fareleira, João M. N. A.; Oliveira, Carla Padrel deThe viscosity of compressed liquid 1,1,1-trifluoroethane (HFC-143a) and pentafluoroethane (HFC-125) has been measured with a vibrating-wire viscometer at five temperatures between (254 and 293) K. The measurements were performed at pressures from above saturation up to 10 MPa, although for the isotherms at about 254 K the maximum pressure was approximately 5 MPa for HFC-143a and 7.4 MPa for HFC-125. For the isotherm at about 263 K, the highest pressure for HFC-143a was of the order of 7.5 MPa. The overall uncertainty of these results has been estimated to be less than ( 1.0 %. The measurements have been correlated using a scheme based on a hard-spheres model. The root mean square deviation of the experimental results from the correlations for HFC-143a and HFC-125 is ( 0.24 % and ( 0.25 %, respectively. The correlation scheme has been used to perform the small extrapolations of the present data to the saturation line to enable comparison with literature results at saturation pressure.
- Thermophysical property measurements: the journey from accuracy to fitness for purposePublication . Oliveira, Carla Padrel de; Wakeham, William A.; Assael, Marc J.; Atkinson, J. K.; Bilek, J.; Fareleira, João M. N. A.; Fitt, A. D.; Goodwin, A. R. H.Until the 1960s much of the experimental work on the thermophysical properties of fluids was devoted to the development of methods for the measurement of the properties of simple fluids under moderate temperatures and pressures. By the end of the 1960s a few methods had emerged that had both a rigorous mathematical description of the experimental method and technical innovation to render measurements precise enough to rigorously test theories of fluids for both gas and liquid phases. These studies demonstrated that, for the gas phase at least, the theories were exceedingly reliable and led to physical insight into simple molecular interactions. The thesis of this paper is, after those early successes, there has been a divergence of experimental effort from the earlier thrust and, in the future, there needs to be focus on in situ measurement of properties for process fluids. These arguments are based upon the balance between the uncertainty of the results and their utility and economic value as well as upon technical developments, which have provided reliable and robust sensors of properties. The benefits accrued from accurate measurements on a few materials to validate predictions of the physical properties, for a much wider set of mixtures over a wide range of conditions, are much less relevant for most engineering purposes. However, there remain some special areas of science where high accuracy measurements are an important goal.
- Viscosity of Di-isodecylphthalate: a potential standard of moderate viscosityPublication . Oliveira, Carla Padrel de; Caetano, Fernando J. P.; Fareleira, João M. N. A.; Wakeham, William A.The paper reports our first measurements of the viscosity of di-isodecylphthalate, which is a candidate for a reference material. At the same time it has a viscosity, which, at room temperature, is around 120mPa · s, so that it can fulfill the need for a reference material more nearly matched to the needs of industry. The present measurements were carried out with a specially designed vibrating-wire viscometer over the temperature range 288–308K and have an estimated uncertainty smaller than ±1.5%, following calibration against the viscosity of toluene. The instrument and results are presented here to encourage other measurements on the same material, by different techniques,which will lead eventually to the establishment of di-isodecylphthalate as a suitable reference material, as well as reference values for its viscosity.
- Diisodecylphthalate (DIDP) — a potential standard of moderate viscosity: surface tension measurements and water content effect on viscosityPublication . Oliveira, Carla Padrel de; Caetano, Fernando J. P.; Fareleira, João M. N. A.; Fernandes, Anabela C.; Serro, Ana P.; Almeida, Inês M. Simões de; Wakeham, William A.Our laboratory, along with several others, has been engaged in a project to determine the suitability of diisodecylphthalate (DIDP) as a standard reference material of viscosity for industrial purposes. As a part of that project, we have undertaken a study of the effects of surface tension and of impurities, of that liquid on viscosity measurements with routine, suspended level capillary instruments. For the former purpose, a set of surface tension measurements is reported here for DIDP and two reference standard mineral oils. In particular, surface tension data for DIDP, obtained using a pendant-drop shape-analysis method, at temperatures from 288K to 308K are presented. The present study suggests that surface tension effects upon viscosity measurements with routine capillary viscometers can be important enough to demand that the surface tension of the reference fluid(s) used for their calibration be specified. It is also evident that the surface tension of materials used for calibration may be relevant to their selection if errors arising from surface tension are to be avoided. In order to fulfil these requirements, the surface tension of the reference material should be included in the definition of the standard. The effect of the water content on the viscosity of a sample of DIDP is assessed as a part of the process of determining its suitability as a standard reference material.
- A new instrument to perform simultaneous measurements of density and viscosity of fluids by a dual vibrating-wire techniquePublication . Oliveira, Carla Padrel de; Mata, José L. Correia da; Fareleira, João M. N. A.; Caetano, Fernando J. P.; Wakeham, William A.A preliminary presentation of a new vibrating-wire instrument for the simultaneous measurement of the viscosity and density of fluids, over wide ranges of temperature and pressure, is made. The new equipment has the capability of measuring modes of oscillation, either in a forced mode or free decay. A theoretical description of both modes enables the simultaneous measurement of the viscosity and density of the fluid, either from an analysis of the resonance curve, or the transient response of the sensor. The consistency of the two approaches, under the same working conditions, and quality of the measuring system, have been assessed with toluene as the test fluid. It is concluded that the new equipment improves the forced mode of operation described earlier and adds a new flexibility to the vibrating-wire method.