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The presence of a gravitational field leads to both practical and fundamental limits of the resolution in critical phenomena experiments in fluids near the gas-liquid critical point. We present equations that yield estimates of the gravitational limitations in a variety of critical phenomena experiments for a large number of fluids and as a function of the magnitude of the gravitational field. Various strategies for improving the resolution of such experiments are discussed, including procedures that remove a fiuid from thermodynamic equilibrium (e.g., stirring). A comparison is made between the gravitational limitations in earth-bound experiments and those at the microgravitational levels that may become accessible in an orbiting laboratory. CONTENTS I. Introduc tion II. Critical Region Parameters for Fluids III. Gravity-Induced Profiles IV. Averaging Errors Due to Finite Sample Height A. Density Ineasurements B. Compre s s ibility measurements C. Specific heat measurements V. Limitations of Optical Experiments A. Beam bending B. Light-scattering experiments VI. Attempts to Suppress Density Gradients Induced by the Earth's Gravitational Field VII. Intrinsic Gravity Effects VIII. Discus sion Acknowledgments References 79 80 83 84 85 85 87 87 87 90 92 94 97 97 97 I~INTRODUCTION Near the gas-liquid critical point, gravity induces a density gradient in the fluid (Gouy, 1892; Teichner, 1904; Baehr, 1954). The resulting inhomogeneous distribution of the local properties of the fluid strongly affects. the interpretation of critical phenomena experiments in fluids (Weinberger and Schneider, 1952). A number of investigators have studied the consequences of gravitationally induced inhomogeneities for the in

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Task specific uncertainty is the measurement uncertainty associated with the measurement of a specific feature using a specific measurement plan. This paper surveys techniques developed to model and estimate task specific uncertainty for coordinate measuring systems, primarily coordinate measuring machines using contacting probes. Sources of uncertainty are also reviewed.