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Subatmospheric vapor pressures evaluated from internal-energy measurements

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Abstract

Vapor pressures were evaluated from measured internal-energy changes in the vapor+liquid two-phase region, ΔU (2). The method employed a thermodynamic relationship between the derivative quantity (ϖU (2)/ϖV) T and the vapor pressure (p σ) and its temperature derivative (ϖp/ϖT)σ. This method was applied at temperatures between the triple point and the normal boiling point of three substances: 1,1,1,2-tetrafluoroethane (R134a), pentafluoroethane (R125), and difluoromethane (R32). Agreement with experimentally measured vapor pressures near the normal boiling point (101.325 kPa) was within the experimental uncertainty of approximately ±0.04 kPa (±0.04%). The method was applied to R134a to test the thermodynamic consistency of a publishedp-p-T equation of state with an equation forp σ for this substance. It was also applied to evaluate publishedp σ data which are in disagreement by more than their claimed uncertainty.

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Abbreviations

C σ :

Saturated liquid heat capacity

C tD v :

Isochoric heat capacity of the ideal gas

M :

Molecular weight

p σ :

Vapor pressure

Q/ΔT:

Amount of energy needed to change the temperature of the sample by 1 K

ρ:

Density

T :

Temperature

τ:

1−T/T c

′:

Saturated liquid

″:

Saturated vapor

(2):

≡{m 1 X′+m g X″}/{m 1+m g }, bulk propertyX (2) in the two-phase region for a specific propertyX, wherem 1 andm g are, respectively, the masses of the liquid and gas

ΔVAP :

Change due to vaporization

ν:

Constant volume (isochoric)

T :

Constant temperature (isothermal)

C:

Critical property

σ:

Saturation property

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Authors and Affiliations

  1. Physical and Chemical Properties Division, Chemical Science and Technology Laboratory, National Institute of Standards and Technology, 80303, Boulder, Colorado, USA

    H. A. Duarte-Garza & J. W. Magee

  2. Texas A&M University, 78363, Texas, Kingsville, USA

    H. A. Duarte-Garza

Authors
  1. H. A. Duarte-Garza
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  2. J. W. Magee
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Duarte-Garza, H.A., Magee, J.W. Subatmospheric vapor pressures evaluated from internal-energy measurements. Int J Thermophys 18, 173–193 (1997). https://doi.org/10.1007/BF02575206

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  • DOI: https://doi.org/10.1007/BF02575206

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