Selected Abstracts (Properties of Fluids)
Click your browser's 'Reload' feature to ensure you are
looking at the most recent version of this page.
Return to [List of topics
| Fluid Metrology Group | Process Measurements Division]
Browse abstracts on [Experimental Techniques |
Properties of Fluids | Critical Phenomena in Fluids | Metrology]
Thermodynamic properties of CHF2-O-
CHF2, bis(difluoromethyl) ether
D. R. Defibaugh, K. A. Gillis, M. R. Moldover, G. Morrison,
and J. W. Schmidt
Physical and Chemical Properties Division, Chemical
Science and Technology Laboratory, National Institute of Standards and
Technology, Gaithersburg, MD 20899, U.S.A.
We have measured the thermodynamic properties of bis(difluoromethyl)
ether, CHF2-O-CHF2, a candidate alternative refrigerant
that is also known as E134. From the data we obtained the coefficients
of a Carnahan-Starling-DeSantis equation of state and a polynomial representation
of the ideal-gas heat capacity. This representation of the thermodynamic
properties of E134 is consistent with the computer package REFPROP distributed
by the National Institute of Standards and Technology to represent the
properties of many candidate refrigerants. The representation is based
on measurements of the refractive index of the saturated liquid and vapor,
and the speed of sound of the dilute vapor. These measurements provide
the boiling point, critical parameters, and the ideal-gas heat capacity
of E134. Measurements on less pure samples were used to estimate the density
of saturated liquid E134 and compressed liquid E134, and the interfacial
tension. The pure samples appeared to be stable during the measurements;
under similar conditions impure samples were not. Azeotropy in mixtures
of E134 with CHF2CH2F (also known as R143) was discovered.
Return to List of topics
Thermodynamic properties of two
gaseous halogenated ethers from speed of sound measurements: difluoromethoxy
- difluoromethane and 2 - difluoromethoxy - 1,1,1 - trifluoroethane
K. A. Gillis
Physical and Chemical Properties Division, Chemical
Science and Technology Laboratory, National Institute of Standards and
Technology, Gaithersburg, MD 20899, U.S.A.
We present measurements of the speed of sound in gaseous
difluoromethoxy - difluoromethane (CHF2-O-CHF2) and
2- difluoromethoxy- 1,1,1- trifluoroethane (CF3- CH2-
O-CHF2). These measurements were performed in an all-metal apparatus
between 255 and 384 K. We have obtained ideal-gas heat capacities and second
acoustic virial coefficients from analysis of these measurements. Two methods
of correlating the second acoustic virial coefficients, a square well model
of the intermolecular interaction and a function due to Pitzer and Curl,
are presented.
Return to List of topics
Thermodynamic properties of seven
gaseous halogenated hydrocarbons from acoustic measurements: CHClFCF3,
CHF2CF3, CF3CH3, CHF2CH3,
CF3CHFCHF2, CF3CH2CF3,
and CHF2CF2CH2F
Keith A. Gillis
Physical and Chemical Properties Division, Chemical
Science and Technology Laboratory, National Institute of Standards and
Technology, Gaithersburg, MD 20899, U.S.A.
Measurements of the speed of sound in 7 halogenated hydrocarbons
are presented. The compounds in this study are 1 - chloro - 1,2,2,2 - tetrafluoroethane
(CHClFCF3 or HCFC-124), pentafluoroethane (CHF2CF3
or HFC-125), 1,1,1 - trifluoroethane (CF3CH3 or HFC-143a),
1,1 - difluoroethane (CHF2CH3 or HFC-152a), 1,1,1,2,3,3
- hexafluoropropane (CF3CHFCHF2 or HFC-236ea), 1,1,1,3,3,3
- hexafluoropropane (CF3CH2CF3 or HFC-236fa),
and 1,1,2,2,3 - pentafluoropropane (CHF2CF2CH2F
or HFC-245ca). The measurements were performed with a cylindrical resonator
at temperatures between 240 and 400 K and at pressures up to 1.0 MPa. Ideal-gas
heat-capacities and acoustic virial coefficients were directly deduced
from the data. The ideal-gas heat-capacity of HFC-125 from this work differs
from spectroscopic calculations by less than 0.2% over the measurement
range. The coefficients for virial equations of state were obtained from
the acoustic data using hard-core square-well intermolecular potentials.
Gas densities that were calculated from the virial equations of state for
HCFC-124 and HFC-125 differ from independent density measurements by at
most 0.15%, for the ranges of temperature and pressure over which both
acoustic and Burnett data exist. The uncertainties in the derived properties
for the other 6 compounds are comparable to those for HCFC-124 and HFC-125.
Return to List of topics
Practical determination of gas densities
from the speed of sound using square-well potentials
Keith A. Gillis and M. R. Moldover
Physical and Chemical Properties Division, Chemical
Science and Technology Laboratory, National Institute of Standards and
Technology, Gaithersburg, MD 20899, U.S.A.
The relationships between the first three density virial
coefficients (B, C, and D) and the first four acoustic
virial coefficients ( 
,
,
,
and 
)
are re-derived and a published error relating D to
is corrected. We observe that even if the nth and higher density
virial coefficients of a hypothetical gas are identically zero, the nth
and higher acoustic virial coefficients are not zero; they depend on the
temperature derivatives of the 1st through (n-1)th density virial
coefficients. Thus, two density virial coefficients may suffice for a fit
to acoustic data with a cubic pressure dependence. These results are exploited
by extending the pressure range of fits to previously published speed-of-sound
data without either introducing additional parameters or degrading the
fits. We deduce gas densities from fits to speed-of-sound data with virial
coefficients having the temperature dependencies calculated from square-well
potentials. The estimated densities differ from independent measurements
by a few tenths of a percent in an important range of conditions. These
estimates require no p-V-T data whatsoever.
Return to List of topics
Thermodynamic properties of CHF2-CF2
-CH2F, 1,1,2,2,3 - pentafluoropropane
D.R. Defibaugh, K.A. Gillis, M.R. Moldover, J.W. Schmidt,
and L.A. Weber
Physical and Chemical Properties Division, Chemical
Science and Technology Laboratory, National Institute of Standards and
Technology, Gaithersburg, MD 20899, U.S.A.
We report the thermodynamic properties of 1,1,2,2,3-pentafluoropropane
(known in the refrigeration industry as HFC-245ca) in the temperature and
pressure region commonly encountered in thermal machinery. The properties
are based on measurements of the vapor pressure, the density of the compressed
liquid, the refractive index of the saturated liquid and vapor, the critical
temperature, the speed of sound in the vapor phase, and the capillary rise.
From these data we deduce the saturated liquid and vapor densities, the
equation of state of the vapor phase, the surface tension, and estimates
of the critical pressure and density. The data determine the coefficients
for a Carnahan- Starlings-DeSantis (CSD) equation of state. The CSD coefficients
found in REFPROP 4.0 are based on the measurements reported here.
Return to List of topics
Thermodynamic properties of CF3-CHF-
CHF2, 1,1,1,2,3,3 - hexafluoropropane
D.R. Defibaugh, K.A. Gillis, M.R. Moldover, J.W. Schmidt,
and L.A. Weber
Physical and Chemical Properties Division, Chemical
Science and Technology Laboratory, National Institute of Standards and
Technology, Gaithersburg, MD 20899, U.S.A.
We report the thermodynamic properties of 1,1,1,2,3,3-hexafluoropropane
(known in the refrigeration industry as R236ea or HFC-236ea) in the temperature
and pressure region commonly encountered in thermal machinery. The properties
are based on measurements of the vapor pressure, the density of the compressed
liquid (PVT), the refractive index of the saturated liquid and vapor,
the critical temperature, and the speed of sound in the vapor phase. The
surface tension was determined from the capillary rise. From these data
we deduce the ideal-gas heat-capacity, the saturated liquid and vapor densities,
the equation of state of the vapor phase, the surface tension, and estimates
of the critical pressure and density. The data determine the coefficients
for a Carnahan- Starlings-DeSantis (CSD) equation of state. The CSD coefficients
found in REFPROP 4.0 database are based on these present measurements.
Return to List of topics
Interaction coefficients for fifteen
mixtures of flammable and non-flammable components
Dana R. Defibaugh and Graham Morrison
Physical and Chemical Properties Division, Chemical
Science and Technology Laboratory, National Institute of Standards and
Technology, Gaithersburg, Maryland 20899
Bubble pressures were measured for fifteen binary mixtures
each comprised mainly of one flammable and one non-flammable component.
The mixtures were trichlorofluoromethane + isopentane, pentafluoroethane
+ 1,1,1-trifluoroethane, 1,1-dichloro-2,2,2-trifluoroethane + {1,1-dichloro-1-fluoroethane
or isopentane} 1,1,1,2-tetrafluoroethane + {1,1-difluoroethane or propane
or cyclopropane or isobutane}, difluoromethane + {pentafluoroethane or
1,1,1,2-tetrafluoroethane or 1,1-difluoroethane}. Also studied were mixtures
of 1,1-difluoroethane + {cyclopropane or propane or butane or isobutane}
which are comprised of two flammable components. The measurements were
made at approximately equimolar compositions using either a vapor-liquid
equilibrium apparatus over a range of temperatures, or a static pressure
measurement at 273.15 K. The bubble pressures were used to determine interaction
coefficients which characterize the non-ideal behavior of these fluid mixtures.
The interaction coefficients are used in equation-of-state models for the
thermodynamic properties of refrigerant mixtures.
Return to List of topics
Thermodynamic properties of 1,1-dichloro-1-fluoroethane
(R141b)
Dana R. Defibaugh, Anthony R. H. Goodwin, Graham Morrison,
Lloyd A.Weber
National Institute of Standards and Technology, Physical
and Chemical Properties Division, Chemical Science and Technology Laboratory,
Gaithersburg, Maryland 20899
Thermodynamic properties of 1,1-dichloro-1-fluoroethane
(R141b) were measured using a vibrating tube densimeter and two different
ebulliometric techniques. The densimeter was used to measure compressed
liquid densities, and the ebulliometers were used to study the vapor pressure.
Measurements of the density were made between 278 K and 369 K and 100 kPa
to 6000 kPa. The vapor pressure was measured from 253 K to 355 K at pressures
from 10 kPa to 449 kPa. Both the compressed liquid and the vapor pressure
results are compared with other published data. Our results for the vapor
pressure have been combined with results already published to obtain a
correlation for the vapor pressure from 253 K to the critical point.
Return to List of topics
Compressed liquid densities, saturated
liquid densities and vapor pressures of 1,1-difluoroethane
Dana R. Defibaugh and Graham Morrison
Physical and Chemical Properties Division, Chemical
Science and Technology Laboratory, National Institute of Standards and
Technology, Gaithersburg, Maryland 20899
The compressed liquid densities and vapor pressures of
1,1-difluoroethane (HFC-152a) have been measured, correlated, and compared
with other data. The liquid densities were measured with a combined standard
uncertainty of ± 0.05% using a vibrating tube densimeter over a
temperature range of 243 K to 371 K and at pressures from near the saturated
vapor pressure to 6500 kPa thus the data extend nearly to the critical
point (TC = 386.41 K and PC = 4514.7
kPa). The vapor pressures were measured with a combined standard uncertainty
of ± 0.02% using a stainless steel ebulliometer in the temperature
range from 280 K to 335 K. Saturated liquid densities were calculated by
extrapolating the compressed liquid isotherms to the saturation pressure.
Download full PDF from ACS
Return to List of topics
Thermodynamic properties of difluoromethane (R32)
Dana R. Defibaugh, Graham Morrison, Lloyd A.Weber
National Institute of Standards and Technology, Physical
and Chemical Properties Division, Chemical Science and Technology Laboratory,
Gaithersburg, Maryland 20899
The pressure-volume-temperature behavior of difluoromethane
(R32) has been measured using a vibrating tube densimeter apparatus and
a Burnett/isochoric apparatus. Liquid PVT data from the vibrating
tube densimeter ranged in temperature from 242 K to 348 K with a pressure
range of 2000 kPa to 6500 kPa. Data from the Burnett apparatus consisted
of 11 isochores for the gas and supercritical fluid, along with vapor pressure
measurements. The temperature ranged from 268 K to 373 K. The gas phase
data are correlated with a virial equation of state. The compressed liquid
data are fit with an abbreviated form of the modified Benedict Webb Rubin
(mBWR) equation. A table of thermodynamic properties is presented for the
saturated liquid and vapor states.
Download full PDF from ACS
Return to List of topics
Alternative refrigerants CH2F2
and C2HF5: critical temperature, refractive index,
surface tension, and estimates of liquid, vapor, and critical densities
James W. Schmidt and Michael R. Moldover
National Institute of Standards and Technology
Refractive index data and capillary rise data are reported
for CH2F2 and C2HF5, which
are denoted as R32 and R125 by the refrigeration industry. For each fluid,
the data extend from 296 K to the critical point and yield the critical
temperature TC and the temperature-dependent capillary
length. The refractive index data were combined with liquid density data
at 303 K to determine the Lorentz-Lorenz constant k. This constant
and the data were used to estimate the liquid, vapor, and critical densities,
and the surface tension up to the critical point. For both fluids, the
surface tension 
is given by the expression
= S0t1.26kBTC(NA/VC)2/3
with S0 = 5.5 for R32 and S0 = 6.0
for R125. Here kB, NA, TC,
and VC are the Boltzmann constant, the Avogadro constant,
the critical temperature, and the molar critical volume, respectively,
and t = (TC - T)/TC.
The present values for S0 are close to the average value
S0 = 5.7 for seven other refrigerants
Download full PDF from ACS
Return to List of topics
Partially halogenated hydrocarbons
CHFCl-CF3, CF3-CH3, CF3-CHF-CHF2,
CF3-CH2-CF3, CHF2-CF2-CH2F,
CF3-CH2-CHF2, CF3-O-CHF2:
critical temperature, refractive indices, surface tension and estimates
of liquid, vapor and critical densities
James W. Schmidt, Ernesto Carrillo-Nava, and Michael R.
Moldover
National Institute of Standards and Technology
For seven partially halogenated hydrocarbons designated
by the refrigeration industry as the refrigerants, R124, R143a, R236ea,
R236fa, R245ca, R245fa, and E125, we determined the critical temperatures
TC, the capillary rise, and the refractive indices of
the liquid and vapor phases in the temperature range from 20C to their
critical points. The refractive indices, when combined with liquid densities
measured by others at lower temperatures, yield estimates of the liquid
and the vapor densities at higher temperatures including the critical temperature.
The densities combined with the capillary rise data give the surface tensions,
,
up to the critical points. The surface tensions of the seven refrigerants
in this study together with nine others measured previously can be represented
by the scaled equation
= 2.51(1 + 0.609
)
kBTC(NA/VC)2/3
t1.26(1 + 0.348t1/2 - 0.487t)
where t = (TC-T)/TC,
is the reduced temperature, and kB, NA,
VC, and
are Boltzmann's constant, Avagadro's constant, the critical volume, and
the acentric factor, respectively. This equation yields values for
within 5% of the measured values for all 16 candidate replacement refrigerants.
Return to List of topics
Virial equation of state of helium,
xenon, and helium-xenon mixtures from speed-of-sound and Burnett P-
-T
measurements
J. J. Hurly1, J. W. Schmidt1, S.
J. Boyes1,2, and M. R. Moldover1
1Physical and Chemical Properties Division,
Chemical Science and Technology Laboratory, National Institute of Standards
and Technology, Gaithersburg, MD 20899, U.S.A.
2Present address: Centre for Quantum Metrology, National
Physical Laboratory, Teddington, Middlesex TW11 0LW, United Kingdom.
The virial equation of state was determined for helium,
xenon, and helium-xenon mixtures for the pressure and temperature ranges
0.5 MPa to 5 MPa and 210 K to 400 K. Two independent experimental techniques
were employed: Burnett P--T
measurements and speed-of-sound measurements. The temperature-dependent
second and third density virial coefficients for pure xenon, and the second
and third interaction density virial coefficients for helium-xenon mixtures
were determined. The density virial equations of state for xenon and helium-xenon
mixtures presented reproduce the speed-of-sound data within 0.01 % and
the P--T
data within 0.02 % of the pressures. All the results for helium are consistent,
within experimental errors, with recent ab initio calculations,
confirming the accuracy of the experimental techniques.
Return to List of topics
Compressed and Saturated Liquid
Densities for 18 Halogenated Organic Compounds
Dana R. Defibaugh and Michael R. Moldover
Physical and Chemical Properties Division, Chemical
Science and Technology Laboratory, National Institute of Standards and
Technology , Gaithersburg, Maryland 20899
The pressure-density-temperature P(,T
) behavior of 18 liquids that are potential working fluids in thermal machinery
has been measured using a vibrating tube densimeter. For each liquid, the
data were taken on isotherms spaced at intervals of 5 K to 10 K spanning
the temperature range 245 K to 370 K. The pressures ranged from just above
the vapor pressure (or the critical pressure) to 6500 kPa. The results
of measurements at more than 12,000 thermodynamic states are summarized
by correlating functions. Comparison with data from other laboratories
indicates that the relative expanded uncertainty in the measured densities
is less than 0.05%, except in the critical region. The repeatability of
the measured densities is on the order of 0.005%. For each fluid, the P(,
T ) data were extrapolated to the vapor pressure to obtain a representation
of the density of the liquid at the vapor pressure. The fluids studied
(and their designations by the refrigeration industry) were: trichlorofluoromethane
(R11); chlorodifluoromethane (R22); 1,1-dichloro-2,2,2-trifluoroethane
(R123); 1,2-dichloro-1,2,2-trifluoroethane (R123a); 1-chloro-1,2,2,2-tetrafluoroethane
(R124); 1,1,2,2-tetrafluoroethane (R134); 1,1,1,2-tetrafluoroethane (R134a);
1,1-dichloro-1-fluoroethane (R141b); 1,1,2-trifluoroethane (R143); 1,1,1-trifluoroethane
(R143a); pentafluorodimethylether (E125); 1,1-difluoroethane (R152a); octafluoropropane
(R218); 1,1,1,2,3,3,3-heptafluoropropane (R227ea); 2-(difluoromethoxy)-1,1,1-trifluoroethane
(E245); 1,1,1,2,2-pentafluoropropane (R245cb); 1,1,1,3,3-pentafluoropropane
(R245fa); and propane (R290).
Download full PDF from ACS
Return to List of topics
Equation of state and ideal-gas
heat capacity of a gaseous mixture of 1,1,1,2-tetrafluoroethane, pentafluoroethane,
and difluoromethane
J.J. Hurly, J.W. Schmidt, and K.A. Gillis
Physical and Chemical Properties Division, Chemical
Science and Technology Laboratory, National Institute of Standards and
Technology, Gaithersburg, MD 20899, USA
We present the gas-phase equation of state and ideal-gas
heat capacity of a ternary mixture (nominal molar concentration) 1,1,1,2-tetrafluoroethane
(35%), pentafluoroethane (30%), and difluoromethane (35%) for temperatures
between 260 and 453 K and pressures between 0.05 and 7.7 MPa. These results
were based on two very different measurement techniques. The first technique
measured the gas density of the mixture in a Burnett apparatus from 313
to 453 K and from 0.2 to 7.7 MPa. The second technique deduced the gas
density and ideal-gas heat capacity from high-accuracy speed-of-sound measurements
in the mixture at temperatures between 260 and 400 K and at pressures between
0.05 and 1.0 MPa. The data from the two techniques were analyzed together
to obtain an equation of state that reproduced the densities from the Burnett
technique with a fractional RMS deviation of 0.038%, and it also reproduced
the sound speeds with a fractional RMS deviation of 0.003%. Finally, the
results are compared to a predictive model based on the properties of the
pure fluids.
KEY WORDS: Burnett, CF3CH2F; CF3CHF2;
CH2F2; difluoromethane; equation-of-state; gas density;
heat capacity; pentafluoroethane; refrigerant; speed-of-sound; tetrafluoroethane;
virial coefficient
Return to List of topics
Virial equation of state and ideal-gas
heat capacities of pentafluoro-dimethyl ether
J.J. Hurly, J.W. Schmidt, and K.A. Gillis
Physical and Chemical Properties Division, Chemical
Science and Technology Laboratory, National Institute of Standards and
Technology, Gaithersburg, MD 20899, USA.
A virial equation of state is presented for vapor-phase
pentafluoro-dimethyl ether (CF3-O-CF2H), a candidate alternative refrigerant
known as E125. The equation of state was determined from density measurements
performed with a Burnett apparatus and from speed-of-sound measurements
performed with an acoustic resonator. The speed-of-sound measurements spanned
the ranges 260 < T < 400 K and 0.05 < P < 1.0
MPa. The Burnett measurements covered the ranges 283 < T <
373 K and 0.25 < P < 5.0 MPa. The speed-of-sound and Burnett
measurements were first analyzed separately to produce two independent
virial equations of state. The equation of state form the acoustical measurements
reproduced the experimental sound speeds with a fractional RMS deviation
of 0.0013%. The equation of state form the Burnett measurements reproduced
the experimental pressures with a fractional RMS deviation of 0.012%. Finally,
an equation of state was fit to both the speed-of-sound and the Burnett
measurements simultaneously. The resulting equation of state reproduced
the measured sound speeds with a fractional RMS deviation of 0.0018% and
the measured Burnett densities with a fractional RMS deviation of 0.019%.
Return to List of topics
The static dielectric constant of liquid water between 274 K and 418 K near the saturated vapor pressure
Jean Hamelin,1 James B. Mehl,1,2 and Michael R. Moldover1
1 Physical and Chemical Properties Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, U.S.A.
2 Physics Department, University of Delaware, Newark, DE 19716
The complex dielectric constant (relative electric permittivity)
= 
' - i"
of liquid water was redetermined in the temperature range 0 °C < t < 145 °C at pressures less that 440 kPa. In this work,
was deduced from measurements of the resonant frequencies 
of a novel, re-entrant, two-mode, radio-frequency resonator. The frequencies ranged from 23 MHz to 84 MHz and were well within the low-frequency limit for
' because (2
max)2 < 5×
10-5 where max = 1.8×
10-11 s is the maximum relaxation time of water under the conditions studied. The data for '
for two water samples differing in conductivity by a factor of 3.6 and for the two resonant modes differing in frequency by a factor of 2.6 were simultaneously fit by the polynomial function '(t) = 87.9144 - 0.404399 t + 9.58726×
10-4 t2 - 1.32802×
10-6 t3
with a remarkably small residual standard deviation of 0.0055. The present data are consistent with previously published data; however, they are more precise and internally consistent. The present apparatus was also tested with cyclohexane and yielded the values
'(t) = 2.0551 - 0.00156 t for 20 °C < t < 30 °C, in excellent agreement with previously published values.
Return to List of topics
Measurement of the dipole moments of seven
partially-fluorinated hydrocarbons with a radio-frequency
reentrant cavity resonator
Anthony R.H. Goodwin1 and James B. Mehl2
Physical and Chemical Properties Division, NIST and
1Center for Applied Thermodynamic Studies, University of
Idaho, Moscow, ID 83844-0905
2Department of Physics and Astronomy, University of
Delaware, Newark, DE 19711-2570
Equilibrium dipole moments of gaseous pentafluoro- dimethylether (HFE-125),
1,1,1,2,3,3,3- heptafluoropropane (HFC-227ea),
1,1,1,2,3,3- hexafluoropropane (HFC-236ea),
1,1,1,3,3,3- hexafluoropropane (HFC-236fa),
1,1,2,2,3- pentafluoropropane (HFC-245ca),
1,1,1,2,2- pentafluoropropane (HFC-245fa),
and
1,1,1,2,2,3,3,4- octafluorobutane (HFC-338mccq)
were obtained from the resonance
frequency of a reentrant cavity at temperatures between 250 and 373
K. The electronic contributions to the polarization were determined
for each fluid from liquid-phase optical index of refraction
measurements at 297 K.
Return to List of topics
Thermodynamic Properties of Sulfur Hexafluoride
John J. Hurly, Dana R. Defibaugh, and Michael R. Moldover
Process Measurements Division, NIST, Gaithersburg, MD 20899-8360
We present new vapor phase speed-of-sound data u(P,T), new
Burnett density-pressure-temperature data 
(P,T), and a
few vapor pressure measurements for sulfur hexafluoride
(SF6). The speed-of-sound data spanned the
temperature range 230 K < T < 460 K and reached
maximum pressures that were the lesser of 1.5 MPa or 80% of
the vapor pressure of SF6. The Burnett (P,T)
data were obtained on isochores spanning the density range
137 mol·m-3 < < 4380 mol·m-3
and the temperature range 283 K < T < 393 K. (The
corresponding pressure range is 0.3 MPa < P < 9.0 MPa.)
The u(P,T) data below 1.5 MPa were correlated using a model
hard-core, Lennard-Jones intermolecular potential for the
second and third virial coefficients and a polynomial for
the perfect gas heat capacity. The resulting equation of
state has very high accuracy at low densities; it is useful
for calibrating mass flow controllers and may be
extrapolated to 1000 K. The new u(P,T) data and the new
(P,T) data were simultaneously correlated with a virial
equation of state containing four terms with the
temperature dependencies of model square-well potentials.
This correlation extends nearly to the critical density and
may help resolve contradictions among data sets from the
literature.
Return to List of topics
Reference Values of the Dielectric Constant of Natural Gas
Components Determined with a Cross Capacitor
M. R. Moldover and T. J. Buckley
Process Measurements Division, NIST, Gaithersburg, MD 20899-8360
A novel toroidal cross capacitor was used to measure
accurately the dielectric polarizability
(p)
(i.e., the dielectric constant as a function of the pressure) of
helium, argon, nitrogen, methane, and carbon dioxide at
T = 50 °C. The data extend up to 7 MPa (5 MPa for CO2) and
may be useful for calibrating on-line, capacitance-based
systems that are designed to measure the heating value of
natural gas. The uncertainties of and p are 4×10-6 and
(3.0×10-5 p + 84 Pa), respectively. The properties of
helium that had been calculated ab initio from quantum
mechanics were used to verify that the cross capacitor
deformed in a predictable manner under hydrostatic (gas)
pressure. Thus, a common cause of systematic errors in
measuring the dielectric constant of gases was avoided.
For helium, the R.M.S. deviation of (p) from the
calculations was only 2.7×10-7. This suggests that the
estimated uncertainty is very conservative.
Return to List of topics
The Viscosity of Seven Gases Measured with a Greenspan Viscometer
J.J. Hurly1, K.A. Gillis1, J.B. Mehl2, and M.R. Moldover1
1Process Measurements Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8360
2Current address: PO Box 307; Orcas, WA 98280-0307.
The viscosity of seven gases (Ar, CH4, C3H8, N2,
SF6, CF4, C2F6) was determined by interpreting
frequency-response data from a Greenspan acoustic viscometer with a detailed model developed
by Gillis, Mehl, and Moldover. The model contains a parameter εr that
characterizes the viscous dissipation at the ends of the viscometer's duct. It was difficult
to determine εr accurately from dimensional measurements; therefore,
εr was adjusted to fit the viscosity of helium on the 298 K isotherm
(0.6 MPa < p < 3.4 MPa). This calibration was tested by additional viscosity
measurements using four, well-studied, polyatomic gases (CH4, C2H6,
N2, and SF6) near 300 K and by measurements using argon in the range
293 K < T < 373 K. For these gases, all of the present results agree with reference
values to within ±0.5% (±0.4% in the limit of zero density). The viscosities of CF4
and C2F6 were measured between 210 K and 375 K and up to 3.3 MPa with
average uncertainties of 0.42% and 0.55%, respectively. At the highest density studied for
CF4 (2746 mol·m-3), the uncertainty increased to 1.9%; of this 1.9%, 0.63%
resulted from the uncertainty of the thermal conductivity of CF4, which other
researchers estimated to be 2% of its value at zero density. As an unexpected bonus, the present
Greenspan viscometer yielded values of the speed of sound that agree, within ±0.04%, with reference
values.
Download Full PDF
Return to List of topics
Dielectric Permittivity of Eight Gases Measured with
Cross Capacitors
J. W. Schmidt and M. R. Moldover
Process Measurements Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8360, U.S.A.
A four-ring, toroidal cross capacitor was used to measure accurately the relative dielectric permittivity ε(p, T) of He, Ar, N2, O2, CH4, C2H6, C3H8, and CO2.
(ε is often called the 'dielectric constant.') The data are in the range from 0 to 50°C and, in many cases, extend up to 7 MPa. The accurate measurement of
ε(p, T) required a good understanding of the deformation of the gas-filled capacitors with applied pressure. This understanding was tested in two ways. First, the experimental values of ε(p, T) for helium were compared with theoretical values. The average difference was within the noise, <εexpt − εtheory> =
(−0.05±0.21)×10−6, demonstrating that the four-ring cross capacitor deformed
as predicted. Second, ε(p, T) of argon was measured simultaneously on three isotherms using two capacitors: the four-ring capacitor, and a 16-rod cross
capacitor made using different materials and a different geometry. The results for the two capacitors are completely consistent, within the specifications of the capacitance bridge. There was a small inconsistency that was equivalent to 1×10−6 of the measured capacitances, or, for argon, 3×10−5Aε, where Aε is the
zero-density limit of the molar polarizability P ≡ (ε−1)/[(ε+2) ρ].
Return to List of topics
The Dielectric Permittivity of Saturated Liquid Carbon Dioxide and Propane Measured Using Cross Capacitors
E. F. May,1,2 M. R. Moldover,1,3 and J. W. Schmidt1
1Process Measurements Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8360, U.S.A.
2Guest Scientist, American Australian Association Fellow.
3To whom correspondence should be addressed. E-mail: michael.moldover@nist.gov
Relative dielectric permittivities er of saturated liquid carbon dioxide (CO2)
and propane (C3H8) were measured using well-characterized cross capacitors
in the range 260 K < T < 300 K. The molar polarizability
℘sat = (εr − 1)/[(εr + 2)ρsat]
of CO2 was calculated using the equation of state of Span and Wagner
to convert our values of the saturation vapor pressure psat to values
of the saturated liquid density ρsat. In the range 260 K < T < 300 K, ℘sat =
(7.659 ± 0.001) cm3·mol−1. The systematic difference between measurements
made with two different capacitors was 0.001 cm3·mol−1; this difference is
equivalent to a shift of 12 mK in the liquid temperature. The uncertainty
of ℘sat from the equation of state of CO2 is approximately 0.002 cm3·mol−1.
Our values of ℘sat for CO2 are consistent with the results of Moriyoshi et
al. and of May et al.; however, our values are 0.5% larger than the values
determined by Haynes. For saturated liquid propane, our values of εr agree
with the values of Haynes and Younglove within the combined uncertainty
of 0.0003.
Return to List of topics
Viscosity and Speed of Sound of Gaseous Nitrous Oxide and Nitrogen Trifluoride Measured with a Greenspan Viscometer
John J. Hurly
Process Measurements Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8360, U.S.A.
The viscosity and speed of sound of gaseous nitrous oxide and nitrogen trifluoride were measured using a Greenspan
acoustic viscometer. The data span the temperature range 225 to 375 K and extend up to 3.4 MPa. The average relative
uncertainty of the viscosity is 0.68 % for N2O and 1.02 % for NF3. The largest relative
uncertainties were 3.09 % and 1.08 %, respectively. These occurred at the highest densities
(1702 mol·m−3 for N2O and 2770 mol·m−3 for NF3).
The major contributor to these uncertainties was the uncertainty of the thermal conductivity. The speeds of sound
measured up to 3.4 MPa are fitted by a virial equation of state that predicts gas densities within the uncertainties
of the equations of states available in the literature. Accurate measurements of the speed of sound in both
N2O and NF3 have been previously reported up to 1.5 MPa. The current measurements agree with
these values with maximum relative standard deviations of 0.025 % for N2O and 0.04 % for NF3.
Return to List of topics
Thermodynamic Properties of Gaseous Nitrous Oxide and Nitric Oxide from Speed-of-Sound Measurements
John J. Hurly
Process Measurements Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8360, U.S.A.
The speed of sound was measured in gaseous nitrous oxide (N2O) and nitric oxide (NO) using an acoustic
resonance technique with a relative standard uncertainty of less than 0.01%. The measurements span the temperature range
200 to 460 K at pressures up to the lesser of 1.6 MPa or 80% of the vapor pressure. The data were analyzed to obtain
the constant-pressure ideal-gas heat capacity Cpo as a function of temperature with a
relative standard uncertainty of 0.1%. For N2O, the values of Cpo agree within
0.1% with those determined from spectroscopic data. For NO, the values of Cpo differ from
spectroscopic results by as much as 1.5%, which is slightly more than the combined uncertainties. The speed-of-sound data
were fitted by virial equations of state to obtain temperature-dependent density virial coefficients. Two virial
coefficient models were employed, one based on square-well intermolecular potentials, and the second based on a hard-core
Lennard-Jones intermolecular potential. The resulting virial equations reproduced nearly all the sound-speed data to within
±0.01% and may be used to calculate vapor densities with relative standard uncertainties of 0.1% or less.
Return to List of topics
Thermophysical Properties of Chlorine from Speed-of-Sound Measurements
John J. Hurly
Process Measurements Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8360, U.S.A.
The speed of sound was measured in gaseous chlorine using a highly precise acoustic resonance technique. The data span the
temperature range 260 to 440 K and the pressure range 100 kPa to the lesser of 1500 kPa or 80% of the sample’s vapor
pressure. A small correction (0.003 to 0.06%) to the observed resonance frequencies was required to account for
dispersion caused by the vibrational relaxation of chlorine. The speed-of-sound measurements have a relative standard
uncertainty of 0.01%. The data were analyzed to obtain the ideal-gas heat capacity as a function of the temperature with
a relative standard uncertainty of 0.1%. The reported values of Cpo are in agreement with those
determined from spectroscopic data. The speed-of-sound data were fitted by virial equations of state to obtain the
temperature dependent density virial coefficients. Two virial coefficient models were employed, one based on square-well
intermolecular potentials and the second based on a hard-core Lennard–Jones intermolecular potential. The resulting virial
equations reproduced the sound speed data to within 0.01% and may be used to calculate vapor densities with relative
standard uncertainties of 0.1% or less.
Return to List of topics
Thermophysical Properties of Nitrogen Trifluoride, Ethylene Oxide, and Trimethyl Gallium from Speed-of-Sound Measurements
John J. Hurly
Process Measurements Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8360, U.S.A.
The speed of sound was measured in gaseous nitrogen trifluoride, ethylene oxide, and trimethyl gallium using a highly precise
acoustic resonance technique. The measurements span the temperature range 200 to 425 K and reach pressures up to the lesser
of 1500 kPa or 80% of the sample vapor pressure. The speed-of-sound measurements have a relative standard uncertainty of
less than 0.01%. The data were analyzed to obtain the constant-pressure ideal-gas heat capacity Cpo
as a function of temperature with a relative standard uncertainty of 0.1%. The values of Cpo are
in agreement with those determined from spectroscopic data. The speed-of-sound data were fitted by virial equations of state
to obtain temperature-dependent density virial coefficients. Two virial coefficient models were employed, one based on
square-well intermolecular potentials, and the second based on a hard-core Lennard-Jones intermolecular potential. The
resulting virial equations reproduced the sound-speed data to within ±0.02%, and may be used to calculate vapor
densities with relative standard uncertainties of 0.1% or less.
Return to List of topics
Browse abstracts on [Experimental Techniques |
Properties of Fluids | Critical Phenomena in Fluids | Metrology]
Return to [Fluid Metrology Group | Process Measurements Division]
Last modified: 11 January 2007.
Send comments to the webmaster at 
.