A Small-Volume Apparatus for the Measurement of Phase Equilibria

An apparatus has been designed and constructed for the measurement of vapor-liquid equilibrium properties. The main components of the apparatus consist of an equilibrium cell and a vapor circulation pump. The cell and all of the system valves are housed inside a temperature controlled, insulated aluminum block. The temperature range of the apparatus is 260 K to 380 K to pressures of 6 MPa. The uncertainty of the temperature measurement is 0.03 K, and the uncertainty in the pressure measurement is 9.8 × 10⁻⁴ MPa. An automated data acquisition system is used to measure temperature and pressure at equilibrium. The apparatus has been performance tested by measuring the vapor pressures of propane, butane, and a standard mixture of propane + butane.     

Calibration Method and Uncertainty Assessment of a High-Temperature GHP Apparatus

In this research, a calibration method of a high-temperature guarded hot plate (GHP) apparatus was proposed in order to improve the measurement accuracy of thermal conductivity. The measurement uncertainties of this GHP apparatus were assessed to validate the reliability of this calibration method. The temperature difference across the guarded gap was set as the bias value to eliminate the heat exchange over the guarded gap. The effects of the thermal expansion and pressure of the apparatus on thickness were investigated to revise the measurement results of in-situ thickness and meter area, respectively. The assessed uncertainty indicated that the related expanded uncertainty approximately increased with the increase in testing temperature and the calibration method should be valid in the temperature range. The contribution of each factor on the combined uncertainty showed that the temperature distribution in plane direction was the main factor in influencing the measurement of thermal conductivity.     

Measuring volume ratios of vacuum vessels using non-evaporable getters

Several methods have been used in vacuum metrology for volume determination. Volume ratios can be determined by different gas expansion methods, which were primarily developed for the precise determination of expansion ratios in static expansion systems. Measurements of the volume ratios using the gas expansion methods below 10⁻² Pa are influenced by outgassing from chamber walls. To reduce outgassing in the expansion chambers during measurements at low pressures, we have installed non-evaporable getter (NEG) pumps, which pump hydrogen and other active gases but have a negligible pumping speed for inert gases. The volume ratio of two chambers of an experimental static expansion system has thus been measured using the inert gases argon, helium and krypton. The obtained results were compared with the measurements without the use of an NEG pump. Measurements of the pressure ratios were performed with a spinning rotor gauge (SRG).     

Measurement of Binary Diffusion Coefficients for Neon–Argon Gas Mixtures Using a Loschmidt Cell Combined with Holographic Interferometry

The paper reports on experimental binary diffusion coefficient data of neon–argon gas mixtures. Measurements were performed in the temperature range between 293.15 K and 333.15 K and for pressures between 1 bar and 10 bar over almost the whole composition range using a Loschmidt diffusion cell combined with holographic interferometry. The thermostated Loschmidt cell is divided into two half-cells, which can be separated and connected by a sliding plate. Prior to the measurements, two different pure gases are filled into the two half-cells. After starting the diffusion process, the temporal change of the partial molar densities, or rather of the refractive index of the gases, is detected in both half-cells using two holographic interferometers. With this apparatus, the temperature, pressure, and concentration dependence of the binary diffusion coefficient can be determined. The relative uncertainty of a diffusion measurement is between 0.4 % and 1.4 % depending on the pressure. The experimental data are compared with data from the literature and with new theoretical data based on quantum-mechanical ab initio calculations combined with the kinetic theory of gases. Due to a systematic error, the concentration dependence determined in the upper half-cell shows deviations from the theoretical values and from most of the literature data. The concentration, temperature, and pressure dependence obtained from the data from the lower half-cell, however, are in very good agreement with available data. The product of the binary gas diffusion coefficient and the molar density of the gas mixture shows no significant dependence on pressure for the studied neon–argon noble gas system.     

A New Calibration Apparatus of Port Quick Leak Detector for Spacecraft

The leak rate of spacecraft port must meet certain standards during the opening and closing processes. The port quick leak detector can be used to on-line measure leak rates of spacecraft port. In this work, a new calibration apparatus is designed and investigated. It can work at different temperatures with different gases in a wide leak rate range. This apparatus consists of the standard leak rate system, calibration system, temperature control system, gas supply and pumping system, which is designed on the basis of direct comparison calibration method and pressure-rising method with a constant volume, and the leak rates are provided by the standard leak rate system. The calibration range of the calibration apparatus is from 2 × 10 ^–6 to 2 × 10 ^–2 Pa m³/s. Its combined standard uncertainty is about 4.7%, and its calibration temperature is from ? 50 to 20 °C. Within the above leak rate and temperature ranges, the correction factor of the port quick leak detector is about 0.88–1.09.     

Thermal conductivity and density of toluene in the temperature range 273–373 K at pressures up to 250 MPa

New experimental data on the thermal conductivity and the density of liquid toluene are presented in the temperature range 0–100°C at pressures up to 250 MPa. The measurements of thermal conductivity were performed with a transient hot-wire apparatus on an absolute basis with an inaccuracy less than 1.0%. The density was measured with a high-pressure burette method with an uncertainty within 0.1%. The experimental results for both properties are represented satisfactorily by the Tait-type equations, as well as empirical polynomials, covering the entire ranges of temperature and pressure. Furthermore, it is found that simple relations exist between the temperature dependence of thermal conductivity and the thermal expansion coefficient, and also between the pressure dependence of thermal conductivity and the isothermal compressibility, as are suggested theoretically.     

Thermal Conductivity of Humid Air

In this article, measurements of the thermal conductivity of humid air as a function of pressure, temperature, and mole fraction of water, for pressures up to 5 MPa and temperatures up to 430 K, for different water contents (up to 10 % vapor mole fraction) are reported. Measurements were performed using a transient hot-wire apparatus capable of obtaining data with an uncertainty of 0.8 % for gases. However, as moist air becomes corrosive above 373 K and at pressures >5 MPa, the apparatus, namely, the pressure vessel and the cells had to be modified, by coating all stainless-steel parts with a titanium nitride thin film coating, about 4 μm thick, obtained by physical vapor deposition. The expanded uncertainty (coverage factor k = 2) of the present experimental thermal conductivity data is 1.7 %, while the uncertainty in the mole fraction is estimated to be better than 0.0006. Experimental details regarding the preparation of the samples, the precautions taken to avoid condensation in the tubes connected to the measuring cell, and the method developed for obtaining reliable values of the water content for the gas mixtures are discussed. A preliminary analysis of the application of the kinetic theory of transport properties in reacting mixtures to interpret the complex dependence of the thermal conductivity of humid air on water composition is addressed.     

Densities of liquid R 114 at temperatures from 310 to 400 K and pressures up to 10 MPa

Density measurements for liquid R 114 (dichlorotetrafluoroethane) have been obtained with a variable-volume method. The results cover the high-density region from 1007 to 1462 kg·m^–3 along ten isotherms between 310 and 400 K at 16 pressures from 0.5 to 10.0 MPa. The experimental uncertainty in the density measurements was estimated to be no greater than 0.2%. Based on the present results the derivatives with respect to temperature and pressure were calculated, and numerical values of the volume expansion coefficient and of the isothermal compressibility are tabulated as a function of temperature and pressure.     

Thermal conductivity andPVT measurements of pentafluoroethane (refrigerant HFC-125)

By means of the transient and steady-state coaxial cylinder methods, the thermal conductivity of pentafluoroethane was investigated at temperatures from 187 to 419 K and pressures from atmospheric to 6.0 MPa. The estimated uncertainty of the measured results is ±(2–3)%. The operation of the experimental apparatus was validated by measuring the thermal conductivity of R22 and R12. Determinations of the vapor pressure andPVT properties were carried out by a constant-volume apparatus for the temperature range 263 to 443 K, pressures up to 6 MPa, and densities from 36 to 516 kg m^–3. The uncertainties in temperature, pressure, and density are less than ±10 mK, ±0.08%, and ±0.1%, respectively.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

PVT Measurements on tetrafluoroethane (R134a) along the vapor-liquid equilibrium boundary between 288 and 373 K and in the liquid state from the triple point to 265 K

For the investigations of the gas-liquid phase equilibria, a new apparatus has been developed capable of simultaneously determining the pressure and the liquid and vapor densities using Archimedes' principle. The relative measurement uncertainties of the liquid and vapor densities of R134a (purity, 99.999%) at 313 K are 2×10^–4 and 7×10^–4, respectively (95% confidence level). For the measurements in the liquid region along nine quasi-isochores at pressures up to 5 MPa, an isochoric apparatus was used. The relative measurement uncertainty ofpv/(RT) is less than 1×10^–3. In addition to the investigation of the (p, v, T) properties, the temperature and pressure at the triple point and the vapor pressure between the triple point and 265 K were measured. On the basis of these data, a vapor pressure correlation has been developed that reproduces the measured vapor pressures within the uncertainty of measurement. The results of our measurements are compared with a fundamental equation for R134a, which is based on the measurements of other research groups.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Compression factor and vapor pressure of bromotrifluoromethane in the temperature range 245–393 K at pressures up to 12 MPa

The compression factors and vapor pressures have been measured on bromotrifluoromethane using a Burnett apparatus. The results on the compression factor cover the range of temperatures 263 to 393 K and of pressures 0.14 to 11.6 MPa, corresponding to a density variation from 7 to 1367 kg· m^–3. The experimental uncertainty of these 176 measurements of compression factor was estimated to be 0.2%. Thirty measurements of vapor pressure were made for temperatures 245 to 339 K, with an experimental uncertainty of 0.1%. Based on these results, the second virial coefficients were determined for temperatures 293 to 393 K.     

Experimental study of the influences of degraded vacuum on multilayer insulation blankets

The paper presented experimental investigation on the heat transfer of MLI with different rarefied gases at different pressures. The investigations were carried out using an innovative static liquid nitrogen boil-off rate measurement system in the case of the small temperature perturbations of cold and warm boundaries. The heat fluxes for a number of inert and some polyatomic gases have been analyzed at different heat transfer conditions ranging from molecular to continuum regime, apparent thermal conductivities of the multilayer insulation were measured over a wide range of temperature (77 K–300 K) and pressure (10⁻³–10⁵ Pa) using the apparatus. The experimental results indicated that under degraded vacuum condition, the influences of rarefied gas on the MLI thermal performance very depend on the gas rarefaction degree which impacted by the MLI vacuum degree. Under the condition of molecular regime heat transfer, the MLI thermal performance was greatly influenced by gas energy accommodation coefficients (EAC), when under the continuum regime, the performances depend on the thermal conductivity of rarefied gas itself. Compared to the results of N₂, Ar, CO₂, Air and He as interstitial gases in the MLI, Ar was the better selection as space gas because of its low EAC and thermal conductivity characteristics on the different vacuum condition ranging from high pressure to vacuum. So different residual gases can be utilized according to the vacuum level and gas energy accommodation coefficient, in order to improve the insulation performance of low vacuum MLI.     

Virial coefficients of methane-ethane mixtures in the temperature range from 0 to 60°C determined with an automated expansion apparatus

An automated expansion apparatus has been set up which can be used for PVT measurements on gases from Il to 60 C at pressures from 2 to 7 NIPa. It consists of two chambers connected by an expansion valve. The ratio of the two volumes is 1:54. The measurements along one isotherm. which take 20 h, can be per formed routinely and without operator attendance. The uncertainty of the com pressibility factor is estimated to be 10^–3. The compressibility factors of binary mixtures of the main components of most natural gases, methane and ethane, were determined on high-grade gas, the mole fractions of ethane being 0.05, 0.15. and 0.25. Seven isotherms were measured of each of the mixtures. Data from the literature for the virial coefficients of the pure substances were used to establish a virial equation of state which approximates the measured com pressibility factors with a standard deviation of 0.4 x 10 '. This is substantially smaller than the uncertainty. The interaction virial coefficients were calculated from this equation of state. The results obtained are in good agreement with other available data.Paper presented al the Twelfth Symposium on Thermophysical Properties. June 19–24, 1994, Boulder, Colorado. U.S.A.     

On the Way to Determination of the Vapor-Pressure Curve of Pure Water

The determination of the physical properties of pure water, especially the vapor-pressure curve of water, is one of the major issues identified by the Consultative Committee for Thermometry of the International Committee for Weights and Measures (CIPM) to improve the accuracy of the national references in humidity. At the present time the saturation-pressure data, corresponding to ice or liquid?Cvapor equilibrium, at low temperature are scarce and unreliable. This study presents new measurements of vapor and sublimation pressures of, respectively, water and ice, using a static apparatus. Prior to saturation-pressure measurements, the temperature and pressure sensors of the static apparatus were calibrated against reference gauges in use at the LNE- CETIAT laboratories. The effect of thermal transpiration has been studied. The explored temperature range lies between 250 K and 374 K, and the pressure range between 70?Pa and 10⁵ Pa. An automatic data acquisition program was developed to monitor the pressure and temperature. The obtained results have been compared with available literature data. The preliminary uncertainty budget took into account several components: pressure measurements, temperature measurements, and environmental error sources such as thermal transpiration and hydrostatic correction.     

GERG Project: Development and Setup of a New Combustion Reference Calorimeter for Natural Gases

Natural gas plays an important role for worldwide energy supply. For billing purposes precise metering of volume and superior calorific value are very important. At present, only a few institutions worldwide are able to determine the superior calorific value (SCV) of gases and their mixtures with an uncertainty of less than 0.2%. Calculations of SCV’s of natural gases using the data of ISO 6976 provides a similar uncertainty as experimental approaches. For this reason a GERG (Groupe Européen de Recherches Gazières) project was initiated to develop a new reference calorimeter for determining the SCV of flammable gases (natural gases), based on the principle of Rossini for a combustion calorimeter. The purpose of such a reference calorimeter is to determine the SCV of pure gases and gas mixtures with an uncertainty of less than 0.05%. The overall uncertainty budget for the SCV is mainly influenced by the mass determination and temperature measurement. An automated weighing and calibration device is used to measure the mass of the combusted gas with an experimental uncertainty of approx. 0.015%. In addition to the experiment, the flow and temperature field in the calorimeter were simulated. These simulations help to reduce each of the combined uncertainties for the combustion and calibration experiment resulting from the temperature measurement. The determination of the adiabatic temperature rise is performed analytically. The assumptions made by early investigators were carefully reconsidered for the first time. The analysis of the temperature–time curves considers (a) the method of evaluation, (b) the interval length of the main period, (c) the location of the heat release during the calibration experiment, and (d) the temperature sensor location.     

Realization of PLTS-2000 for Low-Temperature Resistance Thermometer Calibration Services Below 650 mK

In this study, the Provisional Low Temperature Scale of 2000 (PLTS-2000) was realized below 650 mK for the purpose of launching low-temperature resistance thermometer calibration services in Japan. A Straty–Adams-type \(^{3}\)He melting pressure thermometer (MPT) and a dilution refrigerator were used to realize the PLTS-2000. Offsets due to hydrostatic pressure head in a filling capillary line of the MPT were adjusted using the minimum pressure fixed point on the \(^{3}\)He melting curve. A rather large MPT hysteresis between the decreasing and increasing pressures was observed during pressure calibration of the MPT and was the main source of uncertainty. The combined standard uncertainty (\(k = 1\)) between 50 mK and 650 mK was estimated to be in the range of 0.40 mK to 2.62 mK. The MPT and a number of resistance thermometers with negative temperature coefficients were mounted on the experimental platform with a thermal connection to a mixing chamber and compared in a multiple-temperature-point calibration. The temperature range around the melting pressure minimum, 250 mK to 400 mK, was not used for the calibration. The expanded uncertainty (\(k = 2\)) in the calibration based on realization of the PLTS-2000 between 50 mK and 650 mK was estimated to be in the range of 0.86 mK to 5.25 mK.     

A Capillary Tube Viscometer Designed for Measurements of Hydrogen Gas Viscosity at High Pressure and High Temperature

A capillary tube viscometer was developed to measure the dynamic viscosity of gases for high pressure and high temperature. The apparatus is simple and designed for safe-handling operation. The gas was supplied to the capillary tube from a high-pressure reservoir tank through a pressure regulator unit to maintain a steady state flow. The measurements of a pressure drop across the capillary tube with high accuracy under extreme conditions are the main challenge for this method. A differential pressure sensor for high pressures up to 100 MPa is not available commercially. Therefore, a pair of accurate absolute pressure transducers was used as a differential pressure sensor. Then the pressure drop was calculated by subtracting the outlet pressure from the inlet one with a resolution of 100 Pa at 100 MPa. The accuracy of the present measurement system is confirmed by measuring the viscosity of nitrogen as a reference gas. The apparatus provided viscosities of nitrogen from ambient temperature to 500 K and hydrogen from ambient temperature to 400 K and for pressures up to 100 MPa with a maximum deviation of 2.2 % compared with a correlation developed by the present authors and with REFPROP (NIST).     

Characterization of the Humidity Calibration Chamber by Numerical Simulations

At the Centre for Metrology MIKES of VTT Technical Research Centre of Finland (VTT MIKES), we have been developing a humidity calibration apparatus for radiosondes within an EMRP Project Metrology for Essential Climate Variables. The minimum air temperature and absolute humidity are \(-80\,^{\circ }\hbox {C}\) and 2.576 \(\times \) \(10^{-4} \,\hbox {g}\cdot \hbox {m}^{-3}\) (corresponding the dew-point temperature \(-90\,^{\circ }\hbox {C}\)), respectively. Recent developments for the apparatus extend its pressure operation range down to 7 hPa (abs). When operating in such dry conditions, the efficiency in calibration is highly limited by the time of humidity stabilization in a measurement chamber: Because the water vapor pressure is very low, the adsorption and desorption of water molecules at the chamber walls have a significant effect on the spatial and temporal humidity differences in the chamber. Inhomogeneity in humidity field inside the calibration chamber increases calibration uncertainty. In order to understand how varying parameters such as pressure, temperature, inflow speed and geometry of chamber effect on stabilization time of humidity field, computational fluid dynamics simulations were developed using Comsol software. Velocity and pressure of fluid, water vapor diffusion, temperature as well as adsorption/desorption of water molecules on the chamber walls were included in the simulations. Adsorption and desorption constants for water on the measurement chamber wall were determined experimentally. The results show that the flow speed and the surface area are the dominant parameters affecting the stabilization time of a calibration chamber. It was also discovered that more homogenous water vapor concentration field is obtained at low pressures.     

Isochoricp-ϱ-T measurements on Difluoromethane (R32) from 142 to 396 K and pentafluoroethane (R125) from 178 to 398 K at pressures to 35 MPa

Thep--T-relationships were measured for difluoromethane (R32) and pentafluoroethane (R125) by an isochoric method with gravimetric determinations of the amount of substance. Temperatures ranged from 142 to 396 K for R32 and from 178 to 398 K for R125, while pressures were up to 35 MPa. Measurements were conducted on compressed liquid samples. Determinations of vapor pressures were made for each substance. I have used vapor pressure data and thep--T data to estimate saturated liquid densities by extrapolating each isochore to the vapor pressure, and determining the temperature and density at the intersection. Publishedp--T data are in good agreement with this study. For thep T apparatus. the uncertainty of the temperature is ±0.03 K. and for pressure it is ±0.01%, atp > 3 MPa and ±0.05% atp < 3 MPa. The principal source of uncertainty is the cell volume (28.5193 cm³ at 0 K and 0 M Pa), which has a standard uncertainty of ±0.003 cm³. When all components of experimental uncertainty are considered. the expanded uncertainty (at the two-sigma level) of the density measurements is estimated to be 0.05%.