PvT and thermal-pressure coefficient measurements of diethyl ether (DEE) in the critical and supercritical regions

The pressures ( P ) and its temperature derivatives or thermal-pressure coefficient, γ V  = (∂ P /∂ T ) V , of DEE have been measured in the near- and supercritical regions as a function of temperature along the various liquid and vapour isochores. Measurements were made in the immediate vicinity of the liquid–gas phase transition and the critical points (single- and two-phase regions) using a high-temperature, high-pressure, nearly constant-volume adiabatic piezo-calorimeter. The constant-volume adiabatic calorimeter previously used for C V measurements was additionally supplied with high accurate strain gauge (calibrated piezoelectric transducer) to measure simultaneously the PvT , C V vT , and thermal-pressure coefficient γ V . Measurements were made along 17 liquid and vapour isochores in the range from (212.6 to 534.6) kg · m −3 and at temperatures from (347 to 575) K and at pressures up to 18 MPa. The quasi-static thermo- (reading of PRT, T – τ plot) and barograms (readings of the high accurate strain gauge, P – τ plot) techniques were used to accurate measure of the phase transition parameters ( P S ,  ρ S ,  T S ) and γ V at saturation curve. Temperatures at the liquid-gas phase transition curve, T S ( ρ ), for each measured density (isochore) and the critical parameters ( T C and ρ C ) for DEE were obtained using the quasi-static thermograms technique. The expanded uncertainty of the pressure and its temperature derivative, (∂ P /∂ T ) V , measurements at the 95% confidence level with a coverage factor of k  = 2 is estimated to be 0.05% and (0.12 to 1.5)% (depending on temperature and pressure), respectively. The measured pressures and temperature derivatives, (∂ P /∂ T ) V , have been used to calculate the internal pressure (or energy–volume coefficient) as ∂ U ∂ v T = T ∂ P ∂ T V - P . The effect of pressure and temperature on the internal pressure near the critical point was studied. The measured values of thermal-pressure coefficient, (∂ P /∂ T ) V , were used to determine accurately the behaviour of second temperature derivative (∂ 2 P /∂ T 2 ) V near the critical point and compared with our previous isochoric heat capacity measured results, ∂ 2 P ∂ T 2 ρ = - ρ 2 T ∂ C V ∂ ρ T . The measured and derived thermodynamic properties of DEE near the critical point were interpreted in terms of theory of critical phenomena.