New equation for vapor pressures of difluoromethane (HFC-32)

Critically evaluated experimental vapor-pressure data sets supplemented wit h calculated data for low-temperature region were used in the development o f vapor-pressure equations. The optimum number of terms, coefficients, and exponents of the Wagner-type equation were derived by means of the Setzmann -Wagner program OPTIM based on the combination of the stepwise regression a nalysis and evolutionary optimization method. Equations were checked by the reduced enthalpy of vaporization criterion derived from the Clausius-Clape yron equation and specific volume of ideal gas. An equation developed using 261 experimental data points and low-temperature data calculated by Luddec ke and Magee gives an RMS deviation of 0.102%; a second equation based on t he same experimental data and low-temperature data calculated by Tillner-Ro th gives an RMS deviation of 0.101% from experimental points. The triple-po int pressure extrapolated to the measured temperature T-tp=136.34K is discu ssed. Comparisons with vapor pressure equations by Outcalt and McLinden, Du arte-Garza and Magee, and Kubota et al. are also given.