## 3 Results
### Binary Systems  
We measured many binary systems with both the isochoric and the SLE set up. Regarding low GWP mixtures, our research group focused the research activity on blends with R744, as this fluid is non-flammable and has a GWP = 1. R744 + R1234 yf and R744 + R1234 ze(E) were measured both in  
Figure 3: Vapor pressure (\(P_{\rm sat}\)) and \(P\
u T\) data in the superheated vapor phase measured with the isochoric apparatus for R1234 yf and R1234 ze(E). The vapor pressure of R134a calculated from REFPROP 10.0 is reported. the two-phase and in the superheated vapor region [31; 32]. The results obtained are summarized in Tables 2 and 3 for R744 + R1234 yf and R744 + R1234 ze(E), respectively. The _P-T_ diagram of R744 + R1234 ze(E) is also reported in Figure 4 as an example. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Series** & **N. Points** & \(z_{1}\) & \(v\) **(m\({}^{3}\) kg\({}^{-1}\))** & \(\Delta\)T (K)** & \(\Delta\)P (kPa)** \\ \hline
1 & 17 & 0.051 & 0.026 & 225-373 & 62-976 \\
2 & 17 & 0.160 & 0.029 & 223-373 & 96-973 \\
3 & 16 & 0.266 & 0.027 & 224-373 & 148-1119 \\
4 & 15 & 0.365 & 0.026 & 223–373 & 198–1273 \\
5 & 16 & 0.480 & 0.026 & 225–373 & 271–1381 \\
6 & 16 & 0.574 & 0.029 & 224–373 & 312–1354 \\
7 & 16 & 0.662 & 0.018 & 223–373 & 420–2292 \\
8 & 15 & 0.754 & 0.029 & 224–373 & 460–1667 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Measurements at bulk compositions \(z_{1}\) and average specific volumes \(v\) for R744 (1) + R1234 yf (2) binary system over the temperature range \(\Delta T\) and pressure range \(\Delta P\). Figure 4: Two-phase and vapor phase _PvTz_ data measured with the isochoric apparatus for the R744 + R1234 ze(E) binary system. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Series** & **N. Points** & \(z_{1}\) & \(v\) **(m\({}^{3}\) kg\({}^{-1}\))** & \(\Delta\)T (K)** & \(\Delta\)P (kPa)** \\ \hline
1 & 14 & 0.155 & 0.028 & 233–363 & 104–943 \\
2 & 14 & 0.254 & 0.029 & 233–363 & 149–1001 \\
3 & 14 & 0.331 & 0.029 & 233–363 & 171–1063 \\
4 & 14 & 0.459 & 0.029 & 233–363 & 255–1178 \\
5 & 14 & 0.563 & 0.028 & 233–363 & 340–1366 \\
6 & 14 & 0.642 & 0.030 & 233–363 & 397–1380 \\
7 & 14 & 0.733 & 0.028 & 233–363 & 504–1601 \\
8 & 14 & 0.859 & 0.028 & 233–363 & 634–1909 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Measurements at bulk compositions \(z_{1}\) and average specific volumes \(v\) for R744 (1) + R1234 ze(E) (2) binary system over the temperature range \(\Delta T\) and pressure range \(\Delta P\). For this reason, after measuring the vapor pressures, the superheated vapor data and the triple points of pure fluids, in this paper, we focused on the SLE and the VLE behavior of R744 + R1234 yf and R744 + R1234 ze(E). In fact, to consider R744 as a refrigerant helps in terms of both avoiding flammability and keeping GWP low. In addition, since the main limitations of carbon dioxide as a refrigerant are related to its low critical temperature (very often, it is used as working fluid in trans-critical cycles) and its high triple point (solidification occurs at too high temperatures), to blend it with HFOs would provide improvements for all of these issues. From the elaboration of the _P-T_ curves, it is possible to derive the Vapor -Liquid Equilibrium (VLE) behaviour of the binary system. As explained elsewhere [34], the flash method with the Carnahan-Starling-DeSantis equation of state was used to derive the VLE data from the isochoric measurements. Keeping \(T\), \(z\)1, and the number of moles equal to the experimental values during the calculation, the flash method provides the calculated values of mole fractions of the liquid phase (_x_1) and vapor phase (_y_1) and pressure for each isochoric point. This was obtained by ensuring the isofugacity conditions and minimizing the difference between the calculated volume and the measurement cell volume, determined from the gravimetric calibration.