# An Innovative System of Deep In Situ Environment Reconstruction and Core Transfer
## 4 Temperature and Pressure Decoupling Control Based on the Iso-Density Curves of Water
### P-T Preservation  
Some actions of the SERCT will cause changes in the environment inside the chambers: (1) the push rod extends or the valves close. These actions will increase the pressure and require drainage. (2) The push rod retracts or the valves open. These actions will cause pressure drop and water supplement is required. Because the above two actions have different impacts on the environment, the system handles them in different ways.  
Figure 5 shows the control paths of P-T preservation. Assuming the initial stable state is at point D, when the push rod extends or the valves close, the pressure rises to D\({}_{1}\). Set the fluctuation thresholds of temperature \(\varepsilon_{t}\) and pressure \(\varepsilon_{p}\). When the pressure fluctuation reaches \(\varepsilon_{p}\), the push rod or valve stops and the pressure relief valve starts to work until the pressure returns to \(P_{d}\). This process hardly requires water supplement and has little impact on temperature.  
When the push rod retracts or the valves open, water will be replenished. If the amount of water supplement is too much, the temperature will fluctuate greatly. Therefore, in this process, the water inflow is limited by controlling the moving distance of the push rod or the valve, so as to control the fluctuations of pressure and temperature. In Figure 5, DD\({}_{2}\) is a vertical line and D\({}_{2}\)D\({}_{3}\) is a horizontal line. When the pressure drops to point  
Figure 5: The control paths of P-T preservation.  
\(\mathrm{D}_{2}\) which satisfies the condition of Equation (8), stop the movement of the push rod or the valve.  
\[(T_{d}-T_{d3}=\varepsilon_{t}\ \&\ P_{d}-P_{d2}\ \leq\ \varepsilon_{p}\ )\ ||\ (T_{d}-T_{d3}\ \leq\ \varepsilon_{t}\ \ \&\ P_{d}-P_{d2}= \varepsilon_{p}) \tag{8}\]  
After the push rod or the valve stops moving, the booster valve starts to replenish water into the chamber, which makes the temperature drop while the pressure rise. Assuming the temperature and pressure run along a P-T curve \(\mathrm{D}_{2}\mathrm{D}_{4}\) which is located inside the triangle \(\mathrm{DD}_{2}\mathrm{D}_{3}\), when \(\mathrm{D}_{4}(P_{\mathrm{d4}},T_{\mathrm{d4}})\) satisfies Equation (5), it returns to the iso-density curve where point \(\mathrm{D}\) is located. Then, stop adding water and start to heat, entering an iso-density temperature control process until the state returns to point \(\mathrm{D}\). Because the temperature fluctuation and the amount of water added each time are very small, the density change from \(\mathrm{D}_{4}\) to \(\mathrm{D}\) can be ignored, and the heating process can be regarded as an iso-density heating process.  
The moving distance of the push rod or the valve gate can be controlled by monitoring the pressure change. However, we use the following method to determine this distance, which is stable and reduces the complexity of the control system.  
According to Equation (1), when temperature is given and remains constant, the relationship between density of water and pressure is determined. When \(P_{d}\) and its allowable fluctuation are given, its density \(\rho\) and allowable change \(\Delta\rho\) can be obtained. Ignoring the influence of temperature, the maximum distance of each movement of the push rod or the valve can be obtained by Equation (9):  
\[l=\frac{V\cdot\Delta\rho}{\rho\cdot A} \tag{9}\]  
where \(V\) is the internal volume of the working chamber, and \(A\) is the sectional area of the push rod or the valve gate.