PVT Pressure-Volume-Temperature Tests

The thermal expansion of the microcapsules was measured at several temperatures under several hydrostatic pressures using a PVT (pressure-volume-temperature) apparatus which was made to measure the volume change of materials [14]. In the PVT apparatus (Fig. 34.7), the pressure and the temperature could be controlled independently and the volume change of the specimen could be measured as a function of pressure and temperature. 

The specimen was enclosed in a bellows vessel with silicone oil. This vessel was placed in a pressure vessel filled with another silicone oil and it was heated by an electric heater through the pressure vessel and outer oil jacket. The outer silicone oil was pressurized by a piston and an actuator. Thus at a given temperature, we could apply a hydrostatic pressure to the specimen through the bellows vessel. The volume change of the specimen was calculated from the displacement of the bellows measured using an LVDT (linear variable differential transformer). Inside the transformer, a pin was connected to an iron core and the pin was pushed by the bellows. At a translation of the bellows, the core, which was pressurized in a stainless steel pipe, also moved and the position of the core was detected by an LVDT coil from the outside of the pipe. 

The experiment with the microcapsules was carried out again using a syringe vessel. In this case, only the specimen was enclosed in the syringe, without any silicone oil, so that the change of total volume including the microcapsules and leaked gas could be measured. Because the PVT tests started after pressurization, the gas expanded under a set pressure. The volume fraction of the microcapsules in the syringe was 75%. 

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