This work presents a numerical study, accompanied by experimental validation, on the feasibility of using the capacitance measured between internal mechanisms and the enclosure of a hermetic pressure vessel to estimate the volume of oil inside it. To accomplish this, a study is conducted on the phenomenological model that represents the relationship between the variation in capacitance measured between the core and the enclosure of these equipment and the corresponding variation in the volume of oil. Based on the phenomenological model, a simulation model was built that numerically solves the equations for the geometric model of the pressure vessel in order to estimate the equivalent capacitance corresponding to different oil volumes. Finally, an experimental apparatus was developed to assess the quality of the model. The experimental results allowed the validation of the numerical model and showed that the proposed method can be employed for oil volume estimation without the need for physical modification of the equipment. The estimation confidence interval for the oil volume was 40 mL when considering a general model for a family of pressure vessels, and 4 mL when considering a specific pressure vessel unit.