A sensor apparatus and sensor apparatus system for use in conjunction with a cassette-based peritoneal dialysis system is described, including a disposable cassette. In some embodiments, the cassette includes a thermal well for permitting the sensing of various properties of a dialysate. The thermal well includes a hollow housing of a thermally conductive material. In other embodiments, the cassette includes sensor leads for sensing of various properties of a dialysate. The thermal well has an inner surface shaped so as to form a mating relationship with a sensing probe. The mating thermally couples the inner surface with a sensing probe. In some embodiments, the thermal well is located on a disposable portion and the sensing probe on a reusable portion.