Blood volume in the venous reservoir （VR） is an important parameter that should be continuously monitored in patients with a cardiopulmonary bypass. In this study, we investigated the effects of measurement frequency, hematocrit （Ht）, and blood volume on the capacitance （Cs） of blood present in the venous reservoir by utilizing electrical properties of polycarbonate, a material used in VR.

  In Experiment 1, two electrodes were attached to the VR wall surface and the capacitance Cs［F］ between the electrodes at each liquid level was measured at frequencies from 100 kHz to 1 MHz. VR was filled with physiological saline or bovine blood （Ht 20%, 25%, and 30%）. In Experiment 2, Cs values were measured at a frequency of 1 MHz by attaching an electrode to a cylindrical container and filling it with physiological saline.

  The results of Experiment 1 showed that the Cs value increased with raising liquid level, with increases of 1.30 pF/cm （saline）, 1.26 pF/cm （Ht 20%）, 1.28 pF/cm （Ht 25%） and 1.13 pF/cm （Ht 30%） at a frequency of 1 MHz. In Experiment 2, Cs increased by 1.41 pF/cm. The frequency range of 800 kHz to 1 MHz was found to be stable with minimal effects on the blood, suggesting that the frequency can be used for continuous measurement of the blood volume in VRs of various shapes.