An apparatus for determining the volume of liquid in a probe tip of an aspirating or dispensing probe, such as those used in a diagnostic analyzer, includes: two electrodes on opposite sides of the probe tip and the electrodes and liquid form a variable capacitor; a resistor; a voltage source and the electrodes and resistor are in electrical communication in at least one of series, parallel or series and parallel to form an RC circuit; and a microprocessor in electrical communication with the RC circuit for converting an electrical signal to a volume of liquid in the probe tip. In a preferred embodiment, the probe tip is separate from the probe and is adapted to be fitted on the end of a probe for aspirating and dispensing a liquid. In another preferred embodiment, the apparatus further includes a solenoid, and the voltage source, electrodes, resistor, and solenoid are in electrical communication in at least one of series, parallel or series and parallel to form an RLC circuit, and the microprocessor is in electrical communication with the RLC circuit for converting an electrical signal to a volume of liquid in the probe tip. The apparatus can be used with a liquid have a known or unknown dielectric constant and can be used to determine the presence of bubbles in the liquid.