A method for dynamically determining the volume of a liquid dispensed by a metering system, the metering system including a metering pump, a pump motor, a sensor, and a conduit interconnecting the pump and a metering probe. The method includes the steps of aspirating a quantity of fluid into the metering probe using the metering pump, moving the probe relative to a reaction vessel, and dispensing the fluid. The dispensing step includes the steps of identifying an onset point associated with a parameter which can be measured by the metering system, identifying an end point associated with the measurable parameter, and timing the pump motor during the period between the onset point and the end point in order to determine fluid volume. The sensor, such as a pressure transducer or capacitance change sensor, disposed in relation to the conduit can detect changes in the parameter and identify the onset and end points.