The numerical analysis of an external gear pump with cavitation effects has been validated with experimental data obtained by applying Time-Resolved Particle Image Velocimetry. The effect of inlet and outlet pressure on volumetric efficiency has been studied numerically. First, the Particle Image Velocimetry method was used to analyze the two-dimensional velocity field in the middle plane of the suction chamber of the gear pump. The main improvement, with respect to previous similar analysis is the use of alginate micro particles as tracers. It is seen that the two-dimensional model is able to characterize the flow field of the real pump in the region of the inlet chamber in which cavitation is expected. In a previous study, it was seen that a cavitation cloud acted as a virtual contact point at low pressure, being responsible for an increase on the volumetric efficiency. The first set of simulations represents the pump working with high outlet pressure. Now, the cavitation cloud is not present and cavitation no longer helps to improve the efficiency of the pump. The second set of simulations represents the pump with an inlet loss factor, which implies a mean inlet pressure below atmospheric conditions. This allows cavitation clouds to propagate upstream. Despite the larger cavitation clouds, volumetric efficiency only drops at high operating velocities, when some clouds become trapped between gears and casing and are transported to the pressure side.