In _______ mixing, a measured pressure of oxygen is added to a cylinder, which is then filled to service pressure with air.

Partial pressure mixing involves the addition of a specific pressure of oxygen to a cylinder, followed by filling the rest of the cylinder with air until it reaches the desired service pressure. This method allows divers to create a nitrox gas mixture with the desired oxygen content by controlling the individual pressures of the gases involved.

In this process, the measured pressure of oxygen and the remaining ambient air combine to yield the final gas composition. By accurately controlling the pressure of oxygen and then filling with air, divers achieve a specific blend of oxygen and nitrogen that meets their requirements for dives at various depths. This technique is particularly valuable for ensuring that the partial pressure of oxygen remains within safe limits for the intended dive profile, thereby maximizing the benefits while mitigating the risks associated with high levels of oxygen exposure.

The other options do not accurately describe this mixing process. Density mixing refers to the concentration or mass of molecules in a given volume and is not related to the blending of gases under pressure. Bulk mixing typically refers to the physical combination of large quantities of gases without regard for precise measurements or the pressures involved. Oxygen-enriched could imply a higher concentration of oxygen but does not specify the method of creating the mixture as partial pressure does.