The addition of a small quantity of a fluid or solid particles at a constant rate to a process stream used for accurate determination of flow. Positive displacement metering pumps are generally used for dosing or the dispensing of fluids. They operate by trapping a fixed volume of fluid and moving this fluid via gears, pistons, diaphragms, vanes, or other devices such as peristaltic pumps. They are not particularly sensitive to changes (p. 237) in discharge and suction conditions, and permit flow regulation by adjusting speed and displacement.