To achieve good rangeability and linearity of the valve flow to stem position (installed linearity), the valve trim must be characterized. Thus, at the low load conditions, a valve capacity versus position curve (characterization) will look like that shown in Figure 3. This characterization not only provides better rangeability and control, but also assures that the valve closure member does not operate near the seat and thus minimizes damage to the critical seating surfaces due to excessive fluid velocities. Depending upon the low load pressure drop, other measures may be necessary to limit high fluid velocity erosion such as the use of multi-stage trim designs. These designs become a consideration when pressure ratios, p1/p2 across the valve exceed three or pressure drop across the valve could result in cavitation or excessive noise. Cavitation and excessive noise can occur at pressure drops as low as 30 psi (0.2 MPa) for some fluid conditions. The characterization and trim erosion considerations are very important because they contribute significantly to the most vital function of this valve and that is accuracy of feed control.