269. Various tests while conducting a fuel oil analysis can be performed on board to facilitate proper centrifugation. What test provides the key data for selecting the correct purifier gravity disk for fuel oil centrifugal purifiers?

Fuel oil density determines the correct purifier gravity disk, as it dictates the fuel/water interface position within the purifier bowl. Gravity disks are selected based on fuel density at a specified temperature to ensure proper separation of water and sludge from clean oil; other properties like water content, ignition quality, or calorific value are irrelevant for this selection process.