TM 9-2910-226-34 Figure 1-28. Fuel density compensator (early code F), sectional view. c. The fuel supply is admitted to a pressure regulating valve where the supply pressure is reduced to a constant regulated pressure over the engine operating speed and load range. Fuel flows through the first (viscosity/density sensitive) an- nular orifice formed by a close fitting piston in a cylinder. From the servo pressure cylinder the fuel flows to the second orifice which is a sharp-edged, adjustable needle valve. The two orifices working at widely different restrictions to the flow of fuel form a system sensitive to viscosity changes. The pressure difference between the two orifices will change the servo pressure in direct relationship to the viscosity of the fuel flowing. The position of the spring-loaded servo diaphragm (fig. 1-28) or servo piston (fig. 1-29) is varied in accordance with these servo pressure drops. 1-30 Figure 1-29. Fuel density compensator (code A, B, C, D and E) sectional view.