Fuel oil viscometer

A fuel-oil viscometer is an inline instrument that measures the viscosity of heavy fuel oil continuously and feeds a controller that trims the final heater to hold the fuel at the injection viscosity, typically 10 to 15 centistokes at the engine inlet. Most marine units use an oscillating-piston or capillary-vibration sensor reading actual viscosity, not temperature, so the loop corrects for grade changes and fouling that a fixed-temperature heater would miss. Correct viscosity is what gives good atomization; too thick gives poor spray and afterburning, too thin gives leakage past the injection pump plunger.