The OTC Diesel Fuel Hydrometer, No. 6750, is designed to test #1 and #2 grade diesel fuel composition and combustion characteristics, helping diagnose fuel and drivability issues. The hydrometer measurements are given as an American Petroleum Institute (API) gravity reading when testing diesel fuel composition and combustion characteristics. Fuel is drawn into the hydrometer tube which then floats the internal bulb float. The internal float contains a thermometer that is used to compensate the API reading for the highest accuracy. Instructions are included, with proper measurements noted within. For use on all GM 5/16" EVAP line systems.