Abstract: A thermal oxidation tester is shown for determining thermal stability of a fluid, particularly hydrocarbons when subjected to elevated temperatures. The tendency of the heated fluid to oxidize and (1) form deposits on a surface of a heater tube and (2) form solids therein, are both measured at a given flow rate, temperature and time. The measured results are used to determine whether a fluid sample passes or fails the test. Results of the measurements are recorded. The fluid under test is pumped with a low volume, high pressure, single piston pump with only a small fluctuation (pulsation) in output flow.