An apparatus for testing the thermal stability of a liquid test sample using a source of electric power, said apparatus comprising: a generally cylindrical test chamber; a heater tube contained in said test chamber, said heater tube being connected to said source of electric power to flow current there through to heat a center section of said heater tube to a predetermined temperature; a sample container for holding said liquid test sample; a sample drive pump for slowly pumping said liquid test sample from said sample container and around said center section of said heater tube in said test chamber, wherein said liquid test sample is heated to approximately said predetermined temperature; a test filter for removing solids from said liquid test sample after leaving said test chamber; a differential pressure sensor measuring pressure drop across said test filter; a back pressure regulator for maintaining a high pressure in said test chamber so that said liquid test sample remains in a liquid state during said testing thereon; an aeration circuit for sparging dry air through said liquid test sample in said sample container at the beginning of said testing; a cooling circuit for flowing coolant to each end of said heater tube to maintain said ends near room temperature during said testing; a waste container for receiving said liquid test sample and/or air used during said testing; and a priming circuit for priming said sample drive pump before said slowly pumping, said priming circuit including a priming valve located on an outlet side of said sample drive pump and configured to open until flow is established in said sample drive pump; characterised in that said priming circuit also includes a sample vent valve for preventing pressure from escaping said sample container, said sample vent valve being (a) closed toward the end of said sparging to build pressure in said sample container to force said sample into said sample drive pump before said slowly pumping by said sample drive pump and (b) thereafter opened to allow said sample drive pump to continue drawing said test sample from said sample container. a generally cylindrical test chamber; a heater tube contained in said test chamber, said heater tube being connected to said source of electric power to flow current there through to heat a center section of said heater tube to a predetermined temperature; a sample container for holding said liquid test sample; a sample drive pump for slowly pumping said liquid test sample from said sample container and around said center section of said heater tube in said test chamber, wherein said liquid test sample is heated to approximately said predetermined temperature; a test filter for removing solids from said liquid test sample after leaving said test chamber; a differential pressure sensor measuring pressure drop across said test filter; a back pressure regulator for maintaining a high pressure in said test chamber so that said liquid test sample remains in a liquid state during said testing thereon; an aeration circuit for sparging dry air through said liquid test sample in said sample container at the beginning of said testing; a cooling circuit for flowing coolant to each end of said heater tube to maintain said ends near room temperature during said testing; a waste container for receiving said liquid test sample and/or air used during said testing; and a priming circuit for priming said sample drive pump before said slowly pumping, said priming circuit including a priming valve located on an outlet side of said sample drive pump and configured to open until flow is established in said sample drive pump; characterised in that said priming circuit also includes a sample vent valve for preventing pressure from escaping said sample container, said sample vent valve being (a) closed toward the end of said sparging to build pressure in said sample container to force said sample into said sample drive pump before said slowly pumping by said sample drive pump and (b) thereafter opened to allow said sample drive pump to continue drawing said test sample from said sample container.