Abstract: Liquid testing apparatus includes a testing cell formed by a vertical bore in a metal block. A flow path for liquid being tested is provided in the cell and the liquid is passed through a restriction, such as a mesh filter, obstruction to flow being measured as the temperature is varied. A preferred use of the apparatus is in testing aviation fuels, when the temperature is gradually lowered. When wax crystals begin to form in the fuel, the impeded flow of fuel through the mesh will indicate this change of state, and this temperature will be the minimum usable temperature of the fuel.