Abstract: The invention relates to a test apparatus for measuring frictional properties of a fluid or of a specimen immersed in a fluid. A first specimen holder holds a first specimen in the fluid being tested; the first specimen in contact with a surface of a second specimen, in a second specimen holder. A measurable load is applied between the two specimens, and an oscillatory driving means oscillates the first specimen holder relative to the second specimen holder. Force measuring means measures the frictional force between the two specimens. The specimen holders are in a sealed pressure chamber allowing the test to be carried out under alternative pressures. A push rod provides the oscillating motion of the first specimen holder inside the pressure chamber so the pressure is maintained during the test. The pressure chamber is removable and maintains pressure or vacuum even when removed from the test apparatus.