A steam turbine performance testing system including: an exhaust bypass conduit arranged for selective fluid communication with an exhaust of the steam turbine, the exhaust bypass conduit being arranged to receive a portion of an exhaust of the steam turbine as an exhaust bypass flow; an inlet bypass conduit arranged for selective fluid communication with an inlet of the steam turbine, the inlet bypass conduit being arranged to receive a portion of an inlet steam flow of the steam turbine as an inlet bypass flow; at least one of an inlet measurement station and an inlet bypass measurement station; a mixed flow conduit having a mixing point at one end in fluid communication with the exhaust bypass conduit and the inlet bypass conduit, the mixing point and the mixed flow conduit being arranged to form a mixed flow responsive to the exhaust bypass flow and the inlet bypass flow entering the mixed flow conduit through the mixing point; and a mixed flow measurement station for measuring a mixed flow pressure P Mix, a mixed flow temperature T Mix, and a mixed flow rate Q Mix.; characterized in that a mixed flow temperature control block is provided to ensure that the temperature of the mixed flow, T Mix, is greater than a saturation temperature, T Sat, for the mixed flow pressure, P Mix by adjusting the inlet bypass flow and/or adjusting the exhaust bypass flow, wherein the mixed flow temperature control block is configured to adjust the heating of the exhaust steam sample if the temperature of the mixed flow, T Mix, is less than or equal to the saturation temperature, T Sat, such as by adjusting at least one of the inlet bypass flow and/or the exhaust bypass flow. an exhaust bypass conduit arranged for selective fluid communication with an exhaust of the steam turbine, the exhaust bypass conduit being arranged to receive a portion of an exhaust of the steam turbine as an exhaust bypass flow; an inlet bypass conduit arranged for selective fluid communication with an inlet of the steam turbine, the inlet bypass conduit being arranged to receive a portion of an inlet steam flow of the steam turbine as an inlet bypass flow; at least one of an inlet measurement station and an inlet bypass measurement station; a mixed flow conduit having a mixing point at one end in fluid communication with the exhaust bypass conduit and the inlet bypass conduit, the mixing point and the mixed flow conduit being arranged to form a mixed flow responsive to the exhaust bypass flow and the inlet bypass flow entering the mixed flow conduit through the mixing point; and a mixed flow measurement station for measuring a mixed flow pressure P Mix, a mixed flow temperature T Mix, and a mixed flow rate Q Mix.; characterized in that a mixed flow temperature control block is provided to ensure that the temperature of the mixed flow, T Mix, is greater than a saturation temperature, T Sat, for the mixed flow pressure, P Mix by adjusting the inlet bypass flow and/or adjusting the exhaust bypass flow, wherein the mixed flow temperature control block is configured to adjust the heating of the exhaust steam sample if the temperature of the mixed flow, T Mix, is less than or equal to the saturation temperature, T Sat, such as by adjusting at least one of the inlet bypass flow and/or the exhaust bypass flow.