A method of more accurately measuring steam turbine efficiency is disclosed in which the sealing steam in the steam turbine is re-routed so that a more accurate measure of steam turbine efficiency can be made. Some of the steam entering a turbine goes into the turbine's end pack and then mixes with the steam that goes through the turbine. Piping is added from one of the end pack line's to the condenser. This added line has a valve, pressure, temperature and flow measuring devices. As the valve is opened, the amount of flow going to the end pack line allowing the end pack steam mix with the steam that goes through the turbine is reduced. As the flow in this line is reduced the measured temperature at the turbine exhaust will also decrease. The amount that the valve is opened is increased until either the exhaust temperature has reached a minimum, or the enthalpy in the pipe changes from the initial enthalpy.