It is common to assume that the performance of low-speed turbines depends only on the flow coefficient and Reynolds number. As such, the required operating point is achieved by controlling the values of these two nondimensional quantities by, for example, appropriate choices for the mass flow rate and applied brake torque. However, when the turbine has an atmospheric inlet and uses unconditioned air, variations in ambient pressure, temperature, and humidity are introduced. While it is still possible to maintain the required values for the flow coefficient and Reynolds number, the ambient variations affect additional nondimensional quantities which are related to the blade speed and gas properties. Generally, the values of these additional nondimensional quantities cannot be controlled and, consequently, they affect the turbine performance. In addition, thermal effects, which are exacerbated by the use of plastic blades, can cause changes in the blade row seal clearance and these also affect the performance. Therefore, to obtain measurements with greater accuracy and repeatability, the changes in the uncontrolled nondimensional quantities must be accounted. This paper contains four parts. First, it is described how suitable data acquisition parameters can be determined to eliminate short time scale facility unsteadiness within the measurements. Second, by the analysis of models, the most appropriate forms for the additional nondimensional quantities that influence turbine performance are obtained. Since the variations in the uncontrolled nondimensional quantities affect repeatability, the size of the effect on the turbine performance is quantified. Third, a best-fit accounting methodology is described, which reduces the effects of the uncontrolled nondimensional quantities on turbine performance, provided sufficient directly related measurements are available. Finally, the observations are generalized to high-speed turbomachines.