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It is common to assume that the performance of low-speed turbines depend only on the flow coefficient and Reynolds number. As such the required operating point is achieved by controlling the values of these two non-dimensional 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. Whilst it is still possible to maintain the required values for the flow coefficient and Reynolds number, the ambient variations affect additional non-dimensional quantities which are related to the blade speed and gas properties. Generally, the values of these additional non-dimensional 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 non-dimensional quantities must be accounted. This paper contains four parts. Firstly, it is described how suitable data acquisition parameters can be determined to eliminate short time scale facility unsteadiness within the measurements. Secondly, by the analysis of models, the most appropriate forms for the additional non-dimensional quantities that influence turbine performance are obtained. Since the variations in the uncontrolled non-dimensional quantities affect repeatability the size of the effect on the turbine performance is quantified. Thirdly, a best-fit accounting methodology is described which reduces the effects of the uncontrolled non-dimensional quantities on turbine performance provided sufficient directly related measurements are available. Finally, the observations are generalised to high-speed turbomachines.