The uncertainties in determining the Stern-Volmer coefficients A(T) and B(T) are calibration errors. In a priori PSP calibration in a pressure chamber, the uncertainty is represented by the standard deviation of data collected in replication tests. Because tests in a pressure chamber are well controlled, a priori calibration results usually show a small precision error. However, a significant bias error is found when a priori calibration results are directly used for data reduction in wind tunnel tests due to unknown surface temperature distribution and uncontrollable testing environmental factors. In contrast, in-situ calibration utilizes pressure tap data over a model surface to determine the Stern-Volmer coefficients. Because in-situ calibration correlates the local luminescent intensity with the pressure tap data, it can reduce the bias errors associated with the temperature effect and other sources, achieving a better agreement with the pressure tap data. The in-situ calibration uncertainty, which is usually represented as a fitting error, will be specially discussed in Section 7.3.
7.1.2. Temporal Variations in Luminescence and Illumination
For PSP measurements in steady flows, a temporal change in the luminescent intensity mainly results from photodegradation and sedimentation of dusts and oil droplets on a model surface. The photodegradation of PSP may occur when there is a considerable exposure of PSP to the strong excitation light between the wind- off and wind-on measurements. Dusts and oil droplets in air sediment on a model surface during wind-tunnel runs; the resulting dust/oil layer absorbs both the excitation light and luminescent emission on the surface and thus causes a decrease of the luminescent intensity. The uncertainty in Dt(At) due to the photodegradation and sedimentation can be collectively characterized by the variance var[Dt(At)] ~ [(dV/dt)(At)/V]2. Similarly, the uncertainty in
Dq0(At), which is produced by an unstable excitation light source, is described by var[Dq0(At)] « [(dqo /dt)(At)/qonf ]2.