Pressure Sensitive Paint (PSP)
In the field of optical pressure distribution measurement, the Pressure Sensitive Paint (PSP) technique has been established in recent years as a reliable working tool for major research institutions such as DLR in Göttingen, Germany, ONERA in France, JAXA in Japan and NASA in the USA. This technique uses the deactivation of photochemically excited dye molecules by means of oxygen as an essential measuring principle, i. e. the fact that the fluorescence of an excited dye molecule can be extinguished with the help of oxygen. Since the oxygen concentration in the fluid is in equilibrium proportional to the pressure, this process, known as quenching, can be used directly for spatially resolved pressure distribution measurements. For this purpose, the fluorescent molecules are stored in an oxygen-permeable paint and sprayed onto the surface of the model to be examined after appropriate pre-treatment. The sprayed paint layer must be as thin as possible so that any influence on the flow can be neglected. However, on the other hand, the concentration of the fluorescent molecules - or the thickness of the paint layer - must be sufficiently high to ensure that the signal can be reliably recorded with a camera. After applying the coating and calibrating the measuring system at various reference pressures and temperatures, the molecules are illuminated and the light emitted by the molecules is recorded in the frequency band of interest using suitable cameras. A reference camera is necessary to avoid that, for example, different colour layer thicknesses or a variation of the lighting intensity result in a spatially different sensitivity. In order to be able to reliably register even small differences in intensity, the use of high-quality cameras is mandatory. With a 12bit camera and sufficient temporal integration, a resolution of ±10 mbar or sigma_cp= ±0.05 results and with a camera with 16bit ±1 mbar or sigma_cp =±0.005.
The Pressure Sensitive Paint Measurement method still offers a lot of potential for improvements despite years of development. At the Institute of Fluid Mechanics and Aerodynamics, developments are being pursued particularly in the following areas:
- Data acquisition with CCD and CMOS cameras
- Lighting with flash lamps, diodes and lasers
- Data evaluation and analysis taking into account deformation and vibration of models and temperature drift
- Simultaneous use with other optical measuring techniques such as PIV, deformation measurements and IR thermography