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Dual-Luminophore Paint Detects Pressure and Temperature

Photonics Spectra
Dec 2005
Hank Hogan

Wind tunnels, University of Florida, dual-sensor film that measures pressure and temperature simultaneously. And the approach could be applied elsewhere.

“The methods are widely applicable to any application that uses luminescence to determine a parameter such as oxygen concentration, temperature, pH, metal ion concentration, et cetera,” said Kirk S. Schanze, a chemistry professor at the university and a member of the team that reported on two studies in the Sept. 27 issue of Langmuir.

A critical parameter in wind tunnel testing is the air pressure distribution over a model. To measure it, investigators use pressure-sensitive paints that contain luminophores such as platinum porphyrin or a polypyridine ruthenium complex dispersed in an oxygen-permeable polymer binder. The luminophores react to oxygen, so the higher the concentration, the more that quenching occurs and the lower the luminescence.

Combined with a light source and a scientific-grade CCD camera, pressure-sensitive paints can measure surface air pressure with less cost and labor than other techniques while producing more data.

However, the coatings are affected by temperature, and their readings must be corrected for this variable. One approach has been to use a second luminophore that responds to temperature.

In theory, that is easy. In practice, it is not. When dispersed in a polymer, dissimilar luminophores interact in unpredictable ways.

The emission confusion can be avoided by separating the various luminophores, but for reasons related to the resolution of the systems currently used to image pressure-sensitive paint, the different dyes must be compartmentalized on a scale of 10 µm or less.

Our coating is designed to address this issue,” Schanze said. “With images obtained at several wavelengths, one can determine the full-field temperature and air pressure distribution on a surface.”

The key, the researchers say, is the use of polyacrylonitrile nanospheres that encapsulate the temperature-sensitive luminophore. Averaging 50 nm in size, these keep the dyes separate while being transparent in the near-ultraviolet and visible. Thus, they do not interfere with excitation or emission. They do need to be dispersed throughout the polymer evenly so that their spacing is roughly the resolution of the imaging system. In the case of the Florida investigators’ work, that was on the order of 100 µm.

For their studies, they calibrated the response of the dual-luminophore paint to temperature and pressure using a fluorescence spectrophotometer, with sample excitation at 465 nm. To produce full-field images, they used a Photometrics CCD camera, an array of blue LEDs and Melles Griot 550- and 650-nm bandpass filters.

They tested the performance of the coating by immersing one end of a 4 × 2-in. piece of painted metal in an ice bath while attaching a heater to the other. When they varied the pressure, they succeeded in correcting for the temperature gradient.

Although the results are encouraging, the technique has drawbacks. It is computationally intensive, requiring one to two hours per image to extract temperature and pressure surfaces from the hyperspectral measurements. Better algorithms may help cut this time. The technique also requires a reference image, but Schanze noted that it may not be needed if the group can work out the kinks for a three-dye system.

Ultimately, the plan is to get the dual- or triple-luminophore paint method to work with newer CCD imaging systems to allow the simultaneous acquisition of images at multiple colors or wavelengths.

“Such a combination will facilitate the implementation of our methods and make them essentially routine so that they could be implemented in production wind tunnel facilities,” Schanze said.

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