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Simple Device Measures Film Thickness

Photonics Spectra
Nov 2000
Daniel C. McCarthy

Film thickness between a liquid-vapor interface can reveal much about the performance of a fluid-mechanical system. The thickness of liquid refrigerant film on the interior wall of a tube can help determine heat transfer into and out of the refrigerant and indicate the optimal amount of fluid pressure. Too much fuel injected onto the cylinder walls of an internal combustion engine can signal inefficient operation. Similarly, in steam power plants, a lack of moisture lining the interior pipe walls communicates a hazardous condition.

Conventional methods for measuring film thickness rely on capacitance sensors or fluorescent dyes, but a common problem with these techniques is the need for frequent calibration. Sensors based on these methods can also be bulky, limiting access to small spaces.

An optical technique for measuring film thickness could provide a compact, inexpensive alternative to conventional capacitance sensors used to monitor today's refrigerant systems. Note the ring of light in the camera image on top. The bottom image shows the processed edges of the ring. Courtesy of the University of Illinois.

An optical film-measurement technique developed at the University of Illinois could provide a compact, inexpensive alternative. The system incorporates a high-intensity LED and an inexpensive board camera, and determines thickness accurately to within 0.01 mm. It works by shining the LED onto the outside of a transparent tubular test section. Total internal reflection at the liquid-vapor interface causes the light to form a bright ring on the tube's exterior. Image processing software measures the diameter of the ring to determine film thickness.

Simplicity, compactness and low cost were major goals in the development of the technique, according to mechanical engineering professor Ty Newell, who patented the technology. Depending on the application, it can use LEDs of almost any wavelength, including infrared sources. But, Newell said, it benefits from intensities greater than 1000 mcd. The board camera used in the prototype system costs less than $100, but the researchers also developed an analog version that uses a simple photodiode. The electronics for the analog system cost less than $1. Higher-resolution systems are possible with better-quality -- and more costly -- cameras.

The researchers have not pursued commercial applications for their system. Newell speculated that it could find use in liquid coating processes that require monitoring of film thickness. It also could measure thickness of polymer films.

However, the group's focus is on refrigerant applications, where a prototype installed onto a standard refrigerant "sight glass" showed it was capable of sensing the presence of a liquid film in the compressor suction line.

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