New device determines fluid concentrations

By James E. Kloeppel

UI researchers have invented a simple, low-cost device that monitors the concentrations of fluid mixtures by detecting small changes in the way the mixtures bend light rays. The instrument ­ a new type of refractometer ­ can be used to automatically measure the amount of oil in automobile refrigerants, the concentrations of aqueous coolant brines used in industry and a variety of process fluids.

"The device works by measuring the amount of bending that occurs when light enters a fluid," said Ty Newell, a professor of mechanical engineering and a researcher affiliated with the university's air conditioning and refrigeration center. "The extent to which light rays are bent is determined by a property of the fluid called the refractive index."

To measure the refractive index, Newell and graduate student Evan Hurlburt mounted a light-emitting diode to one side of a semi-transparent piece of glass. The other side of the glass is placed in contact with the fluid to be measured.

"Some of the light rays are transmitted through the fluid," Newell said. "Other rays, depending upon the refractive index of the fluid, are bent back toward the glass. This results in a sharp ring of light being formed at a specific distance from the light source. By measuring the diameter of the ring, we can determine the refractive index of the fluid."

If the fluid concentration changes, the refractive index also will change, causing the ring of light to shift in size and intensity. A cadmium-sulfide photoresistor, cemented to the glass surface with a clear epoxy, can monitor the ring's appearance and provide a continuous measurement of the refractive index. (A temperature sensor, also cemented to the glass, takes into account any temperature variations that can affect the refractive index.)

Both portable and in-line versions of the device have been developed, Newell said. "For example, we constructed the coolant-brine refractometer as a hand-held probe that can be inserted into the coolant, but we built the automotive refractometer into a compact pressure housing designed to withstand harsh operating conditions."

Installed in an automotive air-conditioning system, the refractometer could sense a drop in the amount of oil that circulates with the refrigerant and automatically shut down the system to prevent compressor failure, Newell said.

Because the refractometer can measure subtle changes in fluid concentrations, the device could be used in a number of other applications as well, including monitoring additives in food and beverage processing, measuring the charge content of lead-acid storage batteries and detecting leaks from underground storage tanks.

Researchers have applied for a patent.

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