Inexpensive Refractometer Measures Fluid Concentration In Real Time

 

This versatile and highly efficient refractometer determines the concentration of a fluid by measuring in real time the index of refraction of a fluid. A look-up table can be used to relate the index of refraction to the concentrations of the fluid mixture. Small changes in the concentrations of the fluid can be detected by measuring small changes in the index of refraction. The refractometer can be used as an inline process or hand-held device. The entire system, including the sensor housing, inexpensive charge-coupled device (CCD) camera, and digital image processing software, costs less than $1,000. It is fully immersible and provides an accuracy of 0.1% of scale. Refractive index sensing heads using this technology can be constructed for less than $1 and may be used for remote or in situ refractive index sensing operations (process monitoring in a tank, in a well, in soils, etc.)

Details

A refractometer is a measuring device designed to determine the index of refraction. The refractometer market is very competitive. A wide variety of types and prices for refractometers are available, including hand-held, laboratory, inline process, and automotive at a cost ranging from a few hundred dollars to $12,000. Refractometers are used in the food, beverage, chemical, petrochemical, automotive, and pulp and paper industries. Most inline process refractometers cost $8,000 to $12,000. These refractometers are fully computerized, use expensive CCD arrays, have automatic wash control, and sometimes have redundant sensing heads.

Researchers at the University of Illinois at Urbana-Champaign have invented a lower cost refractometer ideally suited for applications where more expensive refractometers would not be justified (e.g., airplane de-icing, measuring water-soluble oils in machine tool coolant, high-end ink jet printing, controlling the concentration of methanol in fuel cells).

How It Works

This invention works by measuring the amount of bending that occurs when light enters a fluid. The refractometer is an integrated sensor housing containing an LED, a flat (not prism) optical element made of glass or sapphire, and a photodetector. Diffused light from the LED passes through the optical element, the other side of which is in contact with the fluid. A portion of the light is refracted reflected back to the photodetector. This results in a sharp ring of light being formed at a specific distance from the light source that is related to the fluid's refractive index. An inexpensive CCD camera and digital image processing software identifies changes in the light to dark regions of the light ring. By measuring the diameter of the ring, one can determine the index of refraction of the fluid. Indices of refraction from 1.3 to 1.6 can be measured. A look-up table then can be used to relate the index of refraction to the concentrations of the fluid mixture. Small changes in the concentrations of the fluid can be detected by measuring small changes in the index of refraction.

Why It Is Better

Other refractometers that use CCD cameras and have microprocessors for processing the image cost approximately $10,000. This refractometer can be used as an inline process or hand-held device, and the entire system including the sensor housing, inexpensive CCD camera, and digital image processing software costs less than $1,000. The LED (infrared, red, or yellow LEDs may be used), optical element, and photodetector can be molded into a single small sensing package. This sensing package then can be interfaced with other companies' hand-held devices and data acquisition systems. Another advantage of the invention is that it uses a simple flat optical element. For fluids with an index of refraction below 1.5, a flat piece of glass can be used. For fluids with a higher index of refraction, such as refrigerant containing oil, a sapphire optical element can be used. The sapphire optical element is 0.5"-diameter, 1.5-mm- thick, and is capable of measuring the full range of concentration from pure oil to pure refrigerant. Refractometers often are used in the food and beverage processing industries, where small changes in the Brix concentration are critical, and in the pulp and paper industry. This refractometer can measure lubricating oil in refrigerant lines and dissolved solids in cutting fluids with an accuracy of 0.1% of scale. Because it uses an integrated sensor housing, this refractometer is particularly appropriate for applications where the refractometer needs to be immersed into a liquid or placed within a permeable solid.

Applications

  • Food and beverage
  • Chemical and petrochemical
  • Automotive
  • Pulp and paper
  • Monitoring antifreeze/water solutions
  • Monitoring battery acid
  • Monitoring chemical migration through soil
  • Airplane de-icing
  • Measuring water-soluble oils in machine tool coolant High-end ink jet printing
  • Controlling the concentration of methanol in fuel cells
  • Pollutant monitoring in ground surrounding landfills, gas stations, chemical storage depots

Benefits

  • Compact: Can be molded into a small sensing package
  • Simple: Uses an inexpensive CCD camera; light-emitting diodes (LEDs); and a simple, flat optical element made of glass (<1.5 index of refraction) or sapphire (>1.5 index of refraction)
  • Adjustable: Provides automatic temperature compensation
  • Comprehensive: Measures the full range of concentration
  • Immersible: Can be completely immersed in liquid or permeable solid
  • Accurate: Offers accuracy of 0.1% of scale
  • Versatile: Can be a hand-held or inline process
  • Inexpensive: Could cost less than $1,000
  • Flexible: Can be used as a remote sensor