Application of optical refractometers for the control of concentration

The handheld refractometer is applicable to many different types of fluids found in a modern shop, and useful for controlling concentration when the solution is kept clean.

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Optical refractometers, analog and digital, are widely used in industrial practice to measure the concentration of coolants, polymer quenchants, and fire-resistant hydraulic fluids, and also used to determine the sugar content during wine and beer making. The ease of use and the speed of obtaining the percent product in solution are the reasons for its popularity. Specimens can be readily obtained and tested, and the concentration of a solution determined very rapidly. The refractometer is a good tool, but the limitations of this device must be understood.

Theory

A refractometer measures the refractive index of a material. The refractive index is a relative measure of the speed of light through a substance, compared to a reference. The reference can be water, air, or glass. As the optical density decreases, the speed of light increases, and the refractive index decreases.

The refractive index of a material is dependent on temperature and the wavelength of light. For most applications, the standard temperature is 20°C, and a wavelength of 589 nm (or 5890Å) or approximately the wavelength of a sodium-vapor lamp. The color is a yellow-red.

The operation of a refractometer is based on Snell’s Law (Figure 1). This law was first described by Ibn Sahl in Bagdad in 984. This was independently derived by Descartes in his 1637 essay Dioptics. The general relationship of Snell’s Law is:

Where θ i is the angle of incidence; θ x is the angle of refraction; ni is the index of refraction of the incident or reference medium; and nx is the index of refraction of the refractive medium or sample (Figure 1).

Figure 1: Illustration of Snell’s Law and incidence of refraction.

Traditional handheld analog or digital refractometers (Figure 2) are scaled in °Brix, which is a measure of sucrose in water. One degree Brix is 1 gram of sucrose in 100 grams of solution at 20°C. Other scales in use can be percent salinity (used for packaging pickles), or specific gravity (used in checking car batteries).

Figure 2: Typical handheld refractometers used.

The refractometer is based upon the refractive index of a solution and most handheld equipment uses distilled water as zero. The scale is different for each type or model of refractometer. The most common scales are 0 to 10, 0 to 30, and 0 to 50. It would seem logical that the 0 to 10 scale would give more accurate readings, but experience has shown that as the fluid becomes contaminated, the ability to read the refractometer’s line on the scale becomes more difficult with this expanded scale. There are also two types of temperature compensation — one that has automatic temperature compensation in the 60 to 100°F (15.6 to 37.8°C) range, and those that have a correction thermometer on the side of the unit.

When using a refractometer for the control of aqueous quenchants, a multiplier or factor must be used. This factor is different for each product and is only accurate until the quenchant becomes contaminated with a material, which will add to the refractive index of the solution being tested.

The factor of a fluid, such as a polymer quenchant, is straightforward and is determined by the quenchant supplier. A series of known concentrations are created. The kinematic viscosity is determined, and a graph is created of viscosity versus concentration (usually by weight). This master curve measures the viscosity of multiple production lots at several different concentrations.

Kinematic viscosity is used as a check or reference for the refractometer. It is more immune to contamination. A sample of the unknown concentration is taken. The sample kinematic viscosity is determined, and the concentration determined from the viscosity versus concentration bath. Using a handheld refractometer, the Brix reading is measured. The factor F is determined by:

Very clean systems see little change in their factor, while systems with high levels of contamination see major changes to this factor. The factor usually goes down as other dissolved solids add to the reading. Some contaminants have a minimal effect such as carbon fines and most other solid, non-soluble materials. Other materials, such as inorganic salts, liquids soluble in aqueous solutions, oils, and dissolvable solids can have a major effect on the multiplying factor.

Procedure

The procedure for obtaining the concentration of a given product with the refractometer is very simple. A drop of the solution being tested is put on the prism of the refractometer. If the unit has a movable cover, this is closed, and the refractometer is pointed toward a light source. Looking through the eyepiece, the line between the lighter and darker areas is observed and the numerical value is read on the scale. This number is then multiplied by the current factor for the solution to obtain the current concentration. The factor is different for each product and tends to change during use, as mentioned previously.

It is recommended that only °Brix reading refractometers be used for measuring concentration of polymer quenchants. Use of other scales can introduce errors in concentration, as the factors would be different.

It has been found that digital refractometers offer greater reproducibility and accuracy when measuring both clean and contaminated samples. This is due in part to the use of a single wavelength of light when measuring, as well as increased sensitivity.

Conclusions

In this short article, the theory and application of handheld refractometers has been described. The handheld refractometer is applicable to many different types of fluids found in a modern shop, and is useful for controlling concentration, if care is taken to keep the solution clean.

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