Refraction can be defined as the change of radiation direction when passing from one material to another, this phenomenon is associated with the differences in the speed of light propagation in the two media. The method to calculate the refractive index of a sample in order to know its purity is the refractometry and the instrument used to determine it is the refractometer.
Refractometers are devices that can come in various shapes and sizes. Generally, this equipment consists of a light source, a prismatic system, a container where the sample is placed; and a detector to measure the intensity of the light beam.
Types of refractometers
- Abbe refractometers: are tabletop instruments used for more accurate evaluation, can be analog or digital and usually have built-in temperature control of the sample, and an optical arrangement to eliminate errors typical of measuring angles.
- In-line process refractometers: the assembly is usually in the pipe or in a tank, may or may not require a bypass, continuously measuring the refractive index in a fluid as it flows through the sensor. With Bypass, the sample is driven by a bypass to a stainless steel measuring chamber through which small quantities pass, producing a digital and analog output ready for transmission to a PLC.
- Digital refractometer: Often offers twice the resolution of a traditional-style refractometer, producing a much more accurate reading. Generally, it comes with an automatic temperature compensation system. These tools offer a broad scale and better resolution.
- Portable refractometers are an analog instrument mostly used to measure the refractive index of liquids. They are classified to resist water, which improves their durability and simplifies cleaning procedures. No batteries or power supply required. This allows it to be used anywhere in daylight.
Characteristics of a refractometer
- Measuring range: instruments with focusing eyepiece lenses can display the image at scale at various apparent distances from the observer. The focus range is expressed in diopters, the larger the number, the more the apparent distance can be varied.
- Resolution: is the smallest difference in indication of a device that can be perceived significantly. These instruments usually offer it in values in degrees Brix.
- Accuracy: Some portable refractometers incorporate an adjustment screw with a drum calibrated in terms of temperature, which allows the limit line of the scale to be moved according to it. Modern equipment eliminates parameters that in the past limited the measurement procedure, allowing a high-end refractometer to deliver accurate and reproducible results with 5 or even 6 decimal places.
- Temperature measurement (resolution and automatic compensation): this feature is of vital importance if accurate measurements are to be made, since the higher thermal stability the titrations will be more specific. The need for temperature compensation can be avoided by carrying out the procedure at 20 °C, under controlled conditions. When this is not possible, other measurement methods should be used.
- Scales: There are different specific scales according to the application. The most commonly used scale is Degrees Brix (proportion of sucrose or salts in a solution). There are other scales, such as Be (Baume), % of salt, gs, g/dl, nD, % w/w, % vol, % water, mash sac, M-10, MDT, among others. The Zeiss scale is also widely used in the brewing and wine industries because, through formulas, the alcohol content in beers, wines and ciders can be determined. The Butyro Scale is used to measure vegetable oil and animal fats, as well as Ghee in Asia. It is calibrated at 45 °C by the particular characteristics of the oils at this temperature. HERE
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