ACTTR, Published: Friday, 09 June 2017
Refractometer is an optical device that can be used to measure the refractive index of a substance (usually fluid) and, by converting the refractive index, one can get the specific gravity, sugar content (Brix, or Plato).
When electromagnetic waves (including visible light) pass through the interface of two different substance, due to its speed changes, it results the direction of light with an angle. This physics phenomenon is known as “Refraction”. Typically, in practice, it specifically refers to the refraction of visible light.
This is why we saw a broken chopstick in the water, and it is because the refraction of light at the interface of water and air. When the light was reflected from the surface of chopstick and return to the air, the direction of light was changed. So that our eyes saw the ‘virtual’ image on the straight direction of sight, instead of the original location of the chopstick.
Refraction of A Chopstick in The Water
In addition, the refractive index (n) of a specific wavelength of light are different. Today, the standard refractive index, is typically expressed in the unit nD. nD is based on the sodium lamp, which generates a double wavelength light (588.995 nm and 589.592 nm). And we take the center of two wavelengths to determine the absolute refraction, that is, 589.3nm. Earlier, the red hydrogen lamp (656.2725nm, unit nC), or blue hydrogen lamp (486.1327nm, unit nF) was also used. But today, it is not common. We can find nD only, in most of the Refractometer instruments today. Besides, in the new refractometer, it has been replaced the old fashion sodium lamp, by a more stable, longer live LED. It makes the instrument more accurate and more stable in the long run.
Another factor that affects the refractive index (nD) is the specific gravity of the solution, also known as the mass percentage. For example, the sucrose water with different concentrations, will be different on the refractive index, too. Therefore, according to the relationship between specific gravity of sucrose and the refractive index of the known sucrose solution, on can convert the measured refractive index to the gravity of sucrose solution. Usually, we use the unit, Brix, to represent the amount of sugar in the solution. The definition of Brix is: How many grams of sugar per 100 grams of water, at 20 ° C temperature. Another unit, Plato, is also used as a unit of sugar, it is commonly used in the beer industry. Plato represents the sugar in the rye. Therefore, a refractometer, is also known as sugar meter.
The development of the inflection meter has been improved a lot today, no matter the measurement speed, accuracy, stability and longevity of the instrument have been improved greatly. So far, a refractometer has become an indispensable instrument for many researches, or quality control purposes. The modern digital refractometer, had innovated the traditional light-through measurement to total reflection measurement method. The advantage of total reflection is that the transmittance of the sample will not influence the measurement too much, when the light doesn’t need to pass through the sample. So, to those opaque, high viscosity, or dark samples, the new generation of digital refractometer can still work very well.
