Refractometers are very popular in many applications, and each application has slightly different instrument requirements. Therefore, as time goes by, various designs, accuracy, and operation of refractometers can be used. These include Abbe refractometers, handheld refractometers, digital refractometers, automatic refractometers and process refractometers.
The Abbe refractometer, named after its inventor Ernst Abbe (1840-1905), was the first laboratory instrument to accurately measure the refractive index of liquids. The measuring principle of Abbe refractometer is based on the principle of total reflection. Abbe refractometers are used to measure liquids. Reference media glasses (prisms) with high refractive index can be selected. The light from the radiation source is reflected by the mirror and enters the double prism. Place a few drops of sample between the so-called Abbe double prism. Only when the incident angle of the incident beam at the interface is less than the critical angle of total reflection, the incident beam passes through the double prism and is sampled. Use a microscope and mirror with an appropriate mechanism to determine the light / dark boundary line (hatched line).
The Abbe refractometer operator uses a knob to adjust the mirror until the light and dark boundaries are at the intersection of the microscope crosshairs. The corresponding refractive index can then be read from the vernier scale. Since the contrast of the light / dark boundary is very low, it can only be determined manually. The accuracy of the classic Abbe refractometer is nD = 0.0002, where the fourth decimal place is determined by averaging a large number of individual measurements. The result depends on the user's interpretation and usually varies from user to user.
The semi-automatic refractometer is equipped with a digital display of measurement data, so the measurement data can be read more consistently. However, the device still needs to be manually adjusted so that the obtained measurement results continue to depend on the understanding and skills of the person under test.
The multifunctional handheld refractometer is very easy to use. Since there are multiple measuring scales, there are many application possibilities. Beekeepers use a handheld refractometer to determine the water content of honey, winemakers use it to determine the sugar content of fruit and grape juice, and breeders use it to determine the salt content of water in marine aquariums.
A small drop of liquid on the measuring prism is sufficient to determine the refractive index. Leaning against the light source, the measured value can be read on the scale through the eyepiece. The vertical scale and the horizontal boundary line intersect at the measured value. For temperature compensation of liquids, there are handheld refractometers with integrated automatic temperature correction (ATC).
The operation mode of the digital refractometer is the same as the handheld refractometer, but it has the function of automatically determining and reading the boundary line. Compared with manual hand-held refractometers, they reduce variability between operators, have higher accuracy, and usually provide a variety of commonly used scales.
The automatic refractometer completely eliminates differences in measurement results between operators and provides the highest accuracy. In contrast to performing manual refractive index measurements under uncontrolled environmental conditions, for example, the refractive index of a sample can be measured at different temperatures or light wavelengths. Automatic refractometers are mainly used in laboratory applications that require precise measurements under highly controlled conditions.
The process refractometer can continuously analyze the refractive index without moving the sample to a control laboratory. These instruments consist of sensors connected in series or bypass to the control box. The control box usually provides digital readings and outputs. Like other advanced refractometers (ie all refractometers except Abbe and handheld), they operate on the principle of total refraction by determining the critical angle of monochromatic light within the range of incident angles.