Refractometers enjoy popularity in many applications, each of which has slightly different requirements for the instrument. Because of this, and over time, refractometers of varying design, precision and operation have become available. These include the Abbe refractometer, the hand refractometer, the digital refractometer, automatic refractometer, and process refractometer.
The Abbe refractometer, named after its inventor Ernst Abbe (1840-1905), was the first laboratory instrument for the precise determination of the refractive index of liquids. The measuring principle of an Abbe refractometer is based on the principle of total reflection.
Abbe refractometers are used for measuring liquids. The reference media glasses (prisms) can be selected with high refractive indices. The light from a radiation source is reflected by a mirror and hits a double prism. A few drops of the sample are placed between this so-called Abbe double prism. The incident light beams pass through the double prism and sample only if their angles of incidence at the interface are less than the critical angle of total reflection. A microscope and a mirror with a suitable mechanism are used to determine the light / dark boundary line (shadow line).
The operator of the Abbe refractometer adjusts the mirror with the help of a rotary knob until the light / dark boundary is located at the intersection of the microscope’s crosshairs. The corresponding refractive indices can then be read from a Vernier scale. Since the light / dark boundary is very low in contrast, it can be only approximately determined manually. The accuracy of the classical Abbe refractometer is nD = 0.0002, where the fourth decimal place is determined by averaging a large number of individual measurements. The results depend on the interpretation of the user, and often differ between users.
Semi-automatic refractometers are equipped with a digital display of the measurement data and thus allow more consistent reading of measurement data. A manual adjustment of this equipment is still necessary, however, so that measurement results obtained continue to depend on the interpretation and the skill of the measured person.
The versatile handheld refractometers are very easy to use. Because of the availability of a number of measuring scales, many applications are possible. Handheld refractometers are used by beekeepers to determine the water content in honey, by wine producers to determine the sugar content of the fruit and the grape must, and by aquarists to determine the salt content of water in marine aquaria.
A small drop of liquid on the measuring prism is sufficient to determine refractive index. Held against a light source, the measured value can be read via the eyepiece on the scale. The vertical scale is intersected by a horizontal boundary line at the measured value.
For temperature compensation of liquids, there are handheld refractometers with integrated Automatic Temperature Correction (ATC).
Digital refractometers operate in the same way as handheld refractometers, but with an automatic determination and readout of the boundary line. They offer reduced inter-operator variability, and greater precision than manual handheld refractometers, and are typically available with a selection of common scales.
Automatic refractometers completely eliminate differences in measured results between operators, and provide the highest level of accuracy. In contrast to manual refractometric measurement under uncontrolled environmental conditions, it is possible, for example, to measure the refractive index of the sample at different temperatures or light wavelengths. Automatic refractometers are found mainly in laboratory applications, where precise measurement under highly controlled conditions is required.
Modern instruments are equipped with light emitting diodes (LEDs) as light sources, which have largely replaced older tungsten halogen or sodium vapor lamps because of their exceptionally long lifetime of 100 000 hours. An interference filter guarantees the maintenance of the correct wavelength. To enable comparability refractometric measurements, these are usually carried out at the standard wavelength of 589.3 nm (sodium D-line). The measuring principle is the same as other refractometers, where monochromatic light is incident at different angles of incidence on the prism and the sample. The intensity of the reflected light is determined by a charge-coupled device (CCD) scanner and the resulting position of the light / dark boundary is determined automatically.
Dark, cloudy, and even opaque samples such as mustard, ketchup or mayonnaise can easily be measured with the help of fully automated, modern refractometer. For these applications, 2-3 drops or a few grams of sample needed to be applied using a pipette or a spatula onto the measuring prism. To avoid stray light and evaporation, a sample cover is placed over the sample and prism. After measurement, the sample is removed by wiping with a soft paper cloth. The arduous purification of two prism halves (as is necessary for conventional refractometers) is thus avoided.
Automatic refractometers are used in diverse industries in the processing of food and drinks to determine sugar content and for quality control. Refractive index is often represented on the Brix scale (° Brix) for these applications. This value, which is displayed directly on the device, technically refers to the content of sucrose-dry substance in a pure water solution. However, it is also used as the characteristic value in the quality control of juices and other food products, and sometimes even in the analysis of other products, such as petroleum.
Process refractometers allow for the continuous analysis of refractive index, without the need to remove samples to a control laboratory. These instruments are composed of sensors, placed inline or in a bypass, connected to control box. This control box typically provides a digital readout and output. Like the other advanced refractometers (i.e. all except Abbe-type and handheld), they operate on the measuring principle of total refraction by determining the critical angle of monochromatic light of a range of angles of incidence.
Unlike the other refractometers, process refractometers can require zero human involvement during measurement, and can provide real-time measurement data process medium. This makes these instruments useful for large industrial operations, such as those of the food, sugar, pulp and paper, refining and chemical, and pharmaceutical industries. The display and output can be directly displayed in any scale derived from refractive index, as required by the specific industrial application. Process refractometers also have to function under very high pressures.