Refractometers are widely used to measure the amount of dissolved substances in solutions containing mainly sucrose. The relationship between refractive index and the amount of dry substance content is well known for sucrose and is the basis of the BRIX scale, which is the measure of the number of grams of sucrose present per 100 grams of aqueous sugar solution. That is, 50 BRIX means 50 grams of solid per 100 grams of solution. This relationship holds for a large number of similar substances and so the BRIX scale is widely used in the food industry. Unfortunately the refractive index of a sugar solution changes with temperature while its solid content remains the same.
Refractometers measure the critical angle of refraction through a sample. The critical angle of refraction will change with concentration. It will normally increase with an increase in concentration. Color, gas and dissolved particles do not affect measurement results. Refractometers can be designed for either continuous or sample monitoring.
Important specifications for refractometers are the refractive index range or BRIX range, depending on which is being used, and the accuracy. The pressure range of the refractometers is important, as are the maximum flow velocity and the sample size.
Sample temperature control is a popular feature available for some refractometers. Another special feature is an automatic prism wash system, which will clean the prism without having to remove it from the refractometer. Other features include temperature compensation, temperature sensing, sanitary ratings, programmability, self-calibration and alarms.
Common display types for refractometers include analog meters or gauges, digital displays and video terminal displays. They can be handheld, as a field instrument might be, portable with wheels or handles for ease in transporting, or more permanently fixtured instruments. Analog outputs such as current, voltage and frequencies are often available. Computer signals and other digital outputs can be found in these devices as well.