Quick quiz: Which lab instrument can be found in a mechanic’s toolbox, a winery, and even a gemologist’s desk? This resourceful device can also be used by labs to conduct drug testing, but is most often used in the food and beverage industry. Of course, it’s the refractometer, which is so versatile it can test everything from battery acid to the authenticity of emeralds. Both home beer brewers and aquarium keepers rely on a refractometer for very different purposes.
The refractometer requires only a few drops of liquid to measure concentrations of aqueous solutions. Solutions have different refractive indexes depending on their concentration and a refractometer can measure the purity of the composition of the solution. For example, a Brix refractometer can determine the amount of sucrose in fruit juice for quality analysis. Different types of refractometers can measure gases and translucent solids. Most models feature Automatic Temperature Compensation (ATC), which is a built-in measure that ensures accuracy.
When light enters a liquid it changes direction; this is called refraction. Refractometers measure the degree to which the light changes direction, called the angle of refraction. A refractometer takes the refraction angles and correlates them to refractive index (nD) values that have been established. Using these standard values, you can determine the concentrations of solutions.
The prism in the refractometer has a greater refractive index than the solution. Measurements are read at the point where the prism and solution meet. With a low concentration solution, the refractive index of the prism is much greater than that of the sample, creating a large refraction angle and a low reading. The reverse would happen with a high concentration solution.
|Sample Fluid||Temperature||Refractive Index|
|Acetic Acid||25° C||1.370|
|Paraffin Oil||20° C||1.412|
|Palm Oil||20° C||1.456|
|Olive Oil||20° C||1.471|
Brix refractometers measure the sugar content of fruits, beverages, and canned goods, along with the total solid content of syrups, vinegar, and sauces. They can also be used for chemical, agricultural, manufacturing process control, and quality control.
|Sample Fluid||% Brix|
|Cutting Oils||0 to 8|
|Oranges||4 to 13|
|Carbonated Beverages||5 to 15|
|Apples||11 to 18|
|Grapes and Wines||14 to 19|
|Concentrated Juices||42 to 68|
|Condensed Milk||52 to 68|
|Jams and Jellies||60 to 70|
The % Brix Scale shows the concentration percentage of the soluble solids content of a sample (water solution). The soluble solids content is the total of all the solids dissolved in the water,beginning with sugar, salts, protein, acids, etc.; the measurement reading value is the value of the sum total of those. The Brix scale is calibrated to the number of grams of cane sugar contained in 100g of water. When measuring a sugar solution, the % Brix reading should perfectly match the actual concentration.
Gem refractometers are specialized instruments used to determine the quality of fine stones or glass. They may be used in jewelry stores, museums, auctions, and similar venues.
Industrial fluid and coolant/battery acid refractometers monitor the quality of industrial and automotive fluids. This type measures propylene glycol and ethylene glycol concentrations.
Salt refractometers measure salt concentration of brine in salted foods, or salt concentration in aquariums or commercial fisheries. They assess salt content in parts per thousand (ppt) and specific gravity or percent salinity.
Wine refractometers are specifically designed for growers, wineries, and grape must buyers. They are outfitted with a simple scale selection key to deliver immediate Brix, Titre Alcoometrique (T.A.), Oechsle, KMW (babo), and Baume scale readings.
Handheld or manual refractometers are the least expensive choice. Yet, these are also the least precise because they require the user to reference a table to read the results, which invites human error. The refractive index may vary based on the temperature, so ATC is an important feature.
Digital handheld or programmable benchtop refractometers provide a more accurate reading than manual models but are slightly more expensive. The devices work quickly to provide instant measurements. For example, a grape grower can take a handheld model out into the field, squeeze sample grapes, and find out immediately if they are ripe enough for harvest.
Lab or Abbe refractometers offer a high degree of accuracy and better resolution. These are typically benchtop models which are not as portable as the handheld version and are also more expensive. These are ideal for pharmaceutical, industrial fluid, chemical, petrochemical, plastic, and food and beverage applications in which quality control is paramount.
In-line or process versions deliver continuous meaurements so there is no need to take a sample, saving production time. Use these refractometers to monitor or control the concentration of dissolved solids or water soluble liquids in process lines. They can be configured or programmed to collect data.
While some rely on hydrometers to measure sugar content, compensating for temperature variances is difficult with a hydrometer and a much larger sample is required. With ATC built into most refractometers and only a small sample needed, this instrument is much more effective and efficient. With units available for lab and field use, refractometers are the versatile option for a variety of applications