The Measurement Shop’s Guide to Refractometers
What is refractometry?
Refractometry is an analytical process of measuring a substance in accordance with a refractive index in order to determine the composition or purity of the substance.
What do refractometers measure?
Refractometers are measuring instruments used to gauge the extent light refraction (as part of a refractive index) of transparent substances in either a liquid or solid state in order to identify a liquid sample, assess the sample's purity, and determine the amount or concentration of dissolved substances within a sample. As light passes through the liquid from the air it will slow down and create a ‘bending’ illusion, the severity of the ‘bend’ will depend on the amount of substance dissolved in the liquid. For example, the amount of sugar in a glass of water.
There are many types of refractometry scales that are used to measure salt, plasma proteins and more, however the most popular refraction index is commonly referred to as the ‘Brix Scale’. The Brix scale can calculate the number of grams of pure cane sugar dissolved in 100 grams of water.
What is a refractometer used for?
Refractometers are highly useful measuring devices that can used in a variety of applications such as determining the clarity of gems, monitoring saline levels in aquariums, analysing chemicals in laboratories or for making food preserves such as jam, honey and marmalade. Here is a brief list of some of the most common applications that require a refractometer:
One of the primary uses of a refractometer is for measuring the clarity of precious gems such as diamonds, emeralds, sapphires and rubies. Gems are valued according to several criteria including colour, cut/shape, clarity, weight/size and material. A certified jewellery refractometer is frequently used to determine the overall clarity of the gem and is classified in association with its RI (refraction index) value.
Refractometers have recently developed beyond gem refraction and brix index systems, now being adopted for a wide range of applications in the medical field. Refractometers can be used by medical professionals to test the clarity of urine samples in order to determine a patient’s health. In addition, refractometers for clinical use can be used to measure urine-specific gravity as a means to test for tampering of urine drug samples in sports. Doctors and vets may also use medical refractometers to monitor blood plasma or plasma protein.
Automotive refractometers have been designed to ensure the correct levels and freezing point of chemicals, such as glycol, in battery fluid, coolant and vehicle screen wash. Through refraction, a solution can be tested for effectiveness.
Many environmentalists in the forestry and solar industry rely on hand held refractometers as a means of checking for frost protection. Salt water aquarium owners and marine biologists may also use refractometers to monitor and test sodium chloride levels in a given water sample to ensure the health of a water ecosystem. Bee keepers often use honey refractometers to test the purity of produced honey, the clarity of honey is often used as a means to determine the health of a swarm.
Laboratories regularly use refractometers for measuring specific gravity by measuring liquid density in relation to the density of water (water has a specific gravity of 1).
Refractometer have become increasingly popular in cooking as a way of measuring sugar and salt content in food products. Brix refractometers are often used by hobbyists for making preserves including jams, marmalades and honey. Professional kitchens may also use hand held refractometers when preparing marinades or sauces to ensure consistency and quality.
Brewing refractometers are now becoming incredibly popular in home brewing and micro pubs; these refractometers are perfect in the creation of wines and spirits and are used for measuring sugar content in fruit prior to pressing.
How to use a refractometer to measure jam
- Place a small amount (around 30g) of jam/ marmalade into a beaker and mix well in order to produce a testing sample.
- Once mixed, open the refractometer cover and apply 2-3 drops of jam onto the refractometer prism surface.
- Close the plate/ cover and ensure that the jam sample is evenly spread over the whole surface of the prism.
- Look through the refractometer eyepiece and record the value where the dark and light measuring bars meet – This is the Brix percentage value.
- Once you have taken the Brix value, clean the prism using a damp tissue to remove all sample residue.
Things to consider and useful tips:
- This process may differ depending on whether you use an analog or digital refractometer. Therefore you will need to ensure that you are following the correct procedure based on the type of refractometer you are using. (See ‘what is the difference between an analog refractometer and a digital refractometer?’ section below).
- Refer to a brix value conversion table appropriate to the sample you are testing when measuring results.
- A recommended ratio of fruit: sugar is 1:1, however this will differ depending on the amount of pectin within the fruit used.
- The setting point for jam is 105 °C (220F). When starting the jam making process attach a sugar thermometer to the side of a saucepan placing the end into the jam mixture. When reaching the temperature above the jam solution should set.
What is the difference between an analog refractometer and a digital refractometer?
The primary difference between an analog and digital refractometer is the testing process. Analog refractometers require the user to place a sample on a cover plate and prism. You will then need to hold this towards a light source in order to view the scale within the meter. In contrast, digital refractometers require the user to add a drop of the solution to be tested into the well which is then illuminated by an LED light source. The meter will then interpret the light transmitted off the solution into a refraction index.
How to calibrate an analog refractometer
- Place a few drops of liquid (preferably distilled water) onto the refractometer prism.
- Close the cover plate (also known as the ‘daylight plate’) to ensure that the solution spreads across the entire prism surface and is free from bumbles or dry spots.
- Leave the sample on the surface for around 30-40 seconds to ensure the sample has fully adjusted to the temperature of the refractometer.
- Once stabilised, hold the cover plate up to a light source and look through the eyepiece.
- From here you should see a scale with graduations; you may need to focus the eyepiece in order to see these clearly.
- The upper portion of the scale should be show up darker than the lower section (usually blue and white respectively).
- Whilst looking through the eyepiece, you will need to turn the calibration screw located on top of the refractometer until the both the upper and lower boundaries meet exactly on the 0° marker.
Things to consider
- Calibration performed on instruments without automatic temperature calibration (ATC) will only be accurate to the temperature at which the calibration procedure was conducted. Therefore we recommend carrying out the calibration process in the same environment you hope to use your refractometer in.
Caring for your refractometer
Refractometers are relatively hardy devices that require little maintenance. However, it is recommended that the refractometer prism is cleaned after use with a soft lens tissue. Make sure you keep the cover closed to avoid scratching the prism.
Our selection of Kern refractometers
The ORA-W analog brewing refractometer is a convenient, portable device used to measure the sugar content in fruits such as grapes. These devices measure sugar content according to a brix scale in order to ensure the correct consistency and quality. Brewing refractometers are specifically designed for winemakers and brewers that need to measure sugar content in fruits used to make alcoholic beverages such as ciders, wine and brandy.
Kern's ORA-G analog gem refractometer is a compact, portable device ideal for use by jewellers. These devices include a refraction index that can be used to identify precious gems inlcuing diamonds, sapphires, rubies and emeralds.
One of the best refractometers available, the ORA-E expert analog refractometer has a particularly large measuring range for the refraction index as well as large divided scales for measuring brix value. This refractometer has a universal field of application.
Kern's ORT Abbe refractometer is a table-top digital refractometer used to mesure liquid, solid and paste samples in laboratories. The ORT is an extremely reliable products that also includes a built-in themometer for monitoring sample temperature over an examination process. This device is ideal for measuring pharmaceuticals, chemicals, oils and food products. Perfect for oil refineries, chemistry labs and food testing applications.
The ORD-PM is a digital refractometer that has been modified in order to monitor the health of patients in doctors, hospitals and care homes. Urine refractometers can be used to measure plasma proteins in the blood as well as saline levels in urine.
The Kern ORD digital brix/ sugar refractometer is used to monitor the sugar content in fruit and preserves. There are several varities of brix refractometers available from brewing refractometers used to measure sugar content in alcohol, as well as specialist honey refractometers that are often used by beekeepers for checking the health of bee swarms. Home bakers often use these devices when making preserves such as a jam, honey and marmalade.
For a full list of refractometers we offer, please visit our refractometers page.