A Guide to Refractometer Verification and Calibration

Compared to many other instruments in the laboratory or factory, using a refractometer is a very simple process. In most cases you apply a small sample to a cleaned refractometer and take a reading. That’s it! As easy as that is, and as rugged as a refractometer can be, regular verification is always recommended to maximise the accuracy of any readings you take.

So, with this in mind it becomes essential to check all instrumentation, whether they are optical handheld refractometers, digital handheld refractometers, or benchtop refractometers. This is to ensure that there has been no change in the calibration of the device between uses. We do this by checking the accuracy of reading at various points throughout the measuring range, and where necessary, resetting the instrument to read correctly at an appropriate point.

The checking procedure (or verification) involves placing various liquids of known characteristics onto the refractometer prism and checking against the instrument readings. Under normal conditions and expectations, you will find the reading from your refractometer agrees with the value you know to be true. You have verified your refractometer is correct and ready to use.

If there is a discrepancy with the reading and the value you know to be true, then you will need to calibrate your refractometer.

The reading error (or tolerance) taken at a set point (most commonly zero) can be corrected by dialling in the instrument using the capabilities of the refractometer. On a digital model this is often done by pressing the “ZERO” button. On an optical handheld refractometer, you would do this by turning the calibration screw until the borderline is level and reading at your expected reference point.

Calibration Care and Handling

Test liquids (often referred to as calibration materials) are usually solutions where the concentration has been accurately determined. This is done by weighing and, where necessary, relating to Refractive Index (RI – the value given to the ratio or change in the speed of light).

Upmost care must be taken with any refractometer calibration solution to avoid changes in concentration caused by evaporation or deliquescence. This applies at all stages in the handling of the liquids. It even applies to the transferring of the liquid from its container to the prism face. It is recommended that a pipette be used, rather than a rod or spatula, as a means of reducing spoiling. The surface of the test liquid must not be left exposed in an open bottle. Replace that lid as soon as possible and be sure not to introduce contaminants to the calibration liquid by using a pipette more than once!

It is important to note that even when correctly handled and stored, refractometer calibration fluids have a strict shelf-life. From 6 weeks for sucrose solutions, to 12 months for AG Fluids and calibration oils.

It is equally important that strict control of the temperature be maintained when carrying out calibration or zero setting by ensuring the instrument and sample are both stable. Equally, consider any differences between the instrument and calibration materials reference temperature and compensate (electronically or empirically using tables) accordingly.

Traceability Standards

For auditability, many users of refractometers choose to use certified solutions procured by an ISO17025 accredited calibration laboratories (UKAS in the UK, ANAB in the USA, or DAkkS in Germany for example) that provide traceability to internationally recognised standards such as ICUMSA or NIST. If you’re unsure of your local accredited laboratory or inspection body, you can search here on Bellingham + Stanley’s website.

Calibration and verification materials for refractometers, polarimeters and density meters at Refractometer Shop are supplied by Bellingham + Stanley.

For over 100 years Bellingham + Stanley has maintained its position as a leading manufacturer of refractometers and polarimeters to companies globally looking to measure refractive index and optical rotation. These high quality optical instruments have served customers around the world since pioneers Leonard Bellingham and Frank Stanley first introduced them in 1914.

In 2023 Bellingham + Stanley added its first Density Meter to bolster an already impressive roster of instrumentation. Calibration materials for polarimeters and density meters are available to purchase at Refractometer Shop.

With instrumentation being used as a measure for quality and yield control, it is imperative that good laboratory practice is adhered to. Instrument maintenance and a program of calibration forms an important part of this process.

In any industry, particularly those that Bellingham + Stanley provide services to, the accuracy of testing and measurement is of paramount importance.  Andrew Darkins, Technical Services Manager said in an interview with UKAS: “Production and process facilities throughout the world rely on refractometers and polarimeters for quality, compliance and yield control.  No matter what the make or type of instrument, results are only as good as the calibration regime behind the measurement.”

Production quality and control via the use of a high-quality refractometers combined with a verification and calibration plan using UKAS accredited reference materials, goes a long way to ensuring the safety of any end product. 

“Bellingham + Stanley is proud to be a UKAS accredited calibration laboratory (UKAS laboratory No. 0834).  Depending on the instrument type, application and traceability requirement customers are able to make a choice from a range of Certified Reference Materials (CRMs) to verify almost any make of refractometer or polarimeter that they may have.” 

What are the choices – and how to choose?

Careful consideration of Refractive Index or Brix scale range, toxicity and longevity must be taken when selecting the appropriate certified reference material to include in a refractometer users standard operating procedures.

At Refractometer Shop we stock a range of refractometer calibration materials to ensure your readings are as accurate as possible. Refractometer calibration materials are available with up to 5 decimal place RI and up to 2 decimal place Brix.

Calibration Oils

Calibration Oils are organic compounds that cover the range 1.47 -1.56 RI at the upper end of the Brix scale and beyond. Calibration Oils have a shelf life of 12-months. They are ideal for calibrating refractometers that have temperature control in both food and industrial applications.

AG Fluids

AG Fluids are aqueous solutions that cover the range 1.33-1.40 RI (0-40 °Brix equivalent) and have a shelf life of 12-months. They are ideal for calibrating refractometers that have temperature control in both food and industrial applications as well as instruments that have AG Fluid temperature compensation (for example, OPTi handhelds and RFM700 Series benchtop refractometers that operate without temperature control). AG Fluids are unique as they are low Brix and have a long shelf life.

Whilst single bottles can be purchased, customers that prioritise accuracy and calibrate regularly opt for our 5 x 5 ml packs. These packs are available as specific values (5 x 5l of AG 2.5 Brix for example) or as a multi-pack, ideal customers calibrating across a range, such as salespeople and service engineers. These “variety packs” provide complete and a far more complex verification of the linearity across the whole scale range; often this is done as part of an annual instrument validation at a busy food or beverage production plant or a pharmaceutical company (performance verification or PQ). Calibration oils may be included in these multi-packs as appropriate. 

Sucrose Solutions

Sucrose Solutions offer ease of use when calibrating instruments that do not have temperature control but instead rely on temperature compensation such as optical and digital handhelds (although the OPTi Digital Handheld Refractometer does facilitate the use of AG Fluids by offering a unique AG Test Mode that incorporates AG Fluid specific temperature compensation. Find out more about the OPTi here.) They are also preferred by beverage, sugar and food producers as their components typically match the product being tested.  

Where Can I Buy Refractometer Calibration Fluids?

Remember, your readings and results are only as good as your calibration. Refractometers work within fine margins and are delicately balanced scientific instruments. Look after your refractometer and it’ll serve you well for many years to come.

You can buy refractometer calibration materials from our online store. Most of our fluids are available as single items or boxes of 5. If you require a subscription contract or larger packs then please contact us today.

Looking for a new refractometer? Take a look at our range of optical and digital handheld refractometers by answering this simple question: What do you measure?

If you’re looking for a new benchtop refractometer for your laboratory then please take a look at the range of digital refractometers offered by world renowned refractometer pioneers, Bellingham + Stanley. Not only do B+S have over 100 years’ experience in the field of refractometry and polarimetry but are also part of Xylem’s laboratory solutions division, providing customers with know-how from across several like minded brands aimed at the food & beverage, industrial, chemical, pharmaceutical, life science and water industries.

Still want to know more? Please contact us today.