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| HANDHELD REFRACTOMETER OVERVIEW |
Basic Refractometer Principles
Refractometers work according to the principle that when a ray of light passes from one medium to another,
the speed of the light changes according to the density of the transmitting medium. At the interface
between two media, the ray changes direction as its speed suddenly changes. This effect is known as
refraction and is a familiar concept.
The refractive index (RI) of a substance is a measure of the speed of light in a substance relative to that
in a vacuum (very close to the speed in air). The RI is a physical property that depends upon temperature
and the wavelength of the light. For a particular substance the RI is a unique number when measured
using a monochromatic light source (single wavelength) at a fixed temperature.
Handheld refractometers and bench refractometers are devices that measure the RI of a substance, usually a
liquid, but sometimes a solid. Laboratory bench refractometers utilize monochromatic light, usually
that of sodium at 589.3 nm. They also have a means for controlling temperature or at least measuring
it precisely in order to 'compensate' for any variance. A bench refractometer can typically measure
the refractive index to within 0.0001 or better. Thus, the refractive index of water when measured
with sodium light (589.3 nm) at 20 °C is 1.33299.
Scientists may wish to measure the RI when studying the physical properties of different liquids and
solids. However, bench and handheld refractometers are usually used for more pragmatic purposes,
usually to measure the concentration of a dissolved substance.
The simplest and most popular use of a bench or handheld refractometer is in measuring the concentration of
sugar in water. As the concentration of sugar increases the RI increases. A bench or handheld
refractometer can therefore be used to measure concentration of sugar provided the relationship between RI
and concentration (and temperature!) is known.
The Brix scale is the most widely used scale and is based on the relationship between pure sucrose in water
concentration (weight %) and RI. The Brix scale is more popular than RI itself. Brix is used for
testing 'liquid food' products. Even when the food does not just contain sucrose in water, but other
dissolved ingredients, the Brix scale is used as a measure of 'nutritional value'. Thus soft drinks,
juices, sauces, preserves etc. are assigned 'a Brix value' as part of the Quality Assurance for the
product. Indeed, in the juice and soft drink industries, the Brix value is arguably the most important
parameter in quality control.
So, what about handheld refractometers?
The use of a handheld refractometer facilitates convenient and rapid measurement of concentration in a number
of liquid and semi-solid samples. Handheld refractometers are low-cost, simple devices that are popular
in a multitude of applications. Handheld refractometers are popular because they are easy and convenient
to use and cost a fraction of a typical bench instrument. Unlike bench refractometers handheld refractometers
are limited in terms of accuracy and applicability because:
1. They utilize natural (white) light
2. There is no way to control temperature
3. Light must be transmitted by the sample
Using white light means that the handheld refractometer's borderline cannot be as sharp as that obtained in
a laboratory instrument. White light is made up of wavelengths from about 350 to 800 nm (the visible
spectrum). Light of each wavelength travels at a different speed. This manifests itself as blurring
and coloring of the borderline. This is called dispersion (splitting into different wavelengths). Some
substances have high dispersive power and therefore will distort a 'white light' borderline to a greater
extent. This effect gets worse as the RI increases, such that some liquids may not be measurable to an
acceptable accuracy using a handheld refractometer.
The accuracy and precision is also limited by the size and optical arrangement. Typically, a handheld
refractometer can read on an RI scale to about 0.001 units, compared to a resolution of up to 0.00001 on a
bench Abbe or automatic refractometers. These values equate to about °0.2 Brix for a handheld
refractometer versus a resolution to 0.01 Brix on a bench refractometer. Accuracy on the bench refractometer
will vary according to sample type, but can be about °0.02 for a sugar solution or soft drink, i.e. about
100 times more accurate than a handheld refractometer. For information on bench refractometers please
visit Bellingham & Stanley's website.
Despite the fact that some handheld refractometers offer Automatic Temperature Compensation (ATC), there is
no actual control over the sample temperature. The ATC principle (explained below in section) gives a
limited degree of temperature correction, relying upon the sample being at the same temperature as the handheld
refractometer.
Most handheld refractometers are calibrated initially at 20 °C in the factory. When the ambient
temperature changes, the handheld refractometer temperature also changes and the calibration is no longer
valid. Certain handheld refractometers such as the Eclipse
have a 'zero' adjuster. This means the handheld refractometer can be re-set using a reliable standard
such as water. This is in fact a better way to combat changes in temperature with a handheld refractometer
than relying upon ATC. Sudden changes in temperature or applying very hot or very cold samples (e.g.
out of the refrigerator) can incur quite large errors with handheld refractometers. The way to deal
with this is to have good experimental technique/protocol to ensure thermal equilibration.
Because handheld refractometers work with transmitted light, highly colored or opaque samples may not give
very sharp borderlines. The position of the borderline then becomes a subjective judgment of the user,
which reduces the reproducibility of the handheld refractometer. Automatic bench refractometers surmount
this problem by measuring the reflected light from a sample surface, rather than transmitted light. Thus,
for example in the tomato paste industry, automatic bench refractometers are considerably easier to use and
much more reliable than handheld refractometers where borderlines are often not discernible.
In spite of these limitations handheld refractometers have proven to be quite useful in practice and many
users are happy with the lower accuracy of handheld refractometers, accepting that they provide a quick
and convenient way to check their product, rather than a highly precise QA method.
Handheld refractometer food applications include the measurement of sugar in grape must and other fresh
fruit, processed food, sugar solutions and beverages. Starch, honey and salt solutions may also be
measured with handheld refractometers.
Handheld refractometers are also commonly used to control blend ratios of various industrial liquids. Chemical
blends such as glycols, battery acid, heat exchange fluids, automotive antifreeze, aviation wing de-icing
chemicals and jet fuel ice inhibitors can be controlled easily with a handheld refractometer whilst measurement
of industrial fluids such as coolants, quenchants and hydraulic oils are also commonplace.
Global Water Instrumentation offers a complete choice of handheld refractometers starting with the E-line
economy range of optical models for use by hobbyists and within light industrial applications to the latest
OPTi digital models that provide not only the most accurate performance in it's class but also repeatability
when switching between users.
OPTi digital handheld refrectometers
The new OPTi digital handheld refractometers designed in 2011 are manufactured in the UK as highly reliable
premium quality instruments suitable for use in the harshest of environments.
There are many different models within in the OPTi handheld refractometer family covering not only the commonplace
food, beverage and chemical instruments but also life science, automotive, heat transfer and even customer specific
applications.
To see more information on the OPTi digital handheld refractometers, please click
here.
Eclipse "Professional" handheld refractometers
Since its launch in 1999, the Eclipse handheld refractometer has become recognized as one of the World's most
reliable and robust optical refractometers on the market today. Manufactured in the UK using the latest
techniques, Eclipse handheld refractometers feature a bright field of view, a unique sample "dribble feed" and
are sealed against moisture ingress.
A selection of scale types covering a wide range of common applications is available from within the Eclipse
handheld refractometer range as well as the unique Low Volume Brix Models that are favored by zoologists and
entomologists for having the capability of measuring samples of less than 1-microlitre such as nectar.
To see more information on the Eclipse "Professional" handheld refractometers, please click
here.
E-Line "Economy" handheld refractometers
The low cost models within the E-line range of handheld refractometers are specifically suited for light industrial
use within small to medium sized businesses and by hobbyists handling fruit, beverages, confectionery, jam, honey
and other sugar based products as well as for calculating concentration or blend ratios of heat transfer fluids like
glycol, antifreeze, cutting fluids and quenchants.
The e-line gemological handheld refractometer has a refractive index range up to 1.81 RI facilitating the measurement
of precious and semi-precious gemstones.
To see more information on the E-Line "Economy" handheld refractometers, please click
here.
DR-Digital handheld refractometers
DR-Digital instruments provide a digital measurement solution within two discrete applications not yet satisfied by
the OPTi series of handheld refractometers. Brix scale DR-Digital handheld refractometers are available for
concentration measurement of syrups, jams, marmalades and honey. The DR-Digital wine handheld refractometer
models offer multiple scales for measuring grape must.
To see more information on the DR-Digital handheld refractometers, please click here.
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