Refractometer Temperature Compensation

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Temperature Compensation Makes The Difference!

Temperature is one of the single most important factors influencing accurate refractometer readings and is one of the largest sources of error in measurement. Refractive index is VERY dependent on temperature. It is well known that substantially all materials expand when heated (become less dense) and contract when cooled (become more dense). The speed of light in a liquid increases with temperature, and the refractive index, therefore, decreases.

Comparison Graph Showing MISCO Refractometers are Automatically Temperature Compensated

The amount of error per degree Celsius is different for every fluid and differs for different concentrations of the same fluid.

Temperature compensation relieves the user of the responsibility to measure temperature and apply a correction factor when taking readings. Temperature Compensation Makes The Difference!

The chart compares a temperature compensated MISCO Refractometer (red) to a non-compensated competitor (blue). It is plain to see that at 20 'C (68 'F) they are both correct; however, as the temperature deviates from 20 'C, there is a marked error in the non-compensated instrument.


Three Approaches to Temperature Compensation

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Traditional Handheld Refractometer
[Linear]

MISCO traditional handheld refractometers are temperature compensated by means of a bi-metal strip that expands or contracts depending on the temperature of the instrument. This bi-metal strip moves the optics inside the instrument to adjust for temperature variations.

One of the drawbacks to this type of compensation is that the compensation is designed for a single fluid and compensates only in a straight line (linear) not taking into account changes in the temperature coefficient for differing volumes.

Is this compensation accurate? Linear compensation is not quite as accurate as non-linear compensation, but it will give accurate repeatable results for most water-soluble fluids. The largest amount of error in measurement will occur near the limits of the temperature compensation range and for high-fluid concentration.

Digital Handheld Refractometer
[Non-Linear]

The MISCO Digital Fiberoptic Refractometer (DFR) and the MISCO Palm Abbe have a high-precision thermistor that is in contact with the fluid. This thermistor is able to accurately sense the exact temperature of the test sample.

The internal microprocessor interprets the signal from the thermistor and applies a correction to the readings based on the temperature difference and percent volume for the fluid under test.

The most amazing feature is that the digital refractometer can utilize multiple temperature correction tables for different fluids over a wide temperature range.

A digital refractometer can also be custom programed to measure your specific fluids and can have custom temperature compensation tables as well.

Abbe / Laboratory Refractometer
[Controlled]

Abbe / laboratory refractometers typically have a scale that is temperature compensated for Brix using non-linear compensation.

However, the primary method laboratory refractometers use is temperature control. Most precision Lab refractometers have water inlet and outlet ports. By connecting the refractometer to a circulating water bath the temperature can be precisely controlled to 0.1 degree C (see, ARIAS, AR600).

New laboratory refractometers, like the AR700, are available with Peltier temperature conrtol. A Peltier device is a solid-state unit that can electronically heat or cool the prism very precisely. Peltier devices elimate the reliance on external water-baths. Once the Peltier reference temperature has been set, the refractometer will display accurate results regardless of concentration.


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