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.
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.
Frequently Asked Questions
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.
One of the drawbacks to this type of compensation isthat the compensation is designed for a single fluid and compensatesonly in a straight line (linear) not taking into account changes in thetemperature coefficient for differing volumes.
Is this compensation accurate? Linear compensation isnot quite as accurate as non-linear compensation, but it will giveaccurate repeatable results for most water-soluble fluids. The largestamount of error in measurement will occur near the limits of thetemperature compensation range and for high-fluid concentration.
However, the primary method laboratory refractometersuse is temperature control. Most precision Lab refractometers havewater inlet and outlet ports. By connecting the refractometer to acirculating water bath the temperature can be precisely controlled to0.1 degree C (see, ARIAS, AR600).
New laboratory refractometers, like the AR700, areavailable with Peltier temperature control. A Peltier device is a solid-state unit that can electronically heat or cool the prism very precisely. Peltier devices eliminate the reliance on external water-baths.Once the Peltier reference temperature has been set, the refractometer will display accurate results regardless of concentration.