During the 1960s, a significant advance in infrared temperature measuring came about with the introduction of ratio or two-colour infrared thermometry.
This technology uses two infrared sensors within the transmitter, each sensor being sensitive to a specific and different infrared wavelength. The output from the sensors is fed into a microprocessor where an energy 'ratio' is calculated by dividing one sensor signal by the other.
Standard or single-colour infrared thermometers are greatly affected if the IR energy is obscured or attenuated, because they rely for accuracy on the total amount of infrared energy emitted from the product reaching the sensor. In the case of molten metals, specifically irons and steels, the emitted infrared energy tends to be nonlinear and changes with temperature and wavelength, thus making standard thermometers inaccurate for these applications.
Ratio thermometers measure a ratio function or 'slope' between the infrared energy detected by two sensors and are not affected by atmospheric sighting problems, ie dust, steam or particulates which reduce the amount of energy transmitted, because the slope between the sensors does not change.
Since the accuracy of a ratio thermometer is not affected by a reduction in the amount of IR energy reaching the unit, the target field-of-view of the thermometer does not have to be filled, ie the temperature of a product that is smaller than the field-of-view is measured accurately with excellent resolution.
Similarly, when measuring the temperature of products with nonlinear emissivities, the slope is not affected.
There are many two-colour Raytek units installed in South Africa, measuring difficult and demanding applications. Some of these applications are:
* Monitoring klinker temperature and refractory in cement kilns where dust is a problem.
* The nonlinear emissivity problem of monitoring the temperature of steel when poured from a furnace has been solved at two major plants. This solution has also reduced the usage of dip thermocouples.
* A ferro-silicate plant where the sighting of the product is difficult and where there are also problems of nonlinearity of emissivity.
* A ceramic insulation fibre plant where the field-of-view of a stream of 'molten ceramic' at 2000°C is only 5 mm in diameter.
The above are a few examples where ratio (two-colour) infrared thermometers have solved problems that would be impossible for standard (one-colour) thermometers.
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