LumaSense Technologies ISR 320 - Manual (Page 18)

ISR 320 Manual Settings / parameter descriptions 18
Note: Emissivity Slope K Settings: 0.800 to 1.250 in steps of 1/1000 (2-color mode).
4.2.1 Temperature Errors Cause by Non-Graybodies
A graybody target has emissivity that is the same at each of the two wavelengths used for
measurements and is constant throughout the temperature range. The ratio of the emissivities,
1 / 2= 1 and stays constant regardless of the target temperature. When a target deviates from
this, that is when 1 / 2 does not equal 1.0 and a slope adjustment is required. For many
materials, this is a one-time adjustment.
The following table illustrates the ISR 320 reading errors that can occur when the slope setting
differs from the actual material emissivity ratio.
TABLE OF EXPECTED ERROR WHEN EMISSIVITY OF ONE
WAVELENGTH IS 1% DIFFERENT FROM THE SECOND WAVELENGTH
TEMPERATURE ERROR DEG.
°F °C °F °C
1300 700 18 10
1500 815 20 11
1700 926 22 12
1900 1040 25 14
2100 1150 25 14
2300 1260 27 15
2500 1370 29 16
3000 1650 36 20
The table shows typical errors that can result when the emissivity of one wavelength differs
from the other wavelength by only 1%. The errors can get quite large as temperatures increase.
This error can be much larger than a 1-color IR pyrometer would produce for 1% emissivity
change. Therefore, it is important to select the proper mode (2-color vs. 1-color) on the ISR 320
to measure a specific material.
Another source of error is dust or smoke in the optical path which alters the transmission in one
wavelength more than the other. If the “dust” transmits 1% less energy at wavelength 1 than at
wavelength 2, the error table above also applies. Since not all smoke, dust, or dense steam
transmits equally at each wavelength, errors may become larger than expected for a 2-color
instrument. Usually the smoke and dust are the result of the material being processed and can
be cleared from the sight path by a fan or air purge tube.
In some materials, the emissivity may change at different rates with material temperature. Some
materials exhibit great changes in emissivity with temperature or time as oxidation modifies the
surface finish of the material. Such materials are not suited for measurement with 2-color
instruments. When problems are compounded with spectrally absorbing dust or smoke
(described above), obtaining reliable temperature readings with any 2-color instrument may be
impossible. In cases like this, a single color instrument using the shortest wavelength possible
would be the better choice. If this problem is encountered, switch the ISR 320 to 1-color mode.
In some situations, the single color mode will outperform the ratio mode.