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The emissivity of a surface may be measured by
comparison of the infrared thermometer reading with a standard contact
thermometer on the same surface.
A
miniature, thin-film platinum resistance sensor is preferred for the contact
sensor because it has a planar surface that makes better physical contact with
the surface than a conventional probe. A conventional probe may be used if
modest error is acceptable. Follow the manufacturer's recommendations for
mounting any sensor to the test surface. The contact sensor should be as
small as possible and bare (unhoused), for best results.
Once the contact sensor is intimately attached to the Surface
Under Test (SUT) and measuring properly, elevate thee temperature of the SUT and
stabilize at a temperature near the upper end of the infrared thermometer’s span
for approximately one hour for thick sections and proportionately less for thin
sections.
After stabilization, aim the infrared thermometer at a spot
on the SUT as close as possible to the contact sensor. If your instrument has
Visual Light Sighting, refocus the instrument until a ring configuration appears
on the light spot. Position the contact sensor in the dead center section of
the ring. Change the emissivity up or down until the infrared thermometer
display reads the same as the contact thermometer following the instructions in
the Operating Instructions. The resulting emissivity
reading is then equal to the emissivity of the SUT at that temperature and under
those conditions. Record this reading for future reference.
Another simple method of determining emissivity values of
greybody surfaces up to approximately 400°C is to mask off a portion of the base
surface with masking tape or scotch tape and pass the area adjoining the tape
edge through a candle flame or “smoky” torch to deposit hydrocarbon soot on the
area to effect a complete coverage. This soot has a high, well defined
emissivity of 0.955 ± 0.01, and has better thermal conductivity and a thinner
section than paint so as not to affect the thermal balance of the substrate. If
the tape is now pulled off of the target, a sharp line will divide the
soot-covered area from the bare area to be evaluated.
Heat
the object to an appropriate temperature and, using an instrument with Light
sighting with an emissivity setting of 0.95, measure the temperature of the soot
area just adjacent to the bare material line. Next, move the instrument over to
where the light is just on the bare surface adjacent to the boundary and adjust
the emissivity control to produce the same temperature reading that you got on
the soot-covered area. Record the value of this emissivity for future
reference.
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