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TECHNICAL SUPPORT
Emissivity
What
is Emissivity?
Physics
of Emissivity
Affects
of Emissivity
Emissivity
Examples
Increasing
Emissivity
 Calculating
Emissivity
Emissivity
Table
Emissivity
Testing
 support@optotherm.com
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 Calculating Emissivity
OptoTherm Thermalyze™
software has the ability to compensate for emissivity so that accurate
temperature measurements can be made of materials with emissivity below
1.00. The accuracy of the measurement, however, is determined by the
precision to which the emissivity value and ambient temperature are
known. Additionally, the temperature of objects in the environment must
be uniform. Radiance from objects that are hotter or colder than the
surroundings can reflect off of the target and affect the accuracy of
emissivity compensation.
Small changes in an object’s emissivity can result in noticeable affects on
measured temperature. A 0.02 reduction in emissivity, for example, can
decrease the measured temperature of an object at 100°C by approximately
2°C. Likewise, variations in the ambient temperature can affect measured
temperature. An increase in ambient temperature of 5°C, for example, can
increase measure temperature of an object at 40°C with emissivity of 0.80 by
approximately 1°C.
In
order to compensate for the emissivity of an object, its emissivity must
first be determined. There are two basic approaches to determining surface
emissivity; surface treatment or material heating. Surface treatment
involves applying a treatment that is of a known high emissivity (usually
tape or paint) to the surface of the object and then heating the surface.
Material heating involves uniformly heating the object to a known
steady-state temperature that is above ambient temperature. During both
procedures, best results are achieved when the object is heated to a
temperature close to the temperature at which measurements are to be taken
during testing. If performed properly, correct emissivity values can be
obtained using either approach. The chosen method will depend on the
characteristics of the surface and size or shape of the object.
Surface Treatment
This method should be employed when the object’s size and shape facilitates
applying a small section of masking tape. Masking tape is the preferred
surface treatment for object temperatures below 100°C due to its uniform
emissivity (0.95) and thickness. Alternatively, a thin dab of paint or
white-out can be used on objects with small or uneven surfaces where tape
cannot be applied. The disadvantages of using paint or white-out are the
possibility of deviations in coating emissivity and thermal diffusion due to
variations in application thickness. If care is taken during the
application of the coating, however, uniform results can be obtained. To
determine an object’s emissivity using the surface treatment method, follow
these steps:
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Apply a small section
of masking tape to the area of interest making sure to leave a section of
the original surface exposed.
-
Heat the surface to a
temperature that is below 100°C. Heating can be accomplished by different
methods including powering the device or heating the surface using a
heating plate or hot air gun.
-
Capture a thermal image
of the heated surface. Note: Make sure that the heating source is
not reflecting off of the exposed surface when the image is captured.
-
Draw a small region
enclosing the tape and a second small region enclosing the exposed
surface.
-
Set the emissivity of
the region enclosing the tape to 0.95.
-
Adjust the emissivity
of the region enclosing the exposed surface until the temperatures within
both regions are equal. Record the emissivity of the object.
Material Heating
This method should be employed when tape or paint cannot be applied to the
surface due to an object’s small size or surface characteristics. Material
heating can also be used to determine the emissivity of different materials
comprising a complex object with many different surfaces. To determine an
object’s emissivity using the material heating method, follow these steps:
-
Heat the object to a known uniform steady-state temperature. One of the
most common methods of heating small and thin objects, such as
semiconductors chips, is using a heating plate. A thermal chamber can
also be used provided there is an opening or infrared window on the
chamber through which to image the object.
-
Measure the
steady-state temperature of the object by measuring the temperature of a
high emissivity area in the thermal image or by using a contact
temperature probe.
-
Draw a small region
enclosing each different surface to be measured.
-
Adjust the emissivity
of each region until the temperatures within the regions are equal to the
temperature of the object measured in step 2. Record the emissivity of
each different surface.
A Note about
Contact Temperature Probes
If
used in appropriate situations and applied correctly, contact temperature
probes such as thermocouples, thermistors, and RTDs can be used to
accurately measure surface temperature. Small objects and thin surfaces,
however, may not contain enough thermal mass to accurately measure using
these devices. In these cases, contact probes can act as heat sinks and
lower the temperature of the material, creating erroneous readings. Also, a
good thermal bond must exist between the material and contact probe in order
to transfer sufficient thermal energy to heat the probe to the same
temperature as the material. In many cases, poor thermal bonding results in
erroneous temperature measurements that are much lower than true
temperature. Measurement errors due to low thermal mass and poor thermal
bonding can result in errors as great 10, 20, or even 30°C when measuring an
object at 60°C.
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