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Figure 1 -FLOTHERM Model: Front View
Did you ever wonder what effect a thermocouple has on your
experimental data? Can you trust the measured temperature or is there
a hidden source of error you have not considered? Dr. Cathy Biber, Senior
Thermal Engineer with Wakefield Engineering and FLOTHERM user, recently
addressed this topic. She began by building a model of a Wakefield series
275 heat sink attached to a TO-220 power semiconductor dissipating 3 watts
in a natural convection environment. Two thermocouples were added to the
model, one on the heat sink and one on the device tab as shown below.
Dr. Biber's method for modeling a thermocouple (TC) consists
of three parts: the bead, the wire and the contact resistance between
the bead and the surface whose temperature is being measured. The bead
and wire should be modeled as solve-in-solid cuboid blocks, with appropriate
thermal conductivity to account for the type of TC being used in the experiment.
Remember to calculate an equivalent thermal conductivity for the bimetallic
wire. As an example, Dr. Biber used a T type (copper-constantan) thermocouple
which worked out to have an equivalent thermal conductivity of 100 W/mK.
Figure 2 - FLOTHERM Model: Rear View
Finally, an internal plate is used to model the contact
resistance between the bead and the surface. The thickness and thermal
conductivity of the plate are critical in obtaining agreement with test
results. Cathy Biber used the conductivity of air (0.026 W/mK) and a thickness
of 1.5 microns to represent the contact resistance in her model. However,
it is important to model the effects of thermal grease or adhesives if
used. The routing of the TC wire in your model should match that of the
experimental set-up.
Results
Experimental results were compared to FLOTHERM model predictions
with and without the presence of thermocouples. In the model without thermocouples,
the temperature was taken from a cell within the device tab and within
the heat sink. In the model with thermocouples, the temperature was taken
from the cells representing the TC beads. The following table presents
the results.
|
Measured
|
FLOTHERM Model (C)
|
|
(C)
|
No TC's
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With TC's
|
TO-220
Device Tab |
108.1 |
114.0 |
108.4 |
| Heat Sink |
89.8 |
103.0 |
90.6 |
| Max. Temp in Model |
n/a |
119.3 |
118.5 |
Table 1 - Results
As you can see, the model containing thermocouples has
excellent agreement with the experimental data. This model was correlated
to one set of experimental results and then tested against several other
sets of data, all with similar agreement. However, in the model without
TC's, the results showed temperature differences of 6C on the device and
13C on the heat sink!
Figure 3 - Results on the Centerline Plane
Conclusions
Two points are clear as a result of this exercise:
- "Just because you've measured it, that doesn't
make it right". There are errors and inaccuracies involved
with testing as well as with modeling.
- If you are having difficulty correlating your model
with test results, don't dismiss your model as inherently inaccurate.
It may simply be that you have not accurately captured the particulars
of the test setup.
 Author: Dr.
Catharina Biber, Wakefield Engineering
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