C-THERM TCi, Operator'S Manual, Page: 107 / 138

TCi Operator Manual
101
The algorithm to calibrate and calculate k is the m* , which is outlined in US Patent 6,676,287 B1.
It uses an iterative process to calibrate the sensor against measured materials with known
thermal conductivity. The calibration process generates a value called
m* , such that:
A typical k calibration has this form:
Intercept k Slope
m m
  

*
1
(24)
m
*
is found by an iterative process during calibration. To calculate k use:
Slope
Intercept
m m
k



*
1
(25)
Each group of materials has a different k calibration for room temperature. With automatic power
correction and C(T) correction (see below), this calibration can be also used for different
temperatures. The CA correction factor is used for k calculations as well. The calibration curves
for k are all linear. Example is given below.
9.4.1 Limitations for k measurements
The method for measuring k requires different calibration curves for materials with dissimilar
thermo physical properties. For instance, it is necessary to calibrate solids and liquids separately,
and use the applicable calibration when measuring a certain material. Measuring a liquid with a
calibration made for solids, or vice versa, will generate a wrong result. Additionally, certain
materials may exhibit an anomaly relative to others. Those materials normally have different
effusivity to conductivity ratio than the materials used for calibration. For instance, carbon has
high conductivity like high
k metals, but its volumetric heat capacity is much smaller than metals
with similar thermal conductivity.
In avoiding such circumstances, the use of the following guidelines is encouraged:
Always measure a sample with the test method that uses a calibration group, which is closest to
the sample’s type of material.