TA Instruments TGA 2950 - Data Analysis and Plotting

Appendix B
B–32 TA I NSTRUMENTS TGA 2950
Thermocouple
Placement
Generally, the thermocouple should be placed 2
to 5 mm above the bottom of the sample con-
tainer (1 to 4 mm above the edge). Placement
1 mm above the edge is recommended for most
applications. Longer distances will not degrade
heater control and are sometimes helpful in
reducing any adverse effects of sample self-
heating or bubble formation.
Data Analysis
Effects
Hi-Res
TM
TGA, due to its technique of changing
heating rates dynamically, causes very signifi-
cant non-linearity of data points across transition
regions when plotted versus temperature. This
is not a problem when making plots, but when
automatic limit selection is used during data
analysis report generation the analysis program
can sometimes become confused about where to
place the limits. This situation can be identified
by curve tick marks and tangent lines which are
poorly placed on the weight curve or may even
be plotted off of the curve.
The difficulty is caused by the fixed size of the
transition onset and endset windows (the first
and last 12.5% of the analysis region). The data
analysis program assumes that data points are
uniformly distributed throughout this region. If
the number of data points in the baseline portion
of a window is very small compared to the
number of points in the sloped portion of the
window (the usual case with Hi-Res
TM
scans)
then the tangent lines will be excessively