Civil Engineering Reference
In-Depth Information
instruments are based on the Boersma principle in that only the crucibles are
in contact with the thermocouples.
Boersma's DTA configuration, Fig. 1b, can be considered as the
missing link between differential thermal analysis and differential scanning
calorimetry. Some even feel that this configuration is, in fact, a DSC
instrument. This is the major reason behind the confusion as to the
differences between DTA and DSC.
Figure 1.
Schematic diagrams of different instruments used in thermal analysis to detect
energy changes occurring in a sample:
(a)
conventional DTA,
(b)
Boersma
[23]
DTA,
(c)
power-compensation DSC, and
(d)
heat-flux DSC.
The two most crucial differences between the two techniques are:
(a) in DSC, the sample and reference have their own heaters and tempera-
ture sensors as compared to DTA where there is one common heater for
both; (b) DTA measures
∆
T
versus temperature, and, therefore, must be
calibrated to convert
T
into transition energies, while DSC obtains the
transition energy directly from the heat measurement. The confusion is also
partly due to the fact that there are at least three different types of DSC
instruments: a DTA
calorimeter
, a heat-flux type (Fig. 2c), and a power
compensation (Fig. 1d) one. This, in turn, arises from the fact that some
define calorimetry as quantitative-DTA. As opposed to conventional DTA,
the thermocouples in a DSC instrument do not come into contact with either
the sample or reference. Instead, they either surround the sample (thermo-
piles) or are simply outside the sample (thermocouples). Furthermore, the
sample and reference weights are usually under 10 mg.
∆
