THA-SYS
Thin Heater Apparatus for thermal
conductivity measurement
THASYS offers measurement of the
thermal conductivity or total thermal resistance of plastics, composites and
electrical insulation / interface materials. THASYS works according to the ASTM
1114-98 standard; providing an accurate, fast, uncomplicated and absolute
measurement. It consists of a Thin Heater Apparatus (THA01) and a Measurement
and Control Unit (MCU). Employing a specially designed high accuracy thermopile
sensor, THA01 can handle thin sample materials (typically 0.1 to 6 mm, but
sometimes down to 0.01mm). The method is considered superior to procedures
according to ASTM D5470. Using a climate chamber a large temperature range can
be covered, performing measurements at regular intervals. THASYS is fully PC
controlled. For use with high thermal conductivity thin foil materials a
different model, type THISYS, is available.
More details on
the science: thermal
conductivity science. For
a free copy of the product brochure click here
for the PDF version.
Figure 1
THASYS consists of the Thin Heater Apparatus (THA01), (1) and a Measurement and
Control Unit (MCU), (2). It is PC controlled through RS232 (4,5). The PC is not
included. The measurement result appears on screen automatically. The THA01
housing contains two heat sinks in a bath of glycerol. It has a slot on top
through which two equally thick samples are inserted. They are placed on either
side of the thin heater. This heater also contains the hot joints of the
thermopile sensor. The samples plus heater are then pressed together with the
heat sinks using a screw on the side, creating a perfectly symmetrical setup.
The glycerol fluid eliminates the problem of contact resistance.
INTRODUCTION
For determining the thermal conductivity
of materials various types of measurement equipment can be used. Proven methods
are described in standards of the American Society for Testing and Materials
(ASTM).
ASTM C 1114-98 " Standard Test Method"
for "Thin Heater Apparatus" (THA) is a relatively new standard (1998) that
offers the possibility to perform fast measurements with high accuracy, across a
fairly large temperature range.
The THA principle relies on the fact
that a thin heater has negligible lateral heat flow. With a combination of a
very thin heater, two relatively thin samples of similar thickness and two heat
sinks it is possible to generate a homogeneous thermal field with known heat
flux through the samples.
By measuring the flux φ (derived from
heater power), the differential temperature across the samples, ∆Tamp,
and the effective sample thickness, Heff, it is straightforward to
calculate the thermal conductivity λ:
λ = φ Heff / ∆Tamp
The measurements of φ, Heff
and ∆Tamp are all direct measurements of power, dimensions and
temperature. This is contrary to methods like the " guarded hot plate", that
require reference materials or calibrated heat flux sensors. The THA provides an
absolute instrument.
THA01 DESIGN
The technological novelties are a
thin thermocouple thermopile (proprietary Hukseflux design) performing an
accurate and ultra-sensitive differential ∆T measurement, and the fact
that the measurement is performed in a bath of glycerol. The latter
reduces contact resistance and facilitates the THA operation.
Commonly used methods like ASTM D 5470 –
01 have shown to be highly sensitive to contact resistance. THASYS offers a
solution to this problem.
The THA01 can handle samples of 0.01 to 6
mm thickness. Samples typically are sheet materials with a size of 70 by 100 mm.
The measurement accuracy depends on the
total thermal resistance of the samples. In case of very thin samples (0.01mm),
measurement accuracy can be improved by “stacking”, i.e. using four or more
samples instead of the usual two.
The measurement essentially is done at
the temperature of the THA01. If necessary the temperature can be changed by
putting the whole THA01 in a climate chamber, performing measurements while the
instrument temperature changes across the required range. The thermopile is
attached with its cold joints to one of the heat sinks. Its hot joints are
incorporated in the thin heater. The samples are put in between the heat sinks
and the heater. Errors due to contact resistance are minimized by using glycerol
fluid. In a simplified description, the measurement consists of a heating cycle
and a measurement to establish ∆T.
Figure
2 Analysis of several samples of filled plastics using the THASYS. The
graphs represent the signals when putting the heater on. The signal
amplitude is inversely proportional to the thermal conductivity
λ.
MCU01 DESIGN
The MCU performs the functions of
measurement and control as well as data storage. It is PC operated.
Software in a Windows environment is part of the delivery. The parameters
cycle time, sample thickness and heater area are entered and the
experiment is started from the screen.
CALIBRATION
Verification of the stability can
be done by repeated (yearly) testing of Pyrex 7740 samples that are
included in the delivery. The calibration is traceable to NPL. THASYS is
suitable for use by ISO certified laboratories.
MORE INFORMATION / OPTIONS
Please consult the manual for a full
list of THA01 specifications. This manual is available free of charge as a
PDF file via e-mail. The ASTM standard C1114-98 can be obtained from ASTM as
a PDF file. A pressure cell is optional on THASYS. For use with high thermal
conductivity materials the model THISYS is available.
SUGGESTED USE
THA01 SPECIFICATIONS
-
Test
method: Test method ASTM C 1114-98
-
Sensitivity
traceability (∆T): NSI MC96.1-1982
-
Temperature
range: -30 to +120 °C
-
Accuracy
(λ) (examples) all @ 20 °C
@
H/λ> 15 10-3
m2K/W: +/- 3% (2 samples)
@
H/λ> 5 10-3
m2K/W: +/- 6% (2 samples)
@
H/λ> 5 10-3
m2K/W: +/- 3% (4 samples)
@
H/λ> 2.5 10-3 m2K/W:
+/- 3% (6 samples)
@
H/λ> 2.5 10-3 m2K/W:
+/- 3% (6 samples)
@
H/λ> 0.5 10-3 m2K/W:
+/- 14 % (6 samples)
-
Repeatability
(λ):
+/- 1% @ 20 °C
-
Total
measurement time: 3000 s
(typical)
-
Sample
surface A: preferred: 70x 110 mm,
always > 50 x 50 mm
-
Sample thickness:
H = 0.01
- 6 mm (see accuracy for stacking)
-
Traceability:
NPL National Physical Laboratory UK
-
Pressure
cell (optional): 200 N or
max 80.0 kN/m2
MCU01 SPECIFICATIONS
-
Differential
temperature readout: 0.5 µV @ 0 - 30
°C
-
Pt100
readout: +/- 0.2 °C @ 20 °C
-
Voltage
input/output: 220-110 VAC / 15 VDC
-
Further
functionality: Relay function, heater power
measurement,
temporary data storage
-
Communication:
RS232
|
