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THERMAL
CONDUCTIVITY MEASURMENT SYSTEM (THA-SYS-ATX)
THA-SYS-ATX
Thin Heater Apparatus for thermal conductivity measurement
THA-SYS-ATX offers measurement of the thermal
conductivity or total thermal resistance of plastics, composites and electrical
insulation / interface materials. THA-SYS-ATX works according to the ASTM 1114-98
standard; providing an accurate, fast, uncomplicated and absolute measurement.
It consists of a Thin Heater Apparatus (THA-01-ATX) and a Measurement and Control
Unit (MCU). Employing a specially designed high accuracy thermopile sensor,
THA-01-ATX 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. THA-SYS-ATX is fully PC controlled. For
use with high thermal conductivity thin foil materials a different model, type
THISYS, is available.
Figure
1. THA-SYS-ATX consists of the Thin Heater Apparatus (THA-01-ATX),
(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 THA-01-ATX 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.
THA-01-ATX DESIGN
The technological novelties are a thin
thermocouple thermopile (proprietary LICHTZEN AE / AITNIX 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. THA-SYS-ATX offers a
solution to this problem.
The THA-01-ATX 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 THA-01-ATX. If necessary the temperature can be changed by putting
the whole THA-01-ATX 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 THA-SYS-ATX. The graphs
represent the signals when putting the heater on. The signal amplitude is
inversely proportional to the thermal conductivity λ.
MCU-01-ATX 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. THA-SYS-ATX is suitable for use by ISO
certified laboratories.
MORE INFORMATION / OPTIONS
Please consult the manual for a full list of
THA-01-ATX 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 THA-SYS-ATX. For use with high thermal conductivity
materials the model THISYS is available.
SUGGESTED USE
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Plastics and composites thermal analysis
-
Electrical interface materials
THA-01-ATX SPECIFICATIONS
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Test method: Test method ASTM C
1114-98
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Sensitivity traceability (∆T): NSI
MC96.1-1982
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Temperature range: -30 to +120
°C
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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)
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Repeatability (λ): +/- 1% @ 20 °C
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Total measurement time: 3000 s (typical)
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Sample surface A:
preferred: 70x 110 mm, always > 50 x 50 mm
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Sample thickness: H = 0.01 - 6 mm
(see accuracy for stacking)
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Traceability: NPL National
Physical Laboratory UK
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Pressure cell (optional): 200 N
or max 80.0 kN/m2
MCU-01-ATX SPECIFICATIONS
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Differential temperature readout: 0.5 µV @ 0 - 30 °C
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Pt100 readout: +/- 0.2 °C
@ 20 °C
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Voltage input/output: 220-110 VAC / 15 VDC
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Further functionality: Relay function, heater power measurement,
temporary data storage
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Communication: RS232
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