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Table of ContentsThermal
Conductivity Measurement of
Heat FluxThermal Conductivity and System
ConfigurationMethods for Conductivity
Measurement Axial Flow
Methods Absolute Axial Heat Flow
Method Comparative Cut Bar
Method Guarded/Unguarded Heat Flow Meter
Method Guarded Hot Plate
Method Hot Wire
Method The Probe Method
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Consider a sample of cross section A across which a thermal gradient exists.
T2 and T1 are the temperatures measured over a length ÄL.
Let Q be the quantity of heat flowing through A as shown in Fig. 1 below.
Now, thermal conductivity K is given by the ratio of the heat flux Q/A to the
thermal gradient ÄT / ÄL.
Measurement of Heat Flux
The heat flux can be measured directly or indirectly. These methods are:
- The absolute method where the electrical power supplied to the heater is
- The comparative method where a comparative measurement is made
The heat flux has to be uniaxial in all these methods and hence
radial heat loss or gain must be minimized by methods such as insulation.
Thermal Conductivity and System Configuration
The length of a sample is influenced by the magnitude of the thermal
conductivity. When the thermal conductivity of the sample is high, the amount of
heat flowing is high and the heat lost from the sample’s lateral surface is
small. As a high temperature gradient is established in this case, it is
possible to measure it accurately.
On the other hand, samples with low thermal conductivity (and correspondingly low
heat flux) are usually of a smaller thickness, which is sufficient to generate
an accurately measurable thermal gradient. Smaller thicknesses also mean less
lateral losses. Sometimes, self-guarding is provided for lateral surfaces by the
use of additional pieces of the sample material.
For low temperatures, the sample is packed inside insulation to minimize
heat losses or heat gains along the radial direction. Installation of a guard,
which can be controlled to have a temperature gradient same as that across the
sample, is often required at high temperatures.
At such high temperatures, heat losses are difficult to control. Therefore,
the ratio of conductance of the sample to the conductance of lateral insulation
becomes significant, as does the quality of guarding.
Methods for Conductivity Measurement
Here we discuss a few methods of measuring thermal conductivity of solid
materials at temperatures ranging from sub ambient temperatures to 1500°C. These include:
- Axial flow methods
- Absolute axial heat flow method
- Comparative cut bar method
- Guarded/Unguarded Heat Flow Meter Method
- Guarded Hot Plate Method
- Hot Wire Method
- The Probe Method
Axial Flow Methods
Axial flow methods are especially deployed at cryogenic temperatures and
produce consistent, accurate results. Radial heat losses are negligible at such
Absolute Axial Heat Flow Method
For sub ambient environments, the absolute axial heat flow method is
suitable. In this method, the electrical power applied to the heater has to be
Comparative Cut Bar Method
The comparative cut bar method, which is widely used for determining axial
thermal conductivity, is based on the principle of comparing thermal gradients.
In this method, heat flux is passed through samples of known and unknown
A sample of unknown material is sandwiched between two reference samples as
shown in Figure 2. If KR is the thermal conductivity of the reference
samples and KS that of the sample of unknown material, the heat flux
through the unknown sample can be calculated from the expression:
Guarded/Unguarded Heat Flow Meter Method
The guarded/unguarded heat flow meter method, which is deployed in many
testing instruments, is based on a flux gauge, which functions similar to the
reference samples used in the above method. The thermal conductivity of the
sample of unknown material is given by:
Thermal conductivities of building insulation materials are determined using this method.
Guarded Hot Plate Method
Another widely used method for measuring the conductivity of insulations is
the guarded hot plate method. The samples used in this method are larger.
Three different measurement apparatus are required for carrying out thermal conductivity measurements at room temperatures, low temperatures (up to -180°C),
and high temperatures (600°C or greater), respectively. The most common
symmetric configuration in which two specimens sandwich the heater assembly is
shown in Figure 3.
Hot Wire Method
Thermal conductivities of materials such as insulating bricks, powder or
fibrous materials are measured using this method. Thermal properties of liquids
and plastics can also be determined by this method, which requires isotropic
The Probe Method
In this modified method, the thermal conductivity of a sample is determined
by inserting a ‘hypodermic needle probe’ into the sample and measuring its
response. A thermocouple and a heater is attached to the probe. This method is
suitable for measurement of thermal conductivity in materials that are in semi
rigid form, such as soils.
Anter Thermal Analysis Equipment
Anter Corporation manufactures thermal properties analyzers to measure:
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