Choosing the Correct FLEX TPS Sensor Size

Written by Hitoshi Taniguchi, laboratory intern.

Testing a sample using C-Therm’s Trident with the FLEX TPS thermal conductivity sensor

C-Therm’s FLEX Transient Plane Source (TPS) method employs a double-sided hot disc sensor to simultaneously determine thermal conductivity, thermal diffusivity and specific heat capacity (calculated) of materials from a single measurement. This method conforms to ISO standard 22007-2 and it is recommended for more experienced users. FLEX TPS provides the user the greatest flexibility and control over experimental parameters and avoids the use of any contact agents. C-Therm offers three different TPS sensor sizes, and the best option should be chosen depending on the samples to be tested.

C-Therm’s FLEX TPS thermal conductivity sensor sizes
(diameter; from top to bottom): 6mm, 13mm, and 30mm

Although all sensors have the same principles of operation and test setup (the sensor has to be “sandwiched” between two identical samples and a good thermal contact has to be ensured), they differ in the range of sample sizes and thermal properties that they can measure. There are three major points to consider when choosing the sensor size for your measurement: the size of the sample, the thermal conductivity and thermal diffusivity of the material, and the homogeneity of the sample.

Size

Minimum sample size varies depending on the method (see Table 2).

For bulk and anisotropy methods, for example, the standard specification for the sensor size is such that the sample diameter should be no less than 2.5 times that of the sensor diameter, and the sample thickness should be no less than that of the sensor diameter. For instance, the minimum dimensions that a 6mm sensor can test is a sample with diameter of 15mm and a thickness of 6mm.

Appropriate sample size depending on sensor size/diameter (D) for bulk and anisotropic methods

Thermal Conductivity and Thermal Diffusivity

The next factor to check is the estimated thermal conductivity and thermal diffusivity of the material. The range of detectable thermal properties differs depending on the sensor size (see Table 1).

Table 1. Range of Thermal Properties Detectable by Each Sensor Size.

It might seem counter-intuitive to use the estimated thermal properties of a material to choose the right sensor size to actually measure those properties, but generally there is data available. For instance, if the sample is metallic, a 6mm sensor would not be appropriate, since metals usually have thermal conductivities of over 20 Wm^-1K^-1.

Homogeneity

The last point to consider is the homogeneity of the sample. Some samples, such as concrete, have non-homogenous structure within the sample; a certain filler material could have been added to it, it could contain grains inside, etc. For these samples, the use of 30mm sensor is recommended, since the larger sensor area allows the sensor to “average out” the non-homogenous structure within the sample. As a consequence, a larger sample size will be required due to increased sensor size, but this feature of 30mm allows for more accurate measurements of thermal properties for non-homogenous samples.

To conclude, there are three main factors to keep in mind when choosing the right TPS sensor size: the sample size, the thermal conductivity and diffusivity of the sample, and the homogeneity of the sample. Larger sensors allow for larger range of thermal property values to be measured; in particular, the 30 mm sensor is more suitable for non-homogenous materials.

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Additional resources on FLEX TPS:

[Webinar] Thermal Conductivity Characterization of Thin Films and Anisotropic Polymers

[Webinar] Thermally Conductive Polymers: Thermal Conductivity Applications in Isotropic, Anisotropic, Composites, and Thin Films Testing