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Transient Line Source Method

Transient Line Source Method

August 8, 2023

The Transient Line Source (TLS) method is a well-established, absolute technique for measuring the thermal conductivity of soil, rocks, concrete and polymers. Understanding the thermal properties of materials is vital for many different applications and enables one to achieve the best performance out of the materials used in all aspects of life.

This method of testing thermal properties has accelerated development in construction, environmental, renewable energy and even aerospace industries over the past 70 years. The main features that make this method superior to others are its high portability and ease of use.

Let’s look at the Transient Line Source (TLS) method in-depth. Including its interesting history, how it works, advantages/disadvantages concerning other methods and even real-world examples of its incredible versatility.

Understanding Thermal Conductivity

Thermal conductivity refers to a material’s ability to transfer heat. In other words, it is the rate at which heat can pass through a specific material. Knowing a material’s thermal conductivity is crucial for various industries, including aerospace, construction, manufacturing, technology, and medical fields. Thermal conductivity is essential for understanding how well a material will aid or restrict heat flow and how well a material will perform over its expected lifespan.

There are several methods for measuring thermal conductivity. Three standard methods include the Heat Flow Meter (HFM) method, Transient Plane Source (TPS) method, and Transient Line Source (TLS) method. So, what is the Transient Line Source method and how does it work?

The Transient Line Source Method

The Transient Line Source (TLS) method is a straightforward, portable method for testing the thermal conductivity of various materials, mainly soil, rocks, concrete, and polymers. The TLS method utilizes a long, thin needle or probe that acts as a heat source and a resistance thermometer. The needle is inserted fully into the sample, and the material is heated for a set time while temperature measurements are taken at constant intervals. During the cooling period, temperature readings are retaken at the same intervals. Calculate the thermal conductivity of the sample using the obtained data and the following equation:

k= q/4πa

Where k is the thermal conductivity in W/mK, q is the heating power of the needle, and a is the slope of the line for temperature rise over the log of time.

Schleiermacher first reported the earliest mention of the transient line theory in 1888 during his research on the thermal conductivity of gases. In 1931, Stalhane and Pyk developed the first cased probe for the transient hot wire method. However, it would not be until 1950 that Hooper and Lepper elaborated on the theory after recognizing the limitations of the guarded hot plate method. Not long after, Hooper and Chang developed the first official thermal probe in 1953.

Where did Transient line source Originate?

Schleiermacher first reported the earliest mention of the transient line theory in 1888 during his research on the thermal conductivity of gases. In 1931, Stalhane and Pyk developed the first cased probe for the transient hot wire method. However, it would not be until 1950 that Hooper and Lepper elaborated on the theory after recognizing the limitations of the guarded hot plate method. Not long after, Hooper and Chang developed the first official thermal probe in 1953.

How the Transient Line Source Method Works

To set up the measurement with the TLS method, you must create a hole in the sample’s center with the same diameter and length as the needle. You can use a poker tool to make the hole for soft samples like dirt. However, for more challenging samples like rock and concrete, you must use a tiny drill bit to form a hole for the sensor. Thermal paste should be applied to the needle probe when testing a drilled, non-uniform or solid material to minimize any effects of contact resistance.

Once inserted into the sample, allow ten minutes for the sample and sensor to become isothermal. The needle delivers heat into the sample at a constant rate during measurement and records the temperature over a defined period. Afterwards, use the slope of the plot to calculate thermal conductivity (k).

how transient line source method works

Figure 2. How the TLS method works.

Advantages and Applications of the Transient Line Source Method

Scientists predominantly measured the thermal properties of materials using the Guarded Hot Plate (GHP) method before developing the TLS method. This technique must be measured under laboratory conditions, often requiring long test times and large sample sizes; hence, why the concept of a highly portable transient probe that required little setup was highly desirable. The rugged design of the TLS probe allows for laboratory or field testing.

Compared to others, one of the main advantages of the TLS method is its portability. The rugged design allows this method to give accurate in situ and laboratory measurement results with a simple sample setup. Compared to other methods, the TLS technique is quick, easy to use and reliable. This method has proven to work even when testing porous materials containing moisture. Steady-state methods cannot account for the variability that moisture evaporation and condensation have on thermal conductivity, and therefore the transient line method gives the most accurate results.

The Transient Line Source method has applications across a variety of different industries. The most common being construction, electronics, renewable energy, and environmental technology industries. This method is highly preferred in the construction industry as it is less destructive and can account for moisture inside building and insulation materials. Due to its superior soil testing capabilities, this method has also become increasingly popular in environmental studies, including pipelines and underground powerlines

Comparison with Other Thermal Conductivity Measurement Methods

There are several other thermal conductivity measurement methods, including but not limited to Transient Plane Source (TPS), Transient Hot Wire (THW), and Heat Flow Meter (HFM) techniques.

The TPS method incorporates a disk-shaped sensor placed in contact with the surface of the sample material. A heat pulse is sent through the sensor, and the resulting rise is recorded. The thermal conductivity of the sample material is then calculated based on the rate at which the sensor’s temperature returns to its original state. This method works best for testing solids, pastes, and powders.

The THW method consists of a thin heating wire fully immersed in a small volume of the sample to be measured. A current is sent through the wire, heating not only the wire but the sample as well. The electrical resistance in the wire is measured concerning time, and from there, the thermal properties of the test sample can be determined. The THW method tests the thermal properties of liquids and phase-change materials.

The HFM is a steady-state method of determining the thermal conductivity of insulation and construction materials. This method places a sample between two plates of known temperature difference. The rate of heat flow through the sample is then measured, and thermal conductivity can be calculated based on sample dimensions.

Compared to other techniques, TLS can test many materials, including soil, rocks, concrete, polymers, moist and porous materials and even liquids. Another advantage of this method includes that it is highly portable, compact, and inexpensive compared to other techniques. Compared to the steady-state HFM method, TLS does not require in-lab testing or large sample sizes.

Case Studies & Real-World Examples

stone wall

Figure 3. Stone Wall.

The TLS method has broad applicability, as mentioned above. One of the many ways this method is used today is in determining the energy efficiency of old buildings. In 2018, during a study by P.F.G. Banfill, a TLS probe was used to calculate the thermal conductivity of stone walls via the transient line source method. From this information, the research group could then determine how much heat was escaping through the walls of old buildings.

asphalt road

Figure 4. Asphalt road.

In a 2018 study by Byzyka, Rahman, and Chamberlain, the TLS method was used to determine the thermal conductivity of asphalt slabs. Their results showed that thermal conductivity was significantly affected by the presence of air pockets in the slabs. This is important for contractors to know as the inadequate temperature at pothole junctions leads to the asphalt not bonding properly and settling inadequately, causing dangerous roadways.

NASA's Phoenix Mars Lander

Figure 5.  This image, taken by the Surface Stereo Imager on NASA’s Phoenix Mars Lander, shows the lander’s robotic arm scoop positioned over the Wet Chemistry Lab Cell 1 delivery funnel on 6 July 2008, or Sol 41, the 42nd Martian day after landing. Image credit: NASA/JPL-Caltech/University of Arizona/Texas A&M University

The TLS method is so effective that NASA has used it to measure the thermal properties on Mars. In May of 2008, NASA sent its Phoenix Lander to Mars to take measurements of air and soil with a thermal conductivity probe via the transient line method. Due to frigid conditions, the lander roamed the planet making thermal conductivity measurements via the TLS method for six months before ceasing operations.

Best Practices for Conducting Transient Line Source Measurements

To achieve accurate and reliable results, follow these guidelines:
1. Moisture evaporation/condensation may affect the repeatability of the results. The operator may use Saran wrap or something similar if good moisture control is desired.
2. If the sample has been heated or cooled in an oven or other environmental chamber, allow adequate time for the sample to return to ambient conditions before starting measurement.
3. Any space between the sensor and sample will cause contact resistance for non-uniform or solid materials, which introduces errors in the data. Contact resistance can be reduced by ensuring the sensor fits appropriately in the sample and applying thermal paste before insertion.


The Transient Line Source (TLS) Method effectively measures various samples’ absolute thermal conductivity and resistivity, including soil, concrete, and polymers. Compared to others, one of the significant advantages of this method is its fast test time, portability, and ability to test porous and/or moist materials. Knowing and understanding the thermal conductivity of materials enables individuals to achieve the best performance out of items used in everyday life. The TLS method is straightforward and reliable, with endless applications in construction, energy, and environmental industries.

Please visit the applications page for more information about the Transient Line Source (TLS) Method and its applicability.

Frequently Asked Questions

1. What is Transient Line Source?

– Transient Line Source (TLS) is a method of measuring thermal conductivity. It works by fully submerging a needle (probe) into a sample and heating it for a set period. During this period temperature readings are taken at regular constant intervals. Once complete temperature readings are taken at the same intervals during the cooling period. Thermal conductivity is then calculated from the obtained temperature data.

2. What is a Transient Measurement?

– Transient analysis is the evaluation of how a system responds to boundary conditions over time. It involves the determination of temperatures and other thermal quantities that vary with time. So, unlike steady state thermal analysis, time is significant with transient measurements.

3. What is the difference between static and transient analysis?

– Transient analysis involves measurement as a function of time whereas static analysis shows a measurement at a single point in time. Steady state methods require that the sample and reference pieces be at thermal equilibrium prior to measurements beginning. Transient methods do not require this rule to be fulfilled and therefore provide results more quickly.

Learn More About Thermtest TLS-100

Thermtest’s TLS-100 presents field researchers the opportunity to measure the thermal properties of organic materials, such as soils and concrete. This instrument relies on the Transient Line Source (TLS) technique to provide efficient and accurate thermal conductivity readings of soils, powders and solids, ranging in thermal conductivities of 0.1 to 5 W/mK.


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Brostr̲m, T., Nilsen, L., Carlsten, S., & Banfill, P. F. G. (2018). Hygrothermal properties of NHL mortars. In Conference report the 3rd International Conference on Energy Efficiency in historic buildings (pp. 71–79). essay, Uppsala University.

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Byzyka, J., Rahman, M., & Chamberlain, D. A. (2018). An innovative asphalt patch repair pre–heating method using dynamic heating. Construction and Building Materials, 188, 178–197. doi:10.1016/j.conbuildmat.2018.08.086

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