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Transient Line Source Method

Transient Line Source Method

May 3, 2024

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 crucial across various applications and enables one to achieve the best performance out of the materials used.

This method of testing thermal properties has accelerated development in construction, environmental, renewable energy and even the aerospace industries over the past 70 years. Providing accurate data is essential for:

  • Predicting heat transfer: Precise thermal conductivity data is paramount for designing energy-efficient buildings and optimizing cooling systems in electronics.
  • Material Selection and Development: Choosing suitable materials for specific applications often hinges on their thermal properties.
  • Quality Control and Process Monitoring: Reliable thermal conductivity measurements are required to ensure product consistency and adherence to thermal performance solutions.

The TLS method is a powerful and versatile technique for measuring TC across different materials. Let’s explore its principles, applications, and advantages.

Understanding Thermal Conductivity

Thermal conductivity (k) 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. Materials with high thermal conductivity, like metals, conduct heat efficiently, while those with low TC, like insulators, resist heat transfer.

Diagram depicting thermal conductivity

Figure 1. Diagram depicting thermal conductivity.

Precise TC measurements are vital for various industries, including aerospace, construction, manufacturing, technology and medical. TC 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, but the 3 standard methods are:

  1. Heat Flow Meter (HFM) Method
  2. Transient Plane Source (TPS) Method
  3. Transient Line Source (TLS) Method

Introduction to the Transient Line Source Method

History and Origin

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 was not 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.

What Is the TLS 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 needle’s heating power, and a is the slope of the line for temperature rise over the log of time.

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. For soft samples like dirt, you can use a poker tool to make the hole. 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).

A visual guide showing how the TLS method works.

Figure 2. A visual guide showing how the TLS method works.


TLS Quick Guide:

  1. Probe Insertion: A slender probe containing a heating element and a temperature sensor is inserted into the material under test.
  2. Heat Pulse Application: A controlled electrical current is passed through the heating element, generating a short heat pulse within the probe.
  3. Temperature Monitoring: The probe’s temperature sensor records the temperature rise as the heat diffuses into the surrounding material.
  4. Data Analysis: The collected temperature data is analyzed using specialized software to calculate the material’s thermal conductivity.

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 and has several advantages over other TC measurement techniques.

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 TLS method is the preferred method in the construction industry

Figure 3. The TLS method is the preferred method in the construction industry. The Transient Line Source method has applications across various industries, the most common of which are construction, renewable energy, and environmental technology. This method is highly preferred in the construction industry as it is less destructive and can account for moisture inside the 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

While the TLS method offers significant benefits, it’s essential to compare it with other standard techniques like the Transient Plane Source (TPS), Transient Hot Wire (THW), and Heat Flow Meter (HFM) techniques.

Transient Plane Source (TPS)

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.

Transient Hot Wire Method (THW)

The THW method consists of a thin heating wire fully immersed in a small sample volume to be measured. A current is sent through the wire, heating not only the wire but also the sample. The electrical resistance in the wire is measured over 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.

Heat Flow Meter (HFM)

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. This method is also highly portable, compact, and inexpensive. Unlike the steady-state HFM method, TLS does not require in-lab testing or large sample sizes. The TLS method often presents a more versatile and efficient alternative, especially for diverse material types.

Case Studies & Real-World Examples

The TLS method has demonstrably impacted various industries. Here are a few examples highlighting its diverse applications:

Real world applications of the TLS method include energy efficiency in buildings, road safety, and space exploration

Figure 4. Real world applications of the TLS method include energy efficiency in buildings, road safety, and space exploration.

  1. Energy Efficiency in 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.
  2. Road Safety: 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.
  3. Space Exploration: The TLS method is so effective that NASA has used it to measure the thermal properties of 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.
Real world applications of the TLS method include the food science industry and pharmaceutical manufacturing

Figure 5. Real world applications of the TLS method include the food science industry and pharmaceutical manufacturing.

  1. Food Science: Food manufacturers like Nestlé or Unilever use the TLS method to optimize processing temperatures and storage conditions for various food products, minimizing spoilage and ensuring food safety. Additionally, data assists in designing efficient food processing equipment that ensures consistent product quality and minimizes energy consumption.
  2. Pharmaceutical Manufacturing: Pharmaceutical companies like Pfizer or Merck utilize the TLS method to ensure consistent thermal conditions during critical stages of drug production, maintaining product quality and efficacy. It aids in ensuring the quality, efficacy, and safety of pharmaceutical products by preventing thermal degradation.
Real world applications of the TLS method include the electronics industry, geothermal energy exploration and the construction industry.

Figure 6. Real world applications of the TLS method include the electronics industry, geothermal energy exploration and the construction industry.

  1. Geothermal Energy Exploration: Geothermal energy companies use the TLS method to aid in mapping the subsurface TC profile, identifying potential geothermal reservoirs, optimizing placement and design for efficient heat extraction, and evaluating the feasibility and potential energy output of geothermal projects.
  2. Construction Industry: Material manufacturers and construction companies use the TLS method to measure the TC of insulation materials with incredible accuracy. They evaluate the effectiveness of different insulation types and optimize building insulation strategies for maximum energy savings and thermal comfort.
  3. Electronic Device Optimization: Electronics manufacturers employ the TLS method to characterize the TC of electronic components like semiconductors, heat sinks, and packaging materials. They use the information to optimize heat dissipation design in devices to prevent overheating and ensure reliable performance. Additionally, the data collected aids in the development of new materials with superior thermal properties for advanced electronics applications.

Best Practices for Conducting Transient Line Source Measurements

To achieve accurate and reliable results with the TLS method, follow these essential guidelines:

  1. Probe Selection: Choose a probe appropriate for the material type, temperature range, and desired measurement depth. Consider factors like material composition, expected temperature range, and the depth of the measurement point.
  2. Sample Preparation: It is essential to ensure that the sample is uniform and without any air pockets or other irregularities that may affect measurements. This may require pre-treating the sample by removing air pockets, filling voids, or ensuring a smooth surface for probe insertion.
  3. Moisture Control: Moisture evaporation or condensation can impact the repeatability of results. If precise moisture control is necessary, consider using Saran wrap or similar methods to minimize moisture fluctuations during the measurement.
  4. Temperature Equilibrium: If the sample has been pre-heated or cooled in an oven or environmental chamber, allow sufficient time to return to ambient temperature before measuring to ensure accurate thermal conductivity readings.
  5. Contact Resistance Minimization: Any air gap between the sensor and the sample can introduce contact resistance, particularly for non-uniform or solid materials. To minimize this effect, ensure a snug fit of the probe and apply thermal paste before insertion, especially for drilled or uneven samples.
  6. Data Acquisition: Follow the manufacturer’s instructions for proper probe placement, data collection, and analysis procedures, including following recommended data acquisition times and utilizing the appropriate software for accurate calculations.
  7. Calibration: Regularly calibrate the TLS equipment to maintain measurement accuracy and ensure your results’ reliability and consistency over time.

Adhering to these best practices can maximize the effectiveness and reliability of your transient line source measurements, leading to accurate and trustworthy thermal conductivity data.


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 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. As the demand for accurate thermal conductivity data continues to grow, the TLS method is poised to play a vital role in material selection, process optimization, and technological advancements across various fields.

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 evaluates how a system responds to boundary conditions over time. It involves determining 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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