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The Thermtest TLS-100 is a Portable transient line source, thermal conductivity meter for soil thermal conductivity testing as well as rock, concrete, and polymers. The soil conductivity meter TLS-100 follows ASTM D5334, ASTM D5930, IEEE 442-1981 standards.


Ricardo Ibarra

Aurora Geofísica

“One word to describe Thermtest; SERVICE, the company is certainly customer oriented, staff will do anything within their reach to meet the client’s demands and questions. Another word to describe TLS-100; SIMPLICITY, just a few straight forward instructions and you are set to go, manufacturing quality shows, design shows, reliability shows; all in all an excellent instrument to start measuring right after unboxing. “

James Merriman

Mid Pac Engineering

“I was new to thermal resistivity testing. Michael at Thermtest walked me through the process from choosing the proper tester through helping me develop dry out curves. He was available by phone any time I needed him and corresponded multiple times via email to check on my status and make sure I didn’t need additional help. Now days in business service means everything and the customer service I received was top notch. Oh yeah and the TLS-100 worked flawlessly and exactly as advertised. Thanks for everything.”



“The acquired equipment fulfilled the expectations amply. The transaction was very friendly and with an excellent after-sales service for the purchase of spare parts. Widely recommended instrument and seller.”

Jeremy Oglesby

Speedie & Associates

“Thermtest has been wonderful to work with. The staff are incredibly personable, knowledgeable, and helpful. The TLS-100 came very well packaged for the price. The equipment does exactly what we need it to do, and it does it well. Whenever I had questions or encountered difficulties, even difficulties with particular sample types, or with formatting reports (not with the equipment), the people at Thermtest were there to help me along the way, responding to emails very promptly and helping me to understand how to perform and report thermal properties testing. After extensive research in thermal testing equipment, Thermtest demonstrated to me that they offer not only the best value, but also the best customer service. They are our recommended go to for thermal testing.”

Alejandro Ceferino

Geoiberica SpA

“I acquired a portable TLS-100 meter and I am really happy with this unit. In the past, I used another portable thermal resistivity meter, but my decision to work with Thermtest is because of their customer service. They always answer you in a short time and they can solve all the problems that you ask them for. I needed a new testing probe and my custom Thermtest agent did not rest until I had received a new probe in just a couple of days (from Canada to Chile). In summary, the best unit and the best service.”

Juliana Byzyka, PhD Researcher

Brunel University

“The TLS-100 and 50mm needle have allowed us to complete a series of thermal conductivity measurements of a range of materials quickly, reliably and precisely. It is very easy to use, it can be used by students from many engineering departments and give reliable measurements. I was impressed by the abilities of the TLS. Thermtest has highly supported us from the beginning of our communication to the time we started to use TLS and continues providing support whenever it is asked. They responded to all our inquiries with professionalism.”

Zachary Carr, P.E.

FK Engineering

“The TLS-100 Thermal Resistivity Meter was efficient, easy to operate, and got the job done!”

Jon Fredrik Arnesen – Lab Manager

Lindberg & Lund AS

“The TLS-100 from Thermtest is quick and easy to use, and ideal for our needs as we sometimes need to take these measurements in the field. It comes in an extremely rugged case and I do not need to worry about where I bring it with me. In addition working with Thermtest has been a pleasant experience from the start, speedy replies to all question and professional and courteous staff that seem to always give that little extra bit of service.”


Thermtest understands that Soil thermal conductivity is extremely important to environmentally sensitive applications in agricultural and constructional fields. As a general rule, the thermal conductivity of soil increases with increasing moisture content. In the construction industry, buried cables and pipelines pose as possible threats. When construction companies are building underground systems, they must pay close attention to the moisture levels and thermal conductive properties of the surrounding soil. If a low soil thermal conductivity is noted, cables and pipelines may experience overheating and may potentially combust. When dealing with buried hot-water pipes, a surrounding soil of low thermal conductivity is required, to minimize heat loss from the pipes to the ground. Therefore, accurate and proper thermal testing, and ultimately the proper selection of backfilling materials is key for the lasting use of underground systems.

 That is why we developed the TLS-100


At Thermtest we recognized there was a need for a soil testing equipment that was simple to use, versatile, reliable, and accurate. The TLS-100 portable thermal conductivity meter perfectly envelopes these attributes.


Perfect for testing the thermal conductivity of soil, polymers, and other wide selection of soft materials; as well as concrete, rock, stone, or other hard materials.


Testing with the TLS-100 can be performed with the push of a button. The collected data is automatically analyzed and results are displayed immediately.


The TLS-100 boasts a reproducibility typically better than 2% and accuracy typically better than 5% the TPS-100 is on par with bulky laboratory equipment that costs over 10x more.

The Soil Testing Equipment TLS-100 Conforms TO These INDUSTRY STANDARDS

ASTM D5334

This test method presents a procedure for determining the thermal conductivity (λ) of soil and soft rock using a transient heat method. This test method is applicable for both intact and reconstituted soil specimens and soft rock specimens. This test method is suitable only for homogeneous materials. This test method is applicable to dry or unsaturated materials over temperatures ranging from <0 to >100°C, depending on the suitability of the thermal needle probe construction to temperature extremes. However, care must be taken to prevent significant error from: (1) redistribution of water due to thermal gradients resulting from heating of the needle probe; (2) redistribution of water due to hydraulic gradients (gravity drainage for high degrees of saturation or surface evaporation); (3) phase change of water in specimens with temperatures <0°C or >100°C. These errors can be minimized by adding less total heat to the specimen through either minimizing power applied to the needle probe and/or minimizing the heating duration of the measurement.

ASTM D5930

This test method covers the determination of the thermal conductivity of plastics over a temperature range from –40 to 400°C. It is possible to measure the thermal conductivity of filled and unfilled thermoplastics, thermosets, and rubbers in the range from 0.08 to 2.0 W/m.K.

 IEEE Std 442-1981

A method for measurement of soil thermal resistivity that is based on the theory that the rate of temperature rise of a line heat source is dependent upon the thermal constants of the medium in which it is placed is given. This information will enable the user to properly install and load underground cables. The aim is to provide sufficient information to enable the user to select useful commercial test equipment, or to manufacture equipment that is not readily available on the market, and to make meaningful resistivity measurements with this equipment, in the field or on soil samples in the laboratory. Designs for both laboratory and field thermal needles are described.

How It Works

The Transient Line Source (TLS) follows ASTM D5334. The sensor needle consists of a thin heating wire and temperature sensor sealed in a 100 or 50 mm steel tube. The sensor is completely inserted into the sample to be tested. Heat is delivered to the sample using a constant current source (q) and the temperature rise is recorded over a defined period of time. The slope (a) from a plot of temperature rise versus the logarithm of time is used in the calculation of thermal conductivity (k). The higher the thermal conductivity of a sample, the lower the slope. For samples of low thermal conductivity, the higher the slope.

STANDARD 100 MM SENSOR  for soft materials

The TLS-100 comes with the standard 100 mm sensor for testing for the thermal conductivity of soil, soft materials, and easy to drill materials. The needle sensor is fully inserted into an isothermal sample and a measurement is made with the push of a button. In 180 seconds, results are displayed for thermal conductivity and thermal resistivity. 


OPTIONAL 50 MM SENSOR for Hard Materials 

The 50 mm sensor was designed for testing thermal conductivity of concrete, rock, and other hard samples. Drilling the required 4 mm diameter x 50 mm hole in rigid samples is easy with the included masonry drill bit. When testing hard samples, a contact grease is used to improve contact between the sensor and sample.

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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