Frequently Asked Question

Hot Disc TPS

Are testing times long?
Although typical test times are less than 20 seconds, the required length of test time is determined by the thermal diffusivity and dimensions of the sample. The units of thermal diffusivity are mm2/s or area/time, therefore the lower the materials thermal diffusivity, the longer it takes for heat to diffuse into the sample and the opposite is true for materials of high thermal diffusivity.

Thermal Conductivity Tip 1: In the measurement of thermal conductivity and thermal diffusivity, it is universally recommended to penetrate into the sample as deep as possible for true thermal conductivity. This penetration depth is limited to the dimensions of the sample. For control of penetration depth, variable test times are required. The Hot Disc TPS has test times varying from 0.1 sec to 1280 sec, which can be used to test samples of various dimensions from 0.005 to 1800 W/mK and of various dimensions.

Interested in knowing how much test time is required to penetrate into your sample? Learn More

How are test times selected?
The selection of test times is performed to maximize penetration into the sample for measuring true thermal conductivity and thermal diffusivity. As the Hot Disc TPS measures thermal diffusivity, (mm2/s or area/time), the penetration depth (mm) of the test is calculated. Test times can be selected manually or automatically with the iTPS Plug-in.

Thermal Conductivity Tip 2: A wide range of test times is required to test the true thermal conductivity of materials of varying thermal diffusivities and thermal conductivities.

How is sensor size selected?
The iTPS plug-in automatically selects the most appropriate sensor size. That said, the standard Two-sided or Single-sided Hot Disc TPS 5501 sensor can be used to test any sample larger than 20 mm diameter, there is no maximum sample size. In addition, there are available TPS sensors to test samples as small as 2 mm in diameter. Larger TPS sensors are available for samples with poor surface contact, such as large particle loose powders. The larger TPS sensors act to increase sensor to sample contact. Additionally, larger sensors are important for testing heterogeneous materials, as the larger sensors increase the amount of material represented in the measured results, rendering the results more true.

Thermal Conductivity Tip 3: A standard sensor (TPS 5501) can be used for materials larger than 20 mm. The iTPS plug-in can be used to automatically select a sensor when samples are smaller than 20 mm or heterogeneous in nature.

Is the training process complicated?
Like any scientific instrument, there is some training required to in order to be able to operate the instrument properly. The iTPS automates all aspects of measuring thermal conductivity and thermal diffusivity. For more challenging samples, Thermtest offers excellent support, our training corner has received high praise for quick, no-cost, life-time support for basic training, as well as a review of your test results if required.
In addition, as the Hot Disc TPS is the most published method available, our large paper database is a great resource for 100’s of applications across the 1000 papers indexed.

Thermal Conductivity Tip 4: Basic and advanced training is available for no-cost. We are proud to provide high quality support to help our customers get the most out of their instrument.

Is testing of Anisotropic material accurate?
The Hot Disc is able to test bulk (average) as well as directional thermal conductivity and thermal diffusivity for a wide range of materials. With the anisotropic testing module, through thickness and in-plane directional properties can be measured. In addition to the directional testing with anisotropic testing module, isolated directional testing can be accomplished with the one-dimensional and slab testing modules. Accuracy of this capability is well documented in the Hot Disc paper database.

Thermal Conductivity Tip 5: Accurate testing of isotropic and anisotropic samples is possible with the Hot Disc TPS. Here is one example of a research paper from the paper database displaying an excellent correlation between the through thickness (axial) thermal conductivity measurement from a guarded heat flow meter, and the through-thickness (axial) thermal conductivity measurement from the Hot Disc TPS anisotropic testing module. Additionally, the Hot Disc gives results for in-plane (radial) direction.

Is it difficult to test liquids?
When testing the thermal conductivity of any liquid, controlling of convection is paramount. The iTPS plug-in recommends use of short test times and minimal sample volume to control convection.

Thermal Conductivity Tip 6: Accurate testing of liquids is well published.

Explore our Paper Database

Does the Hot Disc TPS require a contact agent?
No, the Hot Disc does not require a contact agent, as the method measures absolute thermal conductivity and thermal diffusivity.  The contact resistance between the sensor and sample can be manually identified and easily removed, or automatically removed with the help of the iTPS plug-in.

Thermal Conductivity Tip 7: No contact agent is required, as the non-linear portion of the raw data, which represents contact resistance is removed post measurement.  This can be accomplished with the iTPS plug-in, or manually performed.

What is difference between two-sided and single-sided Hot Disc TPS sensors?
The two-sided Hot Disc TPS sensor is the most accurate, as 100% of the signal heat introduced to the sample penetrates, with no loses in any direction. The single-sided sensor has a permanently known insulative material affixed to the underside of the Hot Disc TPS sensor, which allows a single sample piece to be tested. The insulation affixed to the underside, has been selected to minimize heat loss in that direction, and force the heat in the direction of the unknown sample.

Thermal Conductivity Tip 8: The two-sided TPS sensor is most popular in the lab setting, while the spring loaded single-sided TPS sensor is best suited for convenient testing of large samples or when only one sample piece is available. Both sensors perform within the instrument’s stated accuracy.

Is the Hot Disc TPS single-sided sensor unique?
Yes, Dr. Silas Gustavsson (Hot Disc Founder) was the first to ever test and publish single-sided thermal conductivity and thermal diffusivity measurements, of solids and liquids more than a decade (1983) before anyone else.

Thermal Conductivity Tip 9: First single-sided thermal conductivity test ever made, published more than a decade before other companies.

Does the Hot Disc TPS measure thermophysical properties in the same way as the C-Therm TCi?
Simply stated no. The C-Therm TCi uses a secondary calibration of multiple materials, measured by an absolute method like the Hot Disc TPS or Laser Flash Apparatus (LFA), covering a range of thermal conductivities or thermal effusivities. The temperature rise vs time slopes from the measured unknown sample is then compared to the interpolated data from the calibration materials. This calibration is performed at a set test time (1 to 3 seconds), power, sensor size, temperature and contact agent type. Different temperature ranges require different contact agents. For range up to 110 W/mK, more than 15 known materials are used to calibrate the instrument. The Hot Disc TPS measures the absolute thermal conductivity and thermal diffusivity. This allows most accurate measurements, as test times and sensors sizes can be selected to maximize penetration into the sample. As the contact resistance between sensor and sample may be identified and removed after the measurement is made, no contact agent is required. The thermal conductivity and thermal diffusivity is then calculated using an iterative mathematical function from the linear portion of the temperature rise vs time grapgh after contact resistance has been removed. Dr. Yi He from Intel does an excellent job of describing in-depth how the method works.

Thermal Conductivity Tip 10: The two methods measure thermal conductivity in very different ways, the Hot Disc measures the absolute thermal conductivity and thermal diffusivity, while the C-Therm TCi measures a comparative value using several known materials measured by other absolute methods.

Heat Flow Meter

What type of materials can be measured for thermal conductivity by the Thermtest HFM 100?

The HFM 100 is specifically designed for testing thermal conductivity and thermal resistance of insulation and common construction materials.

What makes the HFM 100 unique in the market?

The HFM 100 was designed to combine affordability and highest accuracy possible. This was accomplished by focusing on the most common application range of the heat flow meter thermal conductivity method.

How does the HFM 100 work?

A sample is placed between a hot plate and cold plate, which are held at specific temperatures. Flux sensors which are integrated into each plate, are used to monitor flux until steady-state heat flux is achieved. The average heat flux is used to calculate thermal conductivity and thermal resistance according to Fourier’s Law. The basic design of the HFM 100 is based on the Guarded Hot Plate (GHP), which an absolute measurement of thermal conductivity (no calibration). The HFM 100 has a modified design with the use of two flux transducers and calibration to simplify and speed up the measurement.

How is the HFM 100 calibrated?

The HFM 100 uses Standard Reference Materials (SRM) from NIST and IRMM. For expanded operation, the HFM 100 may be user calibrated with Transfer Standards (TS). Thermtest has ability to measure Transfer Standards by absolute methods for use in calibration of the HFM 100.

What standards does the HFM 100 follow?

HFM 100 follows ASTM C518, ISO 8301, and EN 12667 standards.

When to use the HFM 100 to measure thermal conductivity?

HFM 100 was designed to measure through thickness thermal conductivity and thermal resistance of insulation materials which tend to be heterogeneous in nature. The larger sample size of the HFM 100 measures a true thermal conductivity, which is representative of the whole sample.

What is sample size range for the HFM 100?

The sample size range is 6 x 6 inch (150 x 150 mm) up to 12 x 12 inch (300 x 300 mm). Sample thickness up to 4 inch (100 mm) in height.

How is thickness of sample determined?

The thickness of sample is measured with the use of four optical encoders, which are located on each corner of the top sample plate. This ensures the highest accuracy of thickness measurement possible (0.1mm).

What if sample is not ideally flat?

Each optical encoder (4) controls each of the four stepper motors independently. For samples, which are not ideally flat, the independently controlled motors can adjust to slight thickness variations to ensure optimum sample contact.

What is temperature range of the HFM 100?

The maximum temperature range of the upper and lower plates is -20°C to 70°C. Depending on the application or testing standard used, a delta temperature between upper and lower plate is typically 10°C to 20°C. Example, for a mean temperature of 20°C, the user might set upper plate to 30°C and lower plate to 10°C, which follows ASTM C518 which states 20°C delta.