ISO-Standardisation of the Hot Disk^® Method

What is a standard? In the words of the International Organization of Standards (ISO), the leading authority worldwide, a standard may be seen as “a formula that describes the best way of doing something. It could be about making a product, managing a process, delivering a service or supplying materials – standards cover a huge range of activities. Standards are the distilled wisdom of people with expertise in their subject matter and who know the needs of the organizations they represent – people such as manufacturers, sellers, buyers, customers, trade associations, users or regulators.”¹

In our scientific field, an ISO standard approved for a measuring method is a crucial indicator of quality and reliability, specifically of the traceability and stability of the measured thermal conductivity over time. When developing a material to be used in various refined products, employing a recognized international standard in the process is a safeguard against mistakes and mishaps. One example regards construction materials for buildings. Building engineers will typically seek correct data about the conductivity of the construction materials they plan to use. If relevant specifications are sought via measurements by an accepted standard, the engineers can be sure that the data accrued will be trustworthy. They can readily gauge and compare materials tested with the same ISO-approved method, toward making an informed choice.

What makes ISO standards at once so sought-after and popular worldwide, is the careful way they are produced. In our case, when we proposed the Hot Disk® (Transient Plane Source, TPS) method to ISO in 2001, eventually to become ISO Standard 22007-2, we entered a long testing and verification process. After extensive vetting, the first version of the standard was approved by ISO in late 2008. During the years leading up to this stage, intensive work was undertaken within a working group. This was compiled of delegates from several countries having an interest in thermal transport properties in general. But to be accepted as a New Working Item Proposal (this is the formal name initially given to a standard under development), the method in question must show a broad international acceptance already at the outset, among industrial actors and research bodies. Member countries must vote affirmatively that there is a need for the proposed standard. Then, some five countries must each nominate an expert to participate in the relevant working group.

The real work starts upon these formal measures. First a draft standard is produced, which is then discussed and sharpened in several steps and iterations. From the outset to eventual approval, several ballots are held. All member states – not only the designated experts – are encouraged to comment. Eventually, assuming it has cleared a mass of queries and tests, the standard is approved. It is then numbered and published for all interested parties, made available for a modest fee via the ISO website and its Standards Catalogue. After having been in circulation for five years, the standard enters an evaluation and revision stage, to accommodate clarifications and improvements. The first revision of the ISO 22007-2 Standard was initiated in 2013, and after some minor changes it was again approved by ballot in 2015. As of late 2021, a second revision of the ISO 22007-2 Standard is nearing completion, reflecting its durability and applicability.

The initiative in 2001 to start the working group on determining thermal properties of materials was taken by a number of senior ISO members who felt there was a need for proper standards in the industry. The working group considered four different techniques, which had each been in use and established for some time. In turn, these comprised the Guarded Hot Plate method, focusing mainly on insulating materials with very low thermal conductivity; the Laser Flash method, able rapidly to measure thermal diffusivity of relatively small samples up to high temperatures; the Temperature Wave method, applied for thin materials like foils and sheets to measure thermal diffusivity; and the Hot Disk® (TPS) method. The Hot Disk (TPS) method was proven unique in covering a wide range in thermal conductivity, from low-conducting insulators, over plastics and ceramics, to highly-conducting metals. It could also simultaneously measure thermal diffusivity and thermal conductivity. Finally, the Hot Disk (TPS) method remains absolute, meaning that it does not require repeated calibration in order to furnish accurate thermal property data. This inherent simplicity has made for a broad and enthusiastic reception of ISO Standard 22007-2, describing the Hot Disk (TPS) method.

Detailed Information on ISO 22007-2 from www.iso.org

Our Scientific Papers Foundational to ISO 22007-2

Bulk properties:

Transient plane source techniques for thermal conductivity and thermal diffusivity measurements of solid materials
Silas E. Gustafsson
Rev. Sci. Instrum., 62 (3), 797-804 (1991).
Link

Anisotropic materials:

On the Use of Transient Plane Source Sensors for Studying Materials with Direction Dependent Properties
Mattias Karl Gustavsson and Silas E. Gustafsson
Proceedings of the 26th International Thermal Conductivity Conference, Massachusetts, Cambridge, 6–8 August 2005, edited by R. Dinwiddie (Oak Ridge National Laboratory, Oak Ridge, Tennessee, 2004), p. 367–377.
Link

Slab specimens:

Thermal conductivity, thermal diffusivity, and specific heat of thin samples from transient measurements with Hot Disk sensors
Mattias Gustavsson, Ernest Karawacki and Silas E. Gustafsson
Rev. Sci. Instrum. 65 (12), 3856 (1994).
Link

Thin films:

On the Use of the Hot Disk Thermal Constants Analyser for Measuring the Thermal Conductivity of Thin Samples of Electrically Insulating Materials
J.S. Gustavsson, Mattias Karl Gustavsson and Silas E. Gustafsson
Thermal Conductivity 24, Eds. P.S. Gaal, D. E. Apostolescu (Lancaster, MA: Technomic), p. 116-122 (1997).
Link

Footnotes