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.
ISO 22007-2
Development of ISO Standard 22007-2
In 1995, Hot Disk AB introduced to the market unique instruments and sensors for simultaneously and directly testing the two Thermal Transport Properties named Thermal Conductivity and Thermal Diffusivity, based on the in-house developed Hot Disk® method. The Thermal Conductivity of a material is a measure of its ability to conduct heat, while the Thermal Diffusivity of a material is a measure of its interior rate of heat transfer. The simplicity, versatility, and accuracy of testing Thermal Conductivity and Thermal Diffusivity using our instruments and sensors quickly resulted in a broad international recognition of the Hot Disk® method. Just a few years after our launch of Hot Disk® instruments and Hot Disk® sensors, we were urged by the scientific and industrial community to file for an ISO standard.
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 our in-house developed Hot Disk® method (also referred to as the 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. In 2021, a second revision of the ISO 22007-2 Standard was initiated, and approved in 2022, reflecting the standard’s durability and applicability.
The initiative in 2001 to start the working group on determining Thermal Transport 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 thermal conducting insulators, over plastics and ceramics, to highly thermal conducting crystals and 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
| ID-number: | ISO 22007-2:2022 |
|---|---|
| Title: | Plastics — Determination of thermal conductivity and thermal diffusivity — Part 2: Transient plane heat source (hot disc) method |
| Abstract: | This document specifies a method for the determination of the thermal conductivity and thermal diffusivity, and hence the specific heat capacity per unit volume of plastics. The experimental arrangement can be designed to match different specimen sizes. Measurements can be made in gaseous and vacuum environments at a range of temperatures and pressures.
This method gives guidelines for testing homogeneous and isotropic materials, as well as anisotropic materials with a uniaxial structure. The homogeneity of the material extends throughout the specimen and no thermal barriers (except those next to the probe) are present within a range defined by the probing depth(s) (see 3.1). The method is suitable for materials having values of thermal conductivity, λ, in the approximate range 0,010 W∙m−1∙K−1 < λ < 500 W∙m−1∙K−1, values of thermal diffusivity, α, in the range 5 × 10−8 m2∙s−1 < α < 10−4 m2∙s−1, and for temperatures, T, in the approximate range 50 K < T < 1 000 K. NOTE 1 The specific heat capacity per unit volume, C, C = ρ ∙ cp, where ρ is the density and cp is the specific heat per unit mass and at constant pressure, can be obtained by dividing the thermal conductivity, λ, by the thermal diffusivity, α, i.e. C = λ/α, and is in the approximate range 0,005 MJ∙m−3∙K−1 < C < 5 MJ∙m−3∙K−1. It is also referred to as the volumetric heat capacity. NOTE 2 If the intention is to determine the thermal resistance or the apparent thermal conductivity in the through-thickness direction of an inhomogeneous product (for instance a fabricated panel) or an inhomogeneous slab of a material, reference is made to ISO 8301, ISO 8302 and ISO 472. The thermal-transport properties of liquids can also be determined, provided care is taken to minimize thermal convection. |
| Publication date: | 2022-06 |
| Edition: | 3 |
| Number of pages: | 20 |
| Technical Committee: | ISO/TC 61/SC 5 Physical-chemical properties |
| ICS: | 83.080.01 Plastics in general |
| Link for buying: | https://www.iso.org/standard/81836.html |
Our Scientific Papers Foundational to ISO 22007-2
Bulk properties:
Anisotropic materials:
Slab specimens:
Thin films:
Low thermally conducting specimens:
ISO 22007-7
Development of ISO Standard 22007-7
Together with Thermal Conductivity and Thermal Diffusivity, there is a third, less commonly known but important Thermal Transport Property called Thermal Effusivity. The Thermal Effusivity of a material is a measure of its ability to exchange thermal energy with its surroundings. It is used to characterize heat flow through a thin surface-like region. If the Thermal Conductivity and the Thermal Diffusivity of a thin material is known, the Thermal Effusivity can be computed via the ratio of the Thermal Conductivity and the square root of the Thermal Diffusivity. The Hot Disk® method (ISO 22007-2), which simultaneously tests Thermal Conductivity and Thermal Diffusivity, can thus be used to indirectly obtain the Thermal Effusivity. However, following ISO 22007-2 it is difficult to achieve high accuracy in Thermal Conductivity and Thermal Diffusivity when testing thin, low density and low thermal conducting, materials such as textiles.
To more accurately and easily test Thermal Transport Properties of such thin, low density and low thermal conducting materials, Hot Disk AB developed a new technique to directly test the Thermal Effusivity. This method relies on the established Hot Disk® method, while employing some different measurement parameters and calculation routines. This means that the same hardware (Hot Disk® instruments and Hot Disk® sensors) can be readily used also for direct Thermal Effusivity testing. The novel measurement parameters include the use of a very short measurement time together with a larger-diameter Hot Disk® sensor.
The demand for accurate Thermal Effusivity measurements was until recently limited, but in 2018 and 2019 requests from industrial producers of textiles, plastic sheet and films – lacking trusted alternatives – began to reach us. After some discussions in the ISO Working group (TC 61, SC 5, WG 8) around possible methods and suggestions, Hot Disk AB volunteered to write a draft, based on our abovementioned method for direct Thermal Effusivity testing. A working draft was then circulated within the working group in the autumn of 2020, eventually reaching CD ballot in July 2021. A few, mainly editorial, comments were received, and only a year later a modestly revised version was slotted for DIS ballot in late 2022. This new draft also passed without major comments, and already by early 2023 the final FDIS was launched. After the ensuing closing date, the Thermal Effusivity standard 22007-7 was accepted and published. This is probably the quickest standard project that has ever been vetted and recognized by the ISO organisation! From our first draft in late 2020, to final publishing in mid-2023 entails a sum process of just two and a half years. During this period we also endured the Covid pandemic, requiring several regular meetings to be cancelled. All communications were held via Zoom and e-mail exchange, which did not alleviate the process. Our Hot Disk staff of scientists were very determined and goal-oriented in seeing it through, and assisted valuably with expected responses as co-developers during the efficient process.
Detailed information on ISO 22007-7 from www.iso.org
| ID-number: | ISO 22007-7:2023 |
|---|---|
| Title: | Plastics — Determination of thermal conductivity and thermal diffusivity — Part 7: Transient measurement of thermal effusivity using a plane heat source |
| Abstract: | This document specifies a method for the determination of the thermal effusivity. This document is applicable to materials with thermal effusivity in the approximate range 40 W⋅s1/2⋅m−2⋅K−1 <bn < 40000 W⋅s1/2⋅m‑2⋅K‑1, and temperatures in the range of 50 K < T < 1 000 K. |
| Publication date: | 2023-04 |
| Edition: | 1 |
| Number of pages: | 16 |
| Technical Committee: | ISO/TC 61/SC 5 Physical-chemical properties |
| ICS: | 83.080.01 Plastics in general |
| Link for buying: | https://www.iso.org/standard/81222.html |