Thermal conductivity measurement
Thermal conductivity is a material property. It expresses how well a material conducts heat under steady state conditions. In dynamic situations also the heat capacity enters the equations.
In case of a finite object, for example an insulation panel, we may also consider the total thermal resistance of the object.
In many cases we may assume that materials are isotropic, so that the thermal conductivity of a material can be expressed as a single number. However, some materials are non-isotropic, so that thermal conductivity is a 3-dimensional property.
Thermal conductivity measurements are used for many proposes. The main ones are:
- establishing material properties
- quality assurance of insulation materials
What is measured
The measured quantity is thermal conductivity in W/(m·K). Most systems actually measure a thermal resistance and in some cases, when studying a non-homogeneous insulation panel, it is actually the average thermal conductivity over thickness that is determined.
In some cases you may also wish to know the thermal conductivity as a function of temperature or pressure.
The general principle of a thermal conductivity measurement is that you must:
- inject a known amount of heat
- measure a temperature difference in time or over a known distance
- while making sure that you take into account phase transitions, non-isotropy, pressure, absolute temperature, expansion, etc.
For thermal conductivity measurement there is no single measurement solution. The main issues are:
- different materials have very different thermal conductivities, from air to aluminium the difference is a factor 10 000
- specimen size is often non-ideal
- fluids and gasses may start flowing by thermal convection
- moist materials may locally dry out ad be no longer representative
- compressible materials may have properties depending on the pressure they experience
- the nature of the material may pose a problem; common issues are non-isotropy, thermally induced flow (for fluids and gas), evaporation of moisture
- contact resistance between specimen and temperature sensors may be a major source of error
There are several main technologies used:
- steady state techniques, typically relying on a constant heat flux and measuring a temperature difference after a transient interval.
- transient techniques, typically relying on a constant power and analysing the temperature difference between the start of heating and the temperature at later moments
Examples of Steady state techniques are guarded hot plates, heat flux meter apparatus, thin heater apparatus.
Examples of transient techniques are thermal needles and hot disks.
Selecting a sensor or measuring system
a typical checklist is:
- what is the material type, (metal, fluid, insulation material) and expected thermal conductivity range
- what are the available specimen dimensions
- what is the temperature range over which a measurement must be made
- what are the accuracy requirements
- what other things must be measured, such as density, heat capacity
- is any compliance to standards required
What we contribute
Hukseflux has been designing thermal conductivity measurement sensors and systems for over 20 years. In the course of the years, we have added many sensors to our product range. We also offer measuring services.
- sell our “standard” product range of sensors and systems for material characterisation, mainly aimed at plastics, composites and soils. See the thermal conductivity measurement selection guide
- perform “standard” measurements in our material characterisation laboratory on specimens supplied by customers. Common materials are plastics, paints, composites, pastes, powders, fluids, foodstuff and insulation materials. Please ask for our specimen requirements and fill in our request form
- design customer-specific experiments together with our customers to analyse material- or equipment properties and thermal contact resistances, as part of our engineering and consultancy services
- provide expert training in system use, in particular for thermal analyses of soils, thermal route surveys and for measurements of building envelope thermal resistance
- rent out systems, including support for analysing data.
The materials we most often analyse are:
- soils, sediments, concretes, cements - measured with thermal needles and systems of the TP series
- pastes, sludge’s, fluids - measured with needles and systems of the TP series
- plastics and composites - measured with THASYS and THISYS measuring systems, often in multiple directions
- thin foil materials, plastic foils, paints - measured with THASYS
- thin metal graphite, graphene foils - measured with THISYS
- fluids - measured with our thermal needles and special hot wires
- walls – measured with HFP01 and TRSYS
Our systems are optimised for the demands of different applications. We distinguish:
- specimen material type and specimen dimensions
- temperature range
- direction of measurement - in case of non-isotropic materials you may measure in 2 directions
- heating power - in case of use in soils and fluids
The measurement uncertainty of thermal conductivity measurements depends on many factors. For example the nature of the material, the specimen dimensions, the thermal resistance of contact between temperature sensors and specimen. We can assist you making a through uncertainty evaluation.
How to request a thermal conductivity measurement service
- look at our selection guide
- read our specimen requirements document
- fill in our request form
How to rent a thermal conductivity measuring system
- ask for our rental conditions
Hukseflux is a leader in thermal conductivity measurement. We offer:
- assistance: help you select the best sensor or system for your application
- proven performance: in most applications we have experience
- worldwide support – specialists available in the major economies
- compliance with standards: IEEE, ASTM, ...
- good price versus performance; also rental systems
- traceability: formal metrological traceability to international standards
Take a look at our selection of suitable sensors: