Thermal Conductivity
Thermal conductivity ("k value") is a property of a material that determines how much heat is conducted through it for a given temperature difference. However, with a fibrous or porous insulation, which will include both solid material and gas, heat transfer is by a combination of conduction, convection and thermal radiation. "Thermal conductivity" is still normally the quoted property for such insulation products although strictly it is an "effective thermal conductivity".
Units: W/(m.K)
The reciprocal: Thermal resistivity
Measurement techniques: Guarded Hot-plate, Heat-Flow Meter or Hot-Wire
Thermal resistance ("R value") is a property of an object that determines the resistance to heat transfer through it a for a given temperature difference. As the "thermal resistance" refers to an object, as opposed to a particular material, it can be used to describe the insulating effect of products that are constructed of layers of different materials. The resistance of an object to heat flow will be greater if it is thick and made from low thermal conductivity materials.
Units: (m2.K)/W
The reciprocal: Thermal conductance
Measurement techniques: Guarded Hot-plate or Heat-Flow Meter
Thermal transmittance ("U value") is a property of a structure that determines how much heat is transferred from the air space on the hot side of a structure, to the air space on the cold side. Whereas "thermal conductivity" and "thermal resistance" refer to heat flow through homogeneous materials, "thermal transmittance" takes into account all three modes of heat transfer through structures that may have many materials and complex heat flows. It also accounts for the heat transfer between the surrounding air and the structure's outer surfaces. "Thermal Transmittance" would be used to describe heat transfer through building components such as windows, doors, walls and roofs.
Units: W/(m2.K)
Measurement technique: Hot Box
What are thermophysical properties?
Thermophysical properties can be simply defined as material properties that vary with temperature without altering the material's chemical identity. However, it has become customary to limit the scope of the term to properties having a bearing on the transfer and storage of heat.
A more comprehensive definition is that thermophysical properties are all material properties affecting the transfer and storage of heat, that vary with the state variables temperature, pressure and composition (in mixtures), and of other relevant variables, without altering the material's chemical identity. These properties will include thermal conductivity and diffusivity, heat capacity, thermal expansion and thermal radiative properties, as well as viscosity and mass and thermal diffusion coefficients, speed of sound, surface and interfacial tension in fluids.
2: What are the key references for thermophysical property data?
The key references for thermophysical properties data are as follows:
BOOKS
Thermophysical Properties of Matter data series, vols 1-13, eds Y S Touloukian and D P De Witt, IFI/Plenum, 1970.
Somewhat dated but still a key source of thermophysical property data; separate volumes cover thermal conductivity, thermal diffusivity, specific heat capacity, thermal radiative properties, viscosity, thermal expansivity and other properties of solids. Each volume has useful introductory pages on the theory and measurement of the property concerned.
Recommended Thermophysical Properties for Selected Commercial Alloys, K C Mills, Woodhead Publishing Ltd, Cambridge, England ISBN 1 85573 569 5 (2002)
A critical review of available data (heat capacity, density, viscosity, surface tension, thermal diffusivity and thermal conductivity) of selected elements and alloys, aimed at providing the solidification modeller with critically reviewed data. Contains recommended property values with an indication of uncertainties of measurement. Included are aluminium, cobalt, copper, iron, magnesium, nickel, silicon, titanium and zinc and some selected alloys.
WEBSITES
Advanced Materials & Processes Technology Information Analysis Center - access to the properties database is Windows-based and user-friendly. Easy to follow set-up and operating instructions are included in the user manual. The database is also available on CD-ROM. Also publish a newsletter on materials technology developments; has links to US companies and organisations.
THERSYST thermophysical properties database on solids combining data from measurements in the IKE laboratories and from research of the scientific literature.
MEBSP thermophysical property data for casting simulations (Microstructural Engineering By Solidification Processing - MEBSP) and other information and resources, including links to other databases relevant to castings simulation.
The ASHRAE Handbooks on heating, ventilation, air-conditioning and refrigeration (HVACR) are published in a series of four volumes, one of which is revised each year, ensuring that volumes are kept up to date. The volume on Fundamentals covers basic principles, data for HVACR design, refrigeration cycles, fluid flow, heat and mass transfer, farm crops, air contamination, odours, measurement and instruments, airflow around buildings, energy resources, fuels, refrigerants, desiccants, thermal and moisture control and vapour transmission, ventilation, infiltration, heating and cooling loads, fenestration, space air diffusion, duct design, and pipe sizing. Also includes controls, building envelopes, and physical properties of materials. Available via the website or as a book.
An application for determining the physical and transport properties of fluids and fluid mixtures.
3: How can I measure the thermal conductivity of my insulation material and what sort of accuracy can I expect?
Accurate measurement of thermal conductivity is not quite as straightforward as the simple steady-state theory would suggest. For an introduction to this subject refer to the NPL Beginner's Guide to Measuring Thermal Conductivity.
The most commonly used and reliable apparatus for measuring thermal conductivity of insulation and other poor thermal conductors are guarded hot-plates and heat-flow meters. These instruments create a steady-state temperature gradient across a specimen of the material, by sandwiching it between an isothermal heated-plate and cold-plate. Additional thermal guards are positioned so as to minimise any loss or gain of heat from the heated-plate or from the edges of the specimen. The amount of power required for the heated-plate to create a particular temperature gradient will be proportional to the thermal conductivity of the material.
When considering the measurement of insulation products, the low heat-fluxes involved mean that the specimen and plates need to be thermally isolated, to a high degree, from the ambient environment. There are also associated problems such as thickness effects due to heat transfer by radiation, and the effects of moisture content and material homogeneity. With higher thermal conductivity materials such as plastics and masonry products, there are other factors including the effect of contact resistance between the specimen and the temperature sensor, which need to be taken into account.
Each of the four measured parameters (heat flux, temperature drop, specimen thickness and area) are directly related to the calculation of thermal conductivity, which means that any uncertainty in these parameters is transferred directly to the overall measurement uncertainty for thermal conductivity. National laboratories, such as NPL, would expect to measure an insulation material at ambient temperatures and obtain agreement to ± 1%, accredited laboratories within ± 3% and other test laboratories to within ± 5%. For higher conductivity materials and higher temperatures the agreement would not be as good.
4: What are the standard documents that describe the measurement of thermal conductivity of construction materials, refractories and plastics?
TThe following list summarises the main standards documents relating to the measurement of thermal conductivity of construction materials, refractories and plastics. Standards may of course be changed or updated over time so for the latest information visit the BSI or ASTM web sites:
British Standards Institute (BSI)
ISO 8302:1991- Thermal Insulation. Determination of Steady-State Thermal Resistance and Related Properties. Guarded Hot Plate Apparatus.
ISO 8301:1991 - Thermal Insulation. Determination of Steady-State Thermal Resistance and Related Properties. Heat Flow Meter Method
BS EN 12664:2001* - Thermal Performance of Building Materials and Products. Determination of Thermal Resistance by means of Guarded Hot Plate and Heat Flow Meter Methods. Dry and Moist Products of Medium and Low Thermal Resistance.
BS EN 12667:2001* - Thermal Performance of Building Materials and Products. Determination of Thermal Resistance by means of Guarded Hot Plate and Heat Flow Meter Methods. Products of High and Medium Thermal Resistance.
BS EN 12939:2001* - Thermal Performance of Building Materials and Products. Determination of Thermal Resistance by means of Guarded Hot Plate and Heat Flow Meter Methods. Thick Products of High and Medium Thermal Resistance.
ISO 8894:1987 - Refractory Materials - Determination of Thermal Conductivity - Hot-Wire Method.
BS EN 993-14:1998 - Methods of Test for Dense Shaped Refractory Products. Determination of Thermal Conductivity by the Hot-Wire (Cross-Array) Method.
BS EN 993-15:1998 - Methods of Test for Dense Shaped Refractory Products. Determination of Thermal Conductivity by the Hot-Wire (Parallel) Method.
For further detail or to purchase these standard documents contact British Standards Institute at: https://www.bsi-global.com/.
Note: Users may also come across the following two old British standards. These have been withdrawn and replaced by the three standards above identified with an asterisk*.
BS 874-2.1:1986 - Methods for Determining Thermal Insulating Properties. Tests for Thermal Conductivity and Related Properties. Guarded Hot-Plate Method.
BS 874-2.2:1988 - Methods for Determining Thermal Insulating Properties. Tests for Thermal Conductivity and Related Properties. Unguarded Hot-Plate Method.
American Society for Testing and Materials (ASTM)
C177-97 - Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by means of the Guarded-Hot-Plate Apparatus.
C518-98 - Standard Test Method for Steady-State Thermal Transmission Properties by means of the Heat Flow Meter Apparatus.
E1530-99 - Standard Test Method for Evaluating the Resistance to Thermal Transmission of Materials by the Guarded Heat Flow Meter Technique.
E1225-99 - Standard Test Method for Thermal Conductivity of Solids By Means of the Guarded-Comparative-Longitudinal Heat Flow Technique.
D5930-01 - Standard Test Method for Thermal Conductivity of Plastics by means of a Transient Line-Source Technique.
C1113-99 - Standard Test Method for Thermal Conductivity of Refractories by Hot Wire (Platinum Resistance Thermometer Technique).
C201-93 - Standard Test Method for Thermal Conductivity of Refractories.
领英推荐
For further details or to purchase these standard documents, please contact ASTM International.
5: Which is the appropriate heat transfer property to quote for my construction product or structure?
Thermal conductivity ("k value") is a property of a material that determines how much heat is conducted through it for a given temperature difference. However, with a fibrous or porous insulation, which will include both solid material and gas, heat transfer is by a combination of conduction, convection and thermal radiation. "Thermal conductivity" is still normally the quoted property for such insulation products although strictly it is an "effective thermal conductivity".
Units: W/(m.K)
The reciprocal: Thermal resistivity
Measurement techniques: Guarded Hot-plate, Heat-Flow Meter or Hot-Wire
Thermal resistance ("R value") is a property of an object that determines the resistance to heat transfer through it a for a given temperature difference. As the "thermal resistance" refers to an object, as opposed to a particular material, it can be used to describe the insulating effect of products that are constructed of layers of different materials. The resistance of an object to heat flow will be greater if it is thick and made from low thermal conductivity materials.
Units: (m2.K)/W
The reciprocal: Thermal conductance
Measurement techniques: Guarded Hot-plate or Heat-Flow Meter
Thermal transmittance ("U value") is a property of a structure that determines how much heat is transferred from the air space on the hot side of a structure, to the air space on the cold side. Whereas "thermal conductivity" and "thermal resistance" refer to heat flow through homogeneous materials, "thermal transmittance" takes into account all three modes of heat transfer through structures that may have many materials and complex heat flows. It also accounts for the heat transfer between the surrounding air and the structure's outer surfaces. "Thermal Transmittance" would be used to describe heat transfer through building components such as windows, doors, walls and roofs.
Units: W/(m2.K)
Measurement technique: Hot Box
Bibliography
For datasheets on temperature measurement services and calibration artefacts visit: https://www.npl.co.uk/thermal/publns.html#datasheets
'Temperature' by T J Quinn, 2nd Edition, 1990, Academic Press '
The International Temperature Scale of 1990', HMSO 1991
'Theory and Practice of Radiation Thermometry', edited by D P DeWitt and G D Nutter, Wiley Interscience 1989
'Temperature Measurement' by L Michalski, K Eckersdorf and J McGhee, J Wiley and Sons, 2002
'Traceable temperatures' by J V Nicholas and D R White, John Wiley and Sons, 2001
'Fundamentals of heat and mass transfer' by F P Incropera & D P DeWitt, 3rd ed., 1990, John Wiley & Sons
'Thermal radiation heat transfer' by R Siegel & J R Howell, 3rd ed., 1992, Taylor & Francis
'Measurement of thermal radiative properties of materials' by J C Richmond, in 'Compendium of thermophysical property measurement methods', vol. 1, by K Maglic, A Cezairliyan, V Peletsky (eds.), 1984, Plenum Press, New York
'Thermal radiative properties of matter - the TPRC data series', vols. 7-9, by Y Touloukian & D DeWitt, 1970, IFI/Plenum, New York
How do I convert between units of dew point and relative humidity?
Dew point (or dew-point temperature) is the temperature at which dew, or condensation, forms, on cooling a gas. Where the condensate is ice, this is known as frost point.
Relative humidity is the ratio of the amount of water vapour, e, in the air to the amount of water vapour, es, that would be in the air if saturated at the same temperature and pressure, and can be expressed (1)
Unfortunately, there is no simple, direct formula for converting in either direction between dew point and relative humidity. Conversions between these two parameters must be carried out via the intermediate step of evaluating both the actual vapour pressure of water and the saturation vapour pressure at the prevailing temperature, i.e.
To convert from dew point or frost point to relative humidity:
To convert from relative humidity and ambient temperature to dew point or frost point:
Vapour pressure can be calculated using the Magnus formulae:
At a temperature t (in °C), the saturation vapour pressure ew(t), in pascals, over liquid water, is (2)
(ew(t), is in pascals (Pa): 100 Pa = 1 millibar (mbar))
For the range -45 °C to +60 °C, values given by this equation have an uncertainty of less than ±0.6 percent of value, at the 95% confidence level.
Over ice, ei(t) is ?(3)
For the range -65 °C to +0.01 °C, values given by this equation have an uncertainty of less than ±1.0 percent of value, at the 95% confidence level.
A more accurate but complex alternative formula for vapour pressure (in pascals) from dew point (in kelvin) is as follows for water (4)
and for ice (5)
(Formulae due to Sonntag, 1990, updated from formulae given by Wexler, 1976 and 1977.)
The uncertainties associated with these equations are:
at the 95% confidence level.
The accuracy of these calculations depends slightly on the pressure and temperature of the gas concerned. For air near room temperature and atmospheric pressure, the water vapour enhancement factor, affects the result by approximately 0.5 percent of value.
Further information and tables are given in the publication "A Guide to the measurement of Humidity" which is available from the NPL e-store.
2: Can I use salt solutions to generate values of relative humidity? ... How?
Yes - Saturated (or unsaturated) salt solutions, and certain other chemicals, can be used to generate an environment of a particular relative humidity in an enclosed space. The value of relative humidity obtained depends on the particular chemical salt, the concentration of the solution, and the temperature of use, among other things.
Saturated salt solutions, with solid salt present, have the special property that a stable concentration is maintained - and hence a constant relative humidity - even if water migrates to or from the solution. Ready-made capsules are commercially available for a variety of relative humidity "fixed points". Such capsules are designed for insertion of relative humidity probes, with a seal providing a small, enclosed atmosphere of defined relative humidity. Certain of these, if in the form of solution with solid salt present, are re-useable, and can be calibrated to assign a traceable value to the reference humidity they provide.
Other humidity calibration systems employing unsaturated salt solutions exist; for example, in the form of single-use ampoules of solution. These are typically used to soak a pad in a housing designed for exposing a sensor to the humidity generated. These ampoules, too, may be supplied with a traceable calibration, on the basis of tests of samples from batches of ampoules.
As for any other calibration, traceability through a laboratory with UKAS or equivalent accreditation provides the best assurance of a reliable measurement.
Salt solutions can also be used in various ways to provide nominal (uncalibrated) values of humidity. Used in conjunction with a calibrated hygrometer, such nominal humidity environments can also be useful for calibration.
Just as it is for relative humidity in general, temperature stability is critical for calibration with salt solutions, particularly since the properties of the salt solutions change with temperature. Stabilisation after a change in temperature may take an hour, or many hours, depending on the size of the change. For freshly mixed solutions, full stabilisation may take days or weeks.
It is always vital to ensure that no salt contacts the sensing parts of the hygrometer, as this would cause errors in readings, and would destroy some sensors.
Forms of "salt check" offered for sale as a "kit" to be mixed into solution by the user, is not usually suitable for providing any kind of traceable calibration or reliable check by itself, except when used in conjunction with a calibrated hygrometer. Salts used in dry form, in particular, may provide only very approximate values of humidity, and may degrade quickly. Unsaturated solutions also generate atmospheres of relative humidity, but since their concentration gradually changes with repeated use, so does the generated humidity.