How this instrument works
Conductivity k is a property, not a design choice — it belongs to a substance the way density or refractive index does, and rearranging conduction arithmetic is how anyone ever put a figure on one. Two carriers do that work microscopically. Mobile electrons dominate in metals, which is why silver at 429 W/m·K and copper at 401 head most tables. Everywhere else energy hops as phonons, quantised lattice vibrations, and Peter Debye's kinetic estimate k ≈ ⅓·C·v·ℓ says a solid conducts well when it stores heat densely, vibrates fast, and lets those vibrations run far before scattering. Diamond wins on all three counts, reaching roughly 2000 W/m·K while insulating electrically better than most plastics.
Which apparatus produces that number depends on what is being tested. Fluids and pastes go to a transient hot wire, where a filament heats briefly and its own resistance reports how quickly surroundings carry warmth away, standardised as ISO 22007. Metals and ceramics go to laser flash, a method Parker and colleagues published in 1961: pulse one face, watch temperature climb on the other, and thermal diffusivity emerges, with k reconstructed afterwards as α·ρ·c. Coatings a few micrometres deep cannot be sliced or weighed at all, so they get David Cahill's 3ω technique instead. Building products take a steady route much closer to this sheet's own arithmetic — impose a known gap, meter the watts, divide.
That arithmetic assumes k is a single scalar constant, and every word there is negotiable. Conductivity is properly a tensor: pyrolytic graphite passes heat hundreds of times better along its sheets than across them, and timber roughly twice as well along grain. It drifts with temperature, since crystals above their Debye point shed conductivity near 1/T as vibrations begin scattering one another through Umklapp processes, which Rudolf Peierls identified in 1929. Density, porosity and water content all move it. And below roughly a micrometre of thickness k stops behaving as a bulk property at all — phonons cross without scattering, boundary resistance at each interface takes over, and a thin film underperforms its parent material by a wide margin.
- Enter metered power into Heat flow rate, in watts, kilowatts or BTU/h. Use what actually crossed your specimen, not what a heater drew from the wall.
- Give Material thickness as distance heat travelled between hot and cold faces; millimetres and centimetres both sit on that field's menu.
- Put whichever face heat crossed into Area, switching between square centimetres, square metres and square feet as your rig demands.
- Set Temperature difference (K or °C) from surface to surface. One kelvin and one Celsius degree are identical in size, so either scale reads the same here.
- Read Thermal conductivity (W/m·K) and weigh it against published figures for whatever you believe you measured. A mismatch usually indicts your rig, not a new material.
Worked example — naming a batt from 80 watts
A 10 m² test panel holds an unlabelled 100 mm batt. Guard its edges, hold 20 K between both faces, and a calibrated meter settles at 80 W. Heat flow rate = 80, Material thickness = 0.1, Area = 10, Temperature difference (K or °C) = 20. Numerator first: 80 × 0.1 = 8. Denominator: 10 × 20 = 200. Divide, and Thermal conductivity (W/m·K) reads 0.04.
That 0.04 W/m·K names your sample without anyone opening its wrapper — ordinary mineral or glass wool, sitting modestly above still air at 0.026 and well above silica aerogel near 0.013. Read a result like this as a verdict on your apparatus too. Land at 0.06 and something is bypassing that specimen: an edge leak, a crushed corner, damp fibre. Land at 0.004 and no material on Earth matches, so suspect your area or your gap. Genuine insulants cluster tightly between about 0.02 and 0.05, which makes k unusually sharp evidence of a rig behaving itself.
Questions
Why does diamond conduct heat better than copper?
Because heat travels two ways, and diamond exploits whichever one metals neglect. Copper leans on free electrons and reaches 401 W/m·K. Diamond has no free electrons whatever, yet its stiff, light carbon lattice ferries vibrations at roughly 12 km/s with very little scattering, giving around 2000 W/m·K in high-purity stones and more once isotopes are separated. Electrical and thermal conduction therefore come apart completely here: diamond is among nature's finest heat spreaders and simultaneously a fine electrical insulator, which is exactly why it appears as a substrate beneath high-power semiconductor devices.
Is thermal conductivity really one fixed number per material?
Rarely. It is a tensor, so anisotropic materials carry direction-dependent values — pyrolytic graphite moves heat hundreds of times better within its sheets than across them, and timber conducts roughly twice as well along grain as across it. Temperature, density, porosity and moisture content all shift it further. Published tables therefore quote a value at stated conditions, commonly a 10 °C or 25 °C mean, and a damp or compressed specimen will not match. Treat any lone figure as one measurement, in one direction, under one set of conditions.
How do I convert conductivity from imperial units?
Two imperial forms circulate and they differ by a factor of twelve, which catches people out constantly. One BTU per hour foot degree Fahrenheit equals 1.7307 W/(m·K). Its building-industry cousin, BTU·inch per hour square foot degree Fahrenheit, equals 0.14423 W/(m·K), so a foam quoted at 0.15 in those units lands near 0.022 in SI. Do not confuse either with an R-value: k describes a material however thick it happens to be, whereas R describes a particular depth of it and carries different units entirely.
Does this return true conductivity or an apparent value?
Apparent, unless your setup suppresses everything that is not conduction. Radiation crossing pores, convection looping inside a cavity, and contact resistance between specimen and plates all get charged straight to k, because this arithmetic attributes every metered watt to conduction through the sample. Standards accordingly speak of apparent or effective conductivity for porous products. Watch one asymmetric error especially: taking air temperatures instead of surface temperatures inflates your gap, which drags computed k downward and flatters whatever you are testing.
What separates conductivity from thermal diffusivity?
Conductivity says how much heat crosses once conditions have settled; diffusivity α = k/(ρc) says how fast a temperature change spreads. They diverge sharply. Copper reaches steady state several hundred times quicker than water, α ≈ 1.1 × 10⁻⁴ m²/s against 1.4 × 10⁻⁷, and two materials sharing a conductivity will still differ in diffusivity whenever their volumetric heat capacities differ. Transient instruments mostly measure α directly, so k must be rebuilt as α·ρ·c from separately measured density and specific heat — an extra step carrying uncertainty of its own.
Does conductivity change with temperature?
Yes, and not in one direction. Crystalline solids above their Debye point lose conductivity roughly as 1/T, because lattice vibrations increasingly scatter one another. Pure metals stay fairly flat near room temperature yet climb steeply on cooling, with copper at 20 K exceeding its room-temperature value many times over before impurity scattering caps that rise. Gases run the opposite way, conducting better when hot, and cellular insulants follow their trapped gas upward as they warm. That is why any honest datasheet names a mean temperature beside its k.