The unit, answered first
Thermal conductivity is measured in watts per metre per kelvin, written W/(m·K) or W m-1 K-1. It is given the symbol k in most engineering texts and λ in physics and building-materials work, and both symbols mean exactly the same property.
The unit reads as a sentence. A material with a thermal conductivity of one watt per metre per kelvin passes one watt of heat through one square metre of area, across one metre of thickness, when the two faces are held one kelvin apart. It is a property of the substance, not of the particular slab or pipe in front of you. Change the thickness and the heat flow changes; the conductivity does not.
It falls straight out of Fourier’s law for one-dimensional steady conduction:
Q = k · A · ΔT / L, which rearranges to k = Q · L / (A · ΔT)
Q is heat flow in watts, A is the area normal to the flow in square metres, L is the thickness or conduction path length in metres, and ΔT is the temperature difference in kelvin. Substitute the units and you get W·m divided by m²·K, which reduces to W/(m·K). Every apparatus described on this page is a machine for measuring Q, L, A and ΔT accurately enough that k can be recovered from that single rearrangement.
Two things worth settling before the viva:
- Because ΔT is a difference and not an absolute temperature, W/(m·K) and W/(m·°C) are numerically identical. A one kelvin interval and a one degree celsius interval are the same size. Both are arithmetically correct, but kelvin is the SI form and is what belongs in a report or a purchase specification.
- Thermal conductivity is not thermal conductance, not U-value, not R-value and not thermal diffusivity. Students confuse these constantly, and so do some tender documents.
The neighbouring quantities, and how to tell them apart
| Quantity | Usual symbol | SI unit | What it describes |
|---|---|---|---|
| Thermal conductivity | k or λ | W/(m·K) | The material itself, independent of thickness |
| Thermal conductance | C | W/(m²·K) | A particular layer of a particular thickness |
| Overall heat transfer coefficient (U-value) | U | W/(m²·K) | A whole assembly including its surface films |
| Thermal resistance (R-value) | R | m²·K/W | The reciprocal of conductance, so layers can be added |
| Convective heat transfer coefficient | h | W/(m²·K) | The fluid film at a surface, not conduction inside a solid |
| Thermal diffusivity | α | m²/s | How fast a temperature front travels, not how much heat flows |
The older imperial unit still found in some data sheets is BTU per hour per foot per degree Fahrenheit. One W/(m·K) is approximately 0.578 BTU/(h·ft·°F). If a supplier quotes a figure in imperial units without saying so, that is your first hint to check the rest of the sheet carefully.
What a teaching apparatus is actually for
I have been manufacturing this class of equipment since 1993, and the single most useful thing I can tell a head of department is this: a heat transfer laboratory apparatus is a teaching instrument, not a materials testing instrument. Its job is to let a student set up a measurable temperature gradient, record it, apply Fourier’s law and arrive at a number of the right order with an error analysis attached. Its job is not to certify the conductivity of a material.
Departments that buy with that distinction clear in their heads write better specifications and get fewer arguments at inspection. Departments that expect a bench-top trainer to reproduce published property data to two decimal places are disappointed every time.
The four apparatus families, and what each one demonstrates
Guarded hot plate arrangement, for slabs and insulating materials
A flat specimen, or a matched pair of specimens, sits between a central heater and cooled plates. A separate guard heater ring surrounds the central heater and is held at the same temperature, so that heat from the metered central section is forced to travel one-dimensionally through the specimen instead of leaking sideways. Students measure the metered power, the specimen thickness, the metered area and the hot and cold face temperatures.
Where readings scatter: guard balance drift, edge losses at the specimen perimeter, contact resistance between plate and specimen if the material is stiff or uneven, moisture in fibrous insulation, and impatience. Steady state on an insulating slab is slow, and a student who reads at forty minutes when the rig needs ninety will report a conductivity that is too high.
Composite wall and lagged pipe arrangements
The composite wall stacks two or three different slabs in series and asks students to work out the interface temperatures and the equivalent resistance of the stack. The lagged pipe puts insulation around a heated tube and moves the geometry from plane to radial, so the logarithmic form of the conduction equation is needed rather than the linear one.
These two are the best value teaching rigs on the list because they teach series thermal resistance and geometry effects, which is what the syllabus is actually testing, rather than chasing an absolute property value.
Where readings scatter: interface contact resistance dominates the composite wall and is genuinely difficult to control; on the lagged pipe, end losses through the tube and its supports are the usual culprit, along with sensors that have drifted out of their intended radial position.
Metal rod or bar apparatus, for conduction along a bar
A metal bar is heated at one end and cooled at the other, with thermocouples set at intervals along its length. Students plot temperature against distance and take the gradient. With the bar insulated along its length the plot should be close to a straight line; with the lagging removed it curves, which is the point of the experiment because it shows lateral loss.
Where readings scatter: thermocouple seating in the drilled holes, which is the biggest single error source and is entirely a workmanship issue; lagging that has been compressed or lost over successive batches; and cooling water flow that is not steady.
Apparatus for liquids and gases
A thin annular or plane gap of fluid is held between a heated inner surface and a cooled outer one, with the gap kept small enough that convection is suppressed and conduction dominates. Students measure input power and the temperature difference across the gap.
Where readings scatter: this is the most delicate of the four. If the gap is too wide or the temperature difference too large, natural convection starts and the calculated value climbs. Filling without trapping bubbles matters. Concentricity of the annulus matters. If a supplier does not talk about gap tolerance and filling procedure, they have not thought about the physics.
Comparison table
| Apparatus type | Material class it suits | What the student measures | Typical sources of error |
|---|---|---|---|
| Guarded hot plate | Insulating boards, slabs, low-conductivity solids | Metered heater power, hot and cold face temperatures, thickness, metered area | Guard imbalance, edge loss, contact resistance, moisture, readings taken before steady state |
| Composite wall (slabs in series) | Layered solids of differing conductivity | Interface temperatures, total heat flow, individual layer resistances | Interface contact resistance, clamping force variation, sensor placement at the joint |
| Lagged pipe (radial conduction) | Pipe insulation, lagging materials | Radial temperature profile, heater input, inner and outer radii | Axial end losses, support conduction, sensor radial position, uneven packing of the lagging |
| Metal rod or bar | Metals and good conductors | Temperature against distance along the bar, heat input, cooling water rise | Thermocouple seating, damaged or missing lateral insulation, unsteady cooling flow |
| Liquid or gas gap apparatus | Fluids, at modest temperature difference | Heater power, temperature across a narrow gap, gap dimension | Onset of convection, trapped bubbles, non-concentric gap, evaporation at higher temperatures |
If you are putting a full heat transfer bench together rather than one rig, the sequencing question is covered in the mechanical engineering laboratory list, and the equivalent thermal experiments on the process side appear in the chemical engineering laboratory list.
What to write into the specification
This is the part that gets done badly. Most enquiries I receive describe the apparatus by name and nothing else, which means the buyer has handed the design decisions to whoever quotes lowest. The clauses below are the ones that change what arrives in the crate.
| Clause | What to write | Why it matters |
|---|---|---|
| Heater arrangement | Heater type, mounting, whether a separate guard heater is fitted, and how it is powered | A rig sold as guarded but with a single heater circuit cannot balance the guard, and the guarding is decorative |
| Heat input control and measurement | How power is varied and how it is read, with the display resolution stated | Q appears directly in the conductivity calculation, so an imprecise power reading caps the accuracy of the whole experiment |
| Temperature sensing | Sensor type, number of points, where each point sits, and how they are secured | Two sensors give a straight line whether the physics is linear or not. Extra points are what make the plot teach something |
| Insulation and guarding | Insulation material, thickness, and how it is retained during specimen changeover | Lagging that is removed for every experiment and stuffed back loosely is the commonest cause of drifting results over a semester |
| Specimen holding and changeover | Clamping method, repeatability of clamping, time to change a specimen, whether spare specimens are included | A rig that takes twenty minutes to reload cannot serve six batches in an afternoon |
| Instrumentation and logging | Manual selector and digital display, or logging with sampling interval and export format | Logged data lets students see the approach to steady state, which is half the lesson |
| Safety for student operation | Guarding of hot surfaces, earthing, overtemperature cutout, residual current protection, surface labelling | These rigs run hot and are operated by second-year students working in groups |
| Documentation | Manual, experiment sheets, wiring diagram, spare parts list, calibration certificates | Accreditation panels ask for records, not for the machine |
Write those as numbered clauses rather than prose. The method for turning them into a tender-ready document is set out in the specification writing guide, and the surrounding paperwork is listed in the tender documents checklist. Where a standard test method is referenced, check the current designation and revision yourself before you quote a number in a document; the IS and ASTM standards reference explains how to look one up and how a teaching rig relates to a formal test method.
The honest measurement point
A teaching apparatus demonstrates a principle and yields an indicative value. That is what it is designed to do and it does it well. What it does not do is produce a certified material property. If your department, or a project sponsor, needs a defensible conductivity figure for a material, that work belongs with an accredited testing laboratory operating under a recognised test method, with a scope that covers the material and temperature range in question.
To be exact about our own position: Scientico supplies factory calibration certificates with instrumentation. Scientico is not an ISO/IEC 17025 accredited calibration or testing laboratory and is not NABL accredited, and a factory certificate is not a substitute for either. Anyone who blurs that line in a quotation is telling you something about how the rest of their claims should be read. Our quality management system is ISO 9001:2015 certified and CE conformity documentation is available on applicable models; those are documents about how we manufacture, which is a different thing again. The certifications page states exactly what is held, and the calibration and maintenance schedule guide covers what to recalibrate, how often, and how to record it so an inspection panel can follow the trail. Departments preparing documentation should also read the laboratory documentation guide for accreditation.
Practical notes before you raise the purchase order
Buy sensors, not features. Between two rigs at a similar price, the one with more temperature measurement points and better sensor seating will teach more than the one with a larger display or a painted panel. Ask how the thermocouples are fixed into the bar or the plate, and ask for a photograph of that detail rather than of the finished trolley.
Plan the utilities before delivery, not after. Water-cooled rigs need a supply and a drain at the bench, and the drain is the item that gets forgotten. Electrical load, bench depth and clearance for the lagging are the other three. The laboratory setup timeline sets out the order these decisions have to be taken in, and the budget planning guide by intake helps size the quantity to your batch numbers instead of buying one of everything.
Ask about the consumable path. Specimens get damaged, thermocouples fail, heaters eventually burn out. Ask what those cost to replace and how long they take to arrive before you sign, not in year three. The spare parts and after-sales guide explains what a serious support arrangement looks like. If you are evaluating unfamiliar suppliers, the manufacturer due diligence guide gives the checks worth running, and importers buying from India should read the India sourcing guide alongside it. Manuals and reference material sit on the downloads page.
What to send us when you ask for a quotation
A useful enquiry for thermal conductivity equipment is short and specific. Tell us which of the four arrangements your syllabus requires, and whether you need one of each or several of one. Tell us how many student batches run in a week and how many students work on a rig at a time, because that decides how robust the specimen holding has to be and whether a second rig is cheaper than a queue. Tell us your bench dimensions, your electrical supply, and whether cooling water is plumbed to the bench or has to be recirculated.
Say whether you want manual instrumentation with a selector switch and digital display, or logging with export to a computer, and say it explicitly, because the two are priced differently and the difference matters more to the teaching than most of the rest of the specification. If your department is being inspected in the next academic year, say so, because the documentation set matters as much as the hardware and we would rather prepare it properly than send it late.
Scientico has manufactured and exported laboratory and engineering teaching equipment from its own works in Ambala, Haryana, India since 1993, and supplies customers in more than sixty countries under an ISO 9001:2015 certified quality management system. Supply is quote-based, so the specification you send is what your quotation is built against. Send the heat transfer section of your syllabus and the points above, and we will come back with a configuration and a written specification you can paste into your indent. Send us your requirement here.
One honest ask
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