ASTM C177: Thermal Conductivity

ASTM C177 Thermal Conductivity Testing by Guarded Hot Plate

ASTM C177 is used to test the thermal conductivity of materials under steady-state conditions through the use of the guarded hot plate device. The metered heater pushes a certain amount of heat through the material, while the guard surrounding the heater prevents the escape of the heat sideways; hence, the heat that gets into the cold plate flows in one direction only. MatX Lab tests this process on ceramics, refractory, aerogel, and foam glass.

What Is ASTM C177?

ASTM C177 is applied for the measurement of steady-state heat flux and thermal transmission properties by the guarded-hot-plate apparatus. The sample is placed between the two plates: a heated plate and a cooled plate. The power to the metered section of the hot plate is raised until the temperatures on both faces stop drifting, and thermal conductivity follows directly: metered power multiplied by specimen thickness, divided by metered area and the temperature drop across the material.
With the two-sided approach, you have one specimen on top of the hot plate and another one at the bottom. This ensures equal distribution of heat from both sides. The one-sided technique utilizes only one specimen with an insulating layer, ideal for you if materials are limited or the product comes in one panel only.
C177 is an absolute method, so nothing is compared against a reference specimen and no calibration constant sits between the raw measurement and the answer. That property makes it the method other apparatus get checked against.

ASTM C177 Scope and Applications

The technique is applicable for flat, homogeneous, and low conductivity materials in slab form. In reality, guarded hot plate tests are used for materials with thermal conductivity lower than about 1 W/(m.K). As soon as the material becomes a good conductor of heat, edge effects and plate contact resistance become significant enough to skew the results.
Four uses account for most C177 work:

• Design data. Producing apparent thermal conductivity curves against mean temperature for thermal models and furnace calculations.
• Product verification. Confirming that a production lot of insulation board or blanket performs the way the datasheet says.
• Development work. Ranking aerogel formulations, fiber diameters, binder loadings, or density variants during R&D.
• Reference measurement. Establishing values on materials later used to check heat flow meter apparatus.

Insulation pipe and curvature follow ASTM C335. Test whole building assemblies in a hot box per ASTM C1363, and test thermal interface materials per ASTM D5470. The MatX Lab supports commercial aerospace programs through this approach and tests materials on a specimen basis.

Test Method for ASTM C177 Test Procedure

Testing with a guarded hot plate requires patience. Most of the time consumed during the test process involves achieving steady-state conditions, and anything else is the quickest way to come up with an indefensible number.

Limitations you should know before you approve the quotations.

The method demands flat specimens with parallel faces, and a warped board will read high because of the air gap it creates at the plate. Moisture moves under a thermal gradient, so damp specimens drift and never settle. Low-density fibrous and cellular products show a thickness effect, meaning the measured conductivity changes with specimen thickness, so the report always states the thickness tested. Anisotropic materials give you conductivity through the thickness direction only. If you need in-plane values, the specimen has to be cut and stacked differently, and that changes the scope of the job.

ASTM C177 Specimen Requirements and Test Conditions:

For the ASTM C177 specimen, it must cover the metered section and the full guard; otherwise, the one-dimensional assumption breaks.

If sample preparation is required, we will ask to cut and specify a size; otherwise, we will handle the sample preparation. Ceramic fiber board and rigid foam machine cleanly. Blanket products and loose-fill need a frame or a defined compression, and we agree on that detail before work starts.
In the scope of the work, mention the mean temperatures you want on the curve. Three-point cost is higher than one point.

ASTM C177 Test Results and Reporting

The test report has clear thermal conductivity in W/(m.K) for all the mean temperatures tested, with the thermal resistance in m2.K/W being given for the thickness used.
Each of the data points comes with the details behind them, i.e., temperatures of both the hot and cold plates, metered power, thickness of the specimen under load, density, and the moisture state of the test. Steady state conditions and equilibrium time are also included. If you have requested more than one point, then a plot of conductivity versus mean temperature is done.

We quote measurement uncertainty on a per-job basis, and well-behaved specimens on a maintained apparatus generally sit within a few percent. Apparatus performance is verified against certified reference materials such as NIST SRM 1450d fibrous glass board and SRM 1453 expanded polystyrene board.
The timeline varies as per the structure. One mean temperature on a 25 mm ceramic board can reach equilibrium inside a working day, while a five-point curve on thick low-density material occupies the apparatus for most of a week.

ASTM C177 FAQs