Steady-State Thermal Transmission Properties ASTM C518

Steady-State Thermal Transmission Properties ASTM C518

ASTM C518 is used to determine Steady-State Thermal Transmission properties using a Heat Flow Meter Apparatus.


    ASTM C518 is a quick way to determine Steady-State Thermal Transmission properties that guide the design and prototyping of electronic products. 

    Steady-State Thermal Transmission properties of a material change with temperature change. It is important for designing electronics that dissipate fixed amounts of heat in a given time. This ensures that the device stays at the desired temperature and doesn’t overheat under operation. It is also important to model the worst-case power dissipation situations.  

    Laptops and phones now can store way more energy than they could. Electronics designers are therefore under more pressure than ever to design cost-effective products great at dissipating heat. ASTM C518 helps achieve that.

    Test Procedure:

    ASTM C518 involves an extensive calibration procedure. After proper calibrations, a sample of known thickness Δx is placed between a heated and a cooling plate. The temperature difference ΔT between the plates is fixed.

    A temperature gradient is applied to the sample and the resulting heat flowing through the sample is measured. A heat flow meter is used to measure the rate of heat flow q through the test sample. The data from these tests can be used to determine thermal transmittance (U-value) and thermal conductivity.

    Sample size:

    The sample should be between 150 mm and 300 mm square in size and have parallel surfaces. 


    From the measured heat flow q, the thermal transmittance u (U-value) can be calculated by

    U = q/(A × ∆ T )

    Where A denotes the area of the sample and ΔT the applied temperature gradient.

    With the U-value and the thickness d of the sample, the thermal conductivity K can be determined

    K = qD/A∆T


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