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This method is similar to the guarded hot plate except for determination of the heat flow through the test specimen. This is achieved by means of one or two permanently installed heat flux sensors (meters) calibrated in the apparatus using one or more reference materials or certified transfer standards of known thermal resistance. In this way the specimen is self-guarding and the calibration is valid provided the thickness and thermal properties of the test specimens are similar.
A specimen of thermal insulation is sandwiched between the hot and cold plates constructed so that the total blackened surfaces can be maintained at different constant temperatures as monitored by permanently installed temperature sensors. In operation, a calibration specimen is sandwiched under load between the plates and allowed to attain equilibrium (usually within one hour depending on the thickness and density).
The plate temperatures and heat flux sensor output are measured and the sensitivity (s) determined (average of two separate values if two references are used)
where l and e refer to the specimen thickness and sensor output respectively. For an unknown specimen the procedure is repeated using the appropriate dimensional parameters for the specimen and the measured temperature difference and heat flux sensor output together with the calibration constant. |
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Typical single heat flux transducer heat flow
meter apparatus
Key requirements
| 1 |
Not suitable for materials that are hard and have a thermal conductivity ( λ ) in excess of 0.3 W/(m.K)
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| 2 |
Specimen flat and parallel within limits specified in Standard ISO 8301; the thickness and area size limits are the same as those for the guarded hot plate
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| 3 |
Temperature control of plate surfaces needs to be ± 0.1 K |
| 4 |
Heat flux transducer made from highly stable materials – its stability and reproducibility over the temperature range of continuous operation is the CRITICAL factor
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| 5 |
Calibration of apparatus must be undertaken under the same conditions as for the test material
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| 6 |
Regular apparatus calibration, in accordance with guidelines provided in the test standard, to verify stability |
References
| [1] |
J D Verschoor and Ada Wilber, (1954), Trans Am Soc Heat Vent Engrs, 60.
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| [2] |
C M Pelanne and C B Bradley, (1962), Mats Res Stand, 2, 549
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| [3] |
R P Tye, K G Coumou, A O Desjarlais, and D M Haines (1987), Thermal Insulation: Materials and Systems, ASTM STP 922, ASTM Philadelphia | |