Scope:

In ASTM C177, the Guarded-Hot-Plate apparatus is used to measure steady-state heat flow through materials with low thermal conductivity (insulators). In steady state heat transfer, the temperature at any particular point in the system remains constant after equilibrium is attained.

The Guarded-Hot-Plate method is applicable to a wide variety of specimens, from opaque solids to porous or transparent materials, and under a wide range of environmental conditions—from extreme temperature and pressures to various gases.

Test Procedure:

In ASTM C177, the insulator sample is heated from one side by an electrically heated plate. The heated plate is embedded in another metal plate, called Guard, which is separately heated to the same temperature as the plate. The other side of the sample is cooled by a cooled plate. This is called the one-plate setup. 

In the two-plate setup, the hot plate is sandwiched between two samples, with thickness, areas, and densities of samples as identical as possible. The hot plate is embedded in the guard just like in the one plate setup.

In ASTM C177, a fixed rate of heat flow is generated by the electric heater. Because the hot plate and the guard are always at the same temperature, no transfer of heat takes place between them. The heat source nor the sample dissipates heat to the surrounding area. The heat produced by the plate is only transferred through the sample. Thus, unidirectional heat flow is generated. After a steady-state is reached, the heating and cooling plates have stable temperatures. Then, the thermocouples measure the resulting temperature difference over the sample.

The thermal conductivity can be calculated by using the heat input, the thickness of the sample, the area of the heating plate and the temperature difference through the sample.

 Specimen size:

For ASTM C177, the sample should be as big as to cover the guard area. (When the sample is smaller than that, the gap between the guarded-hot-plate and the primary guard will be covered.) It should have a large ratio of area to thickness. A homogeneous specimen with smooth surface, for better plate-to-specimen contact, is required. 

Data:

Following calculations are done for ASTM C177 testing – 

Heat flow:

Q = E × 1

Metered section:

A = A_m +A_g/2

A = A_m +A_gλ_g/2λ

Heat Flux:

q = Q/A

Density:

ρ_m = m/A × L

Conclusion: 

ASTM C177 helps to determine the rate of heat flow through a flat insulator by the Guarded-Hot-Plate apparatus.