What is the guarded heat flow meter?

The guarded heat flow meter (GHFM) is a method derived from the standard heat flow meter, with a much larger range of operation, suitable for testing a wider variety of materials. Guarded heat flow meters can operate over temperatures from 100 °C to 300 °C, and can measure materials with conductivities from 0.1 to 30 W/mK. Similar to the heat flow meter, the guarded heat flow meter can measure the thermal conductivity of solids, though it is not solely restricted to insulators.

Measurements with the guarded heat flow meter

The guarded heat flow meter directly measures thermal resistance, from which thermal conductivity can easily be determined. The apparatus has a similar design to the heat flow meter, however test specimens are much smaller, and a guard of variable temperature surrounds what would be a regular heat flow meter. The guard is in contact with the temperature controlled plates, but not the test specimen. The guard is most often held at the mid-point of the two plate temperatures, and serves to minimize lateral heat loss.

For the guarded heat flow meter, the heat flux is measured in the same way as with the standard heat flow meter; by means of thermocouples on either end of the material. The temperature of each plate is controlled by heaters or Peltier elements, and the thickness of the specimen is determined by the instrument.

Mathematical considerations of the guarded heat flow meter

As with the heat flow meter, the mathematical theory behind the guarded heat flow meter comes from the one-dimensional version of Fourier’s Law.

$q = -K \frac{\partial T}{\partial x}$

Where q is the heat flux, K is the thermal conductivity, and T is the temperature. At equilibrium, the equation will be

$R_{s}=\frac{N(T_{h}-T_{c})}{Q}-R_{o}$

Where $$N$$ and $$R$$ are calibration constants, determined through the physical calibration of the apparatus, $$Q$$ is the heat flow, and T is temperature. Thermal conductivity is the inverse of the determined resistance, and the specific conductivity is the conductivity multiplied by the length of the specimen.

Instrument calibration

The guarded heat flow meter requires a single offset calibration of a reference material with a known thermal conductivity. The software calculates the calibration factor of the heat flux transducer, which can then be used to calculate the thermal conductivity when the corresponding calibration temperature has been determined. For large temperature ranges, multiple calibrations may be required. The guarded heat flow meter is also able to measure strips of materials stacked together, as well as liquids and pastes in specialized cells. Its ability to test a variety of samples over a wider range of thermal conductivities makes the guarded heat flow meter a more versatile instrument than the standard heat flow meter.

Similar to the standard heat flow meter, the guarded heat flow meter is a steady state method. During a steady state measurement, the system must reach a temperature equilibrium prior to performing measurements, which can sometimes result in wait times up to an hour, but yields accuracy levels of ±5%.

Internationally recognized standards

The guarded heat flow meter is covered by ASTM standard E1530, the standard test method for evaluating a material’s resistance to thermal transmission.