### What is a heat flow meter?

Heat flow meters (HFM) are instruments used to measure the thermal resistance and thermal conductivity of bulk samples, particularly insulators and other building materials with low thermal conductivities. As the name implies, the instrument determines thermal conductivity by measuring the heat flux through a sample. The apparatus is simple in design and operation, but test times are lengthier than those used in transient methods, due to the steady-state nature of the instrument, and required calibration. Steady state methods measure thermal conductivity in closed systems, where the samples must reach a constant temperature to achieve accurate results.

### Mathematical considerations of the heat flow meter

The heat flow meter (HFM) has one heating plate, one cooling plate, and two thermocouple heat flow sensors. The apparatus is arranged so that the heat flux of the sample from can be measured on the top or bottom of the sample. The theory behind the calculation is the one-dimensional heat equation, or 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. Once the sample reaches a steady state, the temperature gradient will be linear, so

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

Or

$K = -q \frac{L}{T_{h}{-}T_{c}}$

Where $$L$$ is the length of the sample. This derivation assumes that the heat flow through the sample is purely one dimensional, and that no heat is lost when travelling from one plate to the other. To maintain the plates at a constant temperature, modern methods also employ the Peltier effect, in addition to using the thermocouple heat flux sensor. Peltier elements use an electrical current to transfer heat from one plate to the other, which allows the user to easily change the heating and cooling direction.

### Instrument calibration

The heat flow meter requires a single calibration with a known reference sample to offset the effects of contact resistance; this should be performed periodically as routine maintenance. Because the instrument is calibrated prior to the measurement, the offset is taken into account early in the calculations. In truth, thermal resistance is the property measured during the calibration, which is then used to calculate thermal conductivity.

### Measurements with the heat flow meter

Heat flow meters have a limited range of temperatures, typically operating between -20 and +90 °C, and can measure thermal conductivities from 0.005 to 0.5 W/mK. Common building materials and standard in situ temperatures fall within the testing capabilities of the heat flow meter, so the instruments are continually in demand. Additionally, many devices have high thermal conductivity modules to expand their testing capabilities up to 2.5 W/mK. In these extended modules, thermocouples are placed above and below the test sample and are plugged into the heat flow meter during the measurement. Heat flow meters produce repeatable results, and have an accuracy of ∼±3% when measuring thermal conductivity.

### Internationally recognized standards

Globally, there are multiple heat flow meter manufacturers. Commercialized heat flow meters comply to various internationally recognized standards including the ASTM standard C518, which is the standard test method for steady-state thermal transmission properties, and C1784-14, for measuring the thermal storage properties of phase change materials. The ISO standard 8301 is the procedure used to test the thermal resistance and related properties of thermal insulation with steady-state methods, while the DIN EN standard 12667 is for measuring the thermal resistance of building materials with the heat flow meter and guarded hot plate.

Thermtest Inc. and Linseis each develop their own heat flow meter, referred to as the HFM-100 and the HFM Lambda, respectively. Kyoto Electronics is a Japanese company that manufactures two varieties of heat flow meters: the Multipoint Heat Flow Meter (HFM-215N), and the Portable Heat Flow Meter (HFM-201). FOX is one of the leading manufacturers of heat flow meters, with five different units available, while Taurus Instruments produces three variations of the heat flow meter.