What is the method of the HFM?
The Heat Flow Meter (HFM) is employed for determining the thermal conductivity and thermal transmittance, commonly known as the U-value, especially for materials characterized by low thermal conductivity, such as insulating materials. In this method, the sample is placed between a heated plate and a cooled plate, resembling the configuration of a single-specimen guarded hot plate system. A temperature gradient is then applied to the sample, and the heat flow through the material is measured. Two sensors are strategically positioned between the sample and each plate to quantify the heat flow.
The schematic measurement setup is illustrated below:
The thermal transmittance, denoted as u (U-value), can be calculated from the measured heat flow (q) using the formula:
Here, \(A\) represents the area of the sample, and \(\Delta T\) is the applied temperature gradient. The thermal conductivity (\(\lambda\)) can subsequently be determined using the U-value and the thickness (\(d\)) of the sample:
It’s important to note that the HFM measurement is considered comparative. Calibration of the heat flow sensors with a material of known thermal conductivity is essential. Consequently, the Heat Flow Meter falls under the category of relative measurement methods.
Which properties are determined?
The Heat Flow Meter (HFM) is utilized for determining both the thermal transmittance (U-value) and thermal conductivity. In comparison to the Guarded Hot Plate (GHP) method, HFM measurements provide faster and more efficient results for thermal conductivity. However, it’s worth noting that GHP measurements generally offer higher accuracy, as HFM is a comparative method and entails certain uncertainties.
Which norms count for the heat flow method?
Key standards governing the use of the Heat Flow Meter include:
- ASTM C518 – Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus
- ISO 8301 – Thermal insulation; determination of steady-state thermal resistance and related properties; heat flow meter apparatus
- DIN EN 12667 EN 12 – Thermal performance of building materials and products – Determination of thermal resistance by means of guarded hot plate and heat flow meter methods – Products of high and medium thermal resistance; German version EN 12667:2001.
These standards provide essential guidelines for conducting precise measurements of thermal properties using the Heat Flow Meter, ensuring consistency and reliability in assessing thermal conductivity and thermal transmittance.