Theory & Test Method
Thermal conductivity testing by the flat plate heat flow meter method is grounded in Fourier’s Law of heat conduction, which defines the relationship between heat flux, temperature gradient, and the thermal conductivity of a homogeneous material:
Q = λ × (T₁ − T₂) / d ⇒ λ = Q × d / (T₁ − T₂) [W/(m·K)]
In this relationship, Q is the steady-state heat flux density [W/m²] through the specimen, T₁ and T₂ are the hot and cold plate surface temperatures [K or °C], and d is the specimen thickness [m]. The QualiHFM™ FP 518 heat flow meter measures all three parameters simultaneously — Q through calibrated heat flux transducers in the plate assemblies, temperatures through precision sensors with 0.01°C resolution, and d through the integrated laser ranging sensor — enabling direct, automated computation of λ with ±3% accuracy and R-value over the range 0.02 to 4 m²·K/W.
The flat plate heat flow meter apparatus configuration maintains a steady, one-dimensional temperature gradient through the specimen by clamping it between the hot plate (upper) and cold plate (lower). The dynamic PID temperature control maintains the plates at their target temperatures until thermal equilibrium is confirmed — at which point the heat flux through the specimen is constant and measurable. The precision of the temperature control (±0.02°C for the hot plate) directly determines the uncertainty of the measured temperature differential and, therefore, the accuracy of the thermal conductivity result.
The QualiHFM™ FP 518’s integration of automatic laser thickness measurement and automatic pressure control addresses two critical accuracy-limiting factors in conventional heat flow meter apparatus: thickness uncertainty and variable thermal contact resistance. By measuring thickness in situ to 0.01 mm and applying a controlled, repeatable contact pressure (0–4 kPa), the instrument ensures that both parameters entering the λ calculation are known precisely and reproduced consistently across all test sessions, specimens, and operators.
Step-by-Step Measurement Process
1. Specimen Preparation — Prepare the specimen to 300 × 300 mm × 5–50 mm. Condition to equilibrium per the applicable test standard before loading.
2. Specimen Loading & Clamping — Place the specimen on the cold plate. Activate the one-click sample tank control to automatically close the chamber and bring the hot plate into contact.
3. Automatic Thickness Measurement — The laser ranging sensor automatically measures specimen thickness to 0.01 mm and records the value for the thermal conductivity calculation.
4. Pressure Application — Set the required contact pressure via touch screen or PC software. The automatic pressurization system applies and holds the specified pressure throughout the test.
5. Temperature Programming & Stabilization — Set hot and cold plate temperature targets. The dynamic PID system stabilizes both plates to set points with ±0.02°C hot plate accuracy before measurement commences.
6. Steady-State Measurement — When thermal equilibrium is confirmed — stable temperature gradient and constant heat flux — the system automatically acquires the measurement data and computes λ and R.
7. Automated Results & Reporting — The software automatically analyzes and records results, generates formatted test reports, and stores complete test records in the database.
