The QualiHFM™ Heat Flow Meter thermal conductivity tester series, including the QualiHFM™ 2000 and QualiHFM™ 510A, evaluates thermal conductivity (λ) using a steady-state heat flux method. By establishing a fixed temperature difference (ΔT) across a specimen slab clamped between two temperature-regulated plates, the instrument calculates thermal transport properties based on Fourier’s Law of Heat Conduction, adhering strictly to ASTM C518, ISO 8301, and DIN EN 12667.
Core Measurement Principle
The test specimen, cut to a standard size of 300 mm × 300 mm with a thickness (d) between 10 mm and 60 mm, is loaded between top and bottom plates. Built-in thermoelectric modules regulate plate temperatures across a range of -5°C to 95°C with 0.01°C resolution, creating a one-dimensional temperature gradient.
Thermal energy travels from the warm plate through the material to the cool plate. Once heat transfer reaches steady-state equilibrium, calibrated flux transducers capture the heat flux density (q), and the instrument applies Fourier's equation:
λ = (q × d) / ΔT
Where:
- λ = Thermal Conductivity, measuring across 0.001 to 2.000 W/(m·K) with ±3% accuracy.
- q = Measured rate of heat energy passing per unit surface area.
- d = Real-time sample thickness (standard baseline 25 mm).
- ΔT = Temperature difference between hot and cold plates.
- R = Calculated thermal resistance (≥ 0.02 m²·K/W).
Testing Workflow and Instrument Architecture
The operational method on this Thermal Conductivity Tester combines automated plate positioning with software-guided temperature regulation:
- Specimen Placement: Position flat material samples (such as plastics, rubber, glass, fiberboard, styrene board, extruded board, foamed concrete, wood, or soft materials) onto the lower plate. Sample surface flatness should remain within 0.1 mm.
- Motorized Clamping: Automated plate drives apply a controlled clamping force (≤ 2.5 kPa) to ensure direct, uniform contact and squeeze out insulating air gaps.
- PID Temperature Control: An industrial OMRON PLC CPU unit with dedicated expansion modules runs software-based PID self-tuning to stabilize hot and cold plates at target setpoints.
- Equilibrium Detection: Solid-state contactless switching maintains noise-free isolation control while heat flux sensors monitor transfer rates. The system can evaluate three groups of six specimens to reduce system deviations.
- Automated Calculation: Software records thickness readings, plate temperatures, and transducer voltage signals via an RS-232 connection, displaying thermal conductivity (λ) and resistance (R) in real time.
Error Prevention and Measurement Controls
To ensure high repeatability (±1%), the QualiHFM™ Heat Flow Meter thermal conductivity tester addresses key testing variables directly:
| Testing Factor | Impact on Measurement | System Control Mechanism |
|---|
| Interfacial Contact Resistance | Surface gaps add unwanted thermal resistance, skewing conductivity values lower. | - Motorized plates deliver uniform clamping force (≤ 2.5 kPa)
- Flexible thermal interface pads flatten micro-surface roughness
|
| Side Thermal Losses | Heat escaping horizontally breaks one-dimensional heat transfer assumptions. | Active thermal guard zones surround the 300 × 300 mm testing surface to direct heat strictly top-to-bottom. |
| Temperature Fluctuation | Plate temperature drift delays steady-state conditions and alters sensor outputs. | PID self-tuning software coupled with OMRON PLC modules holds temperature stability down to 0.01°C. |
| Thickness Variations | Inaccurate thickness tracking causes linear calculation errors in Fourier's formula. | Integrated digital encoders track sample thickness throughout the 10 mm to 60 mm range in real time. |
Method Comparison: QualiHFM™ vs. Alternative Rigs
| Instrument Class | Testing Speed | Target Material Types | Operating Advantages |
|---|
| QualiHFM™ Heat Flow Meter | Fast (20 to 60 mins) | Plastics, rubber, glass, fiberboard, foamed concrete (0.001 - 2.000 W/m·K) | High sample throughput, ±1% repeatability, OMRON PLC control, fully compliant with ASTM C518 and ISO 8301. |
| Guarded Hot Plate (GHP) | Slow (4 to 8 hours) | Primary laboratory reference standards | Absolute accuracy, but high equipment costs and long test cycles. |
| Transient Flash Systems | Seconds | Metals, dense ceramics, liquids | Best for high conductors; less practical for low-density insulations. |
