Guarded Heat Flow (GHFM) Testing of Materials
Significance and Purpose
The Guarded Heat Flow or Guarded Heat Flow Meter (GHFM) Technique is a widely used steady-state method for measuring the thermal conductivity and thermal resistance of materials. Its significance lies in its high accuracy and reliability for evaluating the thermal insulation properties of a wide range of solid materials, including polymers, composites, and metals, and is particularly valuable for testing materials with moderate (e.g. ASTM E1530) to low (e.g. ASTM C518) thermal conductivity in applications where heat transfer is an important consideration.
Relevant ASTM and ISO Standards
To ensure uniformity, precision, and accuracy, the guarded heat flow technique is governed by internationally recognized standards:
- ASTM E1530: Standard Test Method for Evaluating the Resistance to Thermal Transmission of Materials by the Guarded Heat Flow Meter Technique.
- 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.
These standards provide detailed guidelines on sample preparation, equipment calibration, testing procedures, and data analysis.
DatapointLabs Tests for Guarded Heat Flow Testing
Tests in the DatapointLabs test catalog that reference guarded heat flow testing are as follows:
General Guarded Heat Flow Testing (inquire regarding material suitability)
| Test | Test Description | Standards |
|---|---|---|
| T-107 | Thermal Conductivity by Guarded Heat Flow Meter (Single Point) | ASTM E1530 |
| T-108 | Thermal Conductivity by Guarded Heat Flow Meter (Scan) | ASTM E1530 |
| T-110 | Thermal Conductivity of Films by Guarded Heat Flow Meter | ASTM E1530 |
| T-150 | Heat Transfer Coefficient | ASTM E1530 |
Principle of Operation
The guarded heat flow technique is based on steady-state heat transfer through a material test specimen under controlled conditions. The key principles include:
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Two Plates with Guarding:
- The sample is sandwiched between a hot plate (heat source) and a cold plate (heat sink).
- Guard rings with low thermal conductivity are used around the plates to prevent lateral heat loss, ensuring one-dimensional heat flow through the sample.
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Heat Flow Measurement:
- A heat flux sensor measures the heat flux (Q), or heat flow rate per area through the sample. This is combined with the measured temperature gradient across the specimen.
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Thermal Property Calculations:
- Thermal resistance (R) is calculated as: R=ΔT/Q, where ΔT is the temperature difference across the sample and Q is the heat flux.
- Thermal conductivity (k) is determined by incorporating the sample thickness (L), where k=L/R
The technique assumes steady-state heat flow and uniform material properties.
Typical Procedure
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Sample Preparation:
- Cut the test sample to the appropriate dimensions, ensuring it is flat, smooth, and has parallel surfaces for proper thermal contact with the plates.
- Thickness is measured accurately, as it directly influences calculations of thermal conductivity.
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Instrument Setup:
- Place the sample between the hot and cold plates.
- Apply thermally conductive grease and a small, controlled contact pressure to ensure good thermal contact.
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Temperature Gradient Establishment:
- Heat the hot plate to a specified temperature and maintain the cold plate at a lower, constant temperature to establish a steady-state temperature gradient across the sample.
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Data Acquisition:
- Monitor the temperature difference (ΔT) across the sample and the heat flux (Q) through the sample.
- Wait for the system to reach steady-state conditions before recording measurements.
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Calculation:
- Use the measured heat flux, temperature gradient, and sample thickness to calculate thermal resistance and thermal conductivity.
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Repeat Measurements:
- Repeat the test with additional samples or different temperatures to ensure repeatability and obtain temperature-dependent data.
Specimen Types
Specimens used by DatapointLabs in guarded heat flow testing are as follows:
| Specimen Type | DatapointLabs Test IDs |
|---|---|
| Discs (50mm Diam.) [Details] | T-107, T-108 |
| Thin Sheets or Film [Details] | T-110 |
| Plaques [Details] | T-150 |
Characterization Measurements
The guarded heat flow technique provides the following thermal properties:
- Thermal Resistance (R): The resistance to heat flow through the sample.
- Thermal Conductivity (k): The ability of the material to conduct heat.
Typical Data Reported (see test descriptions for exact details)
- Thermal Resistance (R): Reported as a single value or as a function of temperature if multiple tests are performed at varying temperatures.
- Thermal Conductivity (k): Reported as a single value or as a function of temperature if multiple tests are performed at varying temperatures. May include sample thickness for detailed heat transfer analysis.
- Temperature Gradient (ΔT): The temperature difference maintained across the sample during testing.
- Heat Flux (Q): The heat flow through the sample per unit area, recorded by the heat flux sensor.
Suitable Material Types
The guarded heat flow technique is versatile and can be applied to a wide range of solid materials, including:
- Polymers and Plastics: Such as thermoplastics, thermosets, and composites.
- Insulation Materials: Foams, lightweight panels, and thermal barriers.
- Metals and Alloys: For measuring moderate to high thermal conductivity.
- Ceramics and Glasses: Materials with low to medium thermal conductivity.
- Composites: Fiber-reinforced or layered materials.
Materials with flat, uniform surfaces and stable properties under steady-state conditions are best suited for this method.
Suitable Applications
The guarded heat flow technique is used in various industries for thermal characterization:
- Construction: Measuring heat resistance of building materials such as composites.
- Electronics: Characterizing materials for heat sinks, thermal interface materials (TIMs), and electronic packaging.
- Automotive and Aerospace: Analyzing heat shields, thermal barriers, and structural components for high-temperature applications.
- Energy: Assessing thermal performance of solar panel materials, battery components, and heat exchangers.
- Material Development: Developing and optimizing new materials with tailored thermal properties.
- Quality Control: Ensuring consistency in thermal performance during manufacturing.
Conclusion
The guarded heat flow or guarded heat flow meter (GHFM) technique is a reliable method for measuring the steady-state thermal conductivity and thermal resistance of materials. With its ability to test a wide range of solid materials, the method provides precise and repeatable data critical for material selection, quality control, and thermal management in various industries. By ensuring one-dimensional heat flow through the sample, this technique is particularly well-suited for applications requiring detailed and accurate thermal property measurements. By following ASTM E1530, ASTM C518, and ISO 8301 standards, the technique delivers reproducible and standardized data for a wide range of applications.
