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D2149-13 – Standard Test Method Technical Guide
📐 Scope and Significance
This test method, D2149-13 (Reapproved 2021), details the determination of the relative permittivity (dielectric constant) and dissipation factor of solid dielectrics over a frequency range of 50 Hz to 10 MHz and a temperature range of −80 °C to 500 °C. The standard is essential for evaluating the electrical properties of insulating materials under environmental and operational stress. As noted in the significance section, permittivity and dissipation factor are highly sensitive to changes in chemical composition, impurities, and homogeneity, making them useful for quality control and for determining the effect of environments such as moisture, heat, or radiation.
Two distinct procedures are provided to accommodate different specimen types and measurement requirements:
- Procedure A: Using Micrometer Electrode.
- Procedure B: Using Precision Capacitor.
⚙️ Test Methods and Circuit Requirements
The standard references Test Methods D150 for suitable measuring circuits. For measurements from 50 Hz to 100 kHz, a substitution method using a low-voltage capacitance bridge is recommended. For measurements at 1 MHz and above, a resonant-circuit susceptance variation method is prescribed. The Q of the circuit must be at least 200, except for very low loss materials for which a Q of 500 or higher is desirable to ensure adequate measurement resolution.
The test fixture requires a Hartshorn-Ward type specimen holder adapted to a temperature-controlled test enclosure. Terminal connections must be located away from the temperature zone to prevent measurement drift. Where applicable, the requirements for a Grade A enclosure as specified in Specification E197 should be used.
| 🟦 📏 Frequency Band | 🎯 Recommended Method | 📐 Circuit Criteria |
|---|---|---|
| 50 Hz to 100 kHz | Substitution Method (Low-Voltage Capacitance Bridge) | Standard bridge accuracy |
| 1 MHz to 10 MHz | Resonant-Circuit Susceptance Variation | Q ≥ 200 (≥ 500 for very low loss) |
✔️ Best Practice: When testing near the upper temperature limit of 500 °C, ensure that your Hartshorn-Ward fixture and connecting cables are properly rated to avoid thermal damage and ensure accurate capacitance readings.
📊 Key Measured Properties and Terminology
Accurate definitions from Terminology D1711 are critical for applying this standard correctly. The core electrical properties are defined as follows:
- Permittivity (Dielectric Constant): The ratio of the capacitance between two conductors when embedded in the material to the capacitance between the same configuration of conductors in a vacuum (or air).
- Dissipation Factor: The ratio of the resistive to capacitive currents in the dielectric.
- Loss Index: The product of the permittivity and dissipation factor, representing the total energy lost in the dielectric.
| ⚡ Parameter | 📐 Definition |
|---|---|
| Permittivity (ε) | Capacitance (material) / Capacitance (vacuum) |
| Dissipation Factor (D) | Resistive Current / Capacitive Current |
| Loss Index (ε”) | ε × D |
⚠️ Critical Requirement: The standard emphasizes that for high-temperature measurements, the test enclosure must be specifically adapted to the specimen holder. A Grade A enclosure is recommended to minimize thermal gradients and maintain the specimen at the target temperature uniformly.
❓ Frequently Asked Questions
🔍 What is the specific scope of ASTM D2149?
ASTM D2149 covers the measurement of relative permittivity (dielectric constant) and dissipation factor of solid dielectrics. It applies specifically to a frequency range of 50 Hz to 10 MHz and a temperature range of −80 °C to 500 °C.
💡 What is the difference between Procedure A and Procedure B?
Procedure A utilizes a Micrometer Electrode for direct contact measurement on the specimen. Procedure B utilizes a Precision Capacitor in a substitution arrangement. The selection depends on the geometry of the specimen and the specific requirements of the test fixture.
⚡ What circuit is recommended for measurements at 1 MHz and above?
For frequencies of 1 MHz and above, a resonant-circuit susceptance variation method is required. The circuit must have a minimum Q of 200, although a Q of 500 or higher is required for very low loss materials to achieve sufficient accuracy.
📌 How is the loss index calculated according to this standard?
The loss index is calculated as the mathematical product of the permittivity (dielectric constant) and the dissipation factor at a given frequency and temperature. This derived value quantifies the total energy dissipated as heat within the dielectric material.