Overview
ISO 7991:1987 specifies a standardized laboratory method for determining the coefficient of mean linear thermal expansion of glass in its elastic, solid-like state (well below the transformation temperature). The nominal reference temperature is 20 °C, so expansion results are reported as a(20 °C; t). The method describes specimen preparation, dilatometer apparatus, furnace and temperature-measurement requirements, correction procedures and calculation formulae. An annex illustrates self-adjusting alignment devices for specimen and push‑rod axes.
Key topics and technical requirements
- Scope: Applies to common bulk-production glasses (excludes fused silica, glass-ceramics or very low-expansion glasses in normal cases).
- Specimen: Rod-shaped glass specimens (e.g., 5 mm diameter or 5×5 mm square, 25–100 mm length common). Two specimens are tested for reproducibility.
- Apparatus
- Push-rod dilatometer (simple or differential) with resolution ~2×10‑5 L (example: 2 μm per 100 mm).
- Length-measuring device accuracy ~0.1 %.
- Contact force of extensometer ≤ 1 N with spherical or planar contacts.
- Furnace compatible with dilatometer; maintain specimen temperature uniformity and repeatability ±0.5 mm position. Temperature stability ±2 °C over specimen length.
- Temperature sensing: thermocouples (type E, J or K) capable of ±2 °C accuracy per IEC.
- Test procedures
- Preferred measurement range: start ~20 °C and preferred final nominal temperatures: 100, 200, 300, 400 °C (preferred actual ranges given, e.g., 290–310 °C for 300 °C nominal).
- Heating and annealing programs specified (heating rate up to (5 ± 1) °C/min; annealing cooling ~2 ± 0.2 °C/min).
- Corrections: dilatometer blank correction (AzB), specimen‑holding assembly expansion (Az0) and thermocouple/specimen temperature lag corrections are required.
- Calculations & reporting
- Final length and a(20 °C; t) calculated using formulas in the standard with specified significant-figure rules.
- Performance checks with certified reference materials (e.g., annealed vitreous silica, sapphire, chemically pure platinum).
Applications and users
- Who uses it: Materials scientists, glass manufacturers, quality control labs, research institutes and calibration labs performing thermal property characterization of commercial glasses.
- Practical uses:
- Design and reliability assessment where dimensional changes with temperature matter (optical components, seals, display substrates).
- Material selection for assemblies requiring matched thermal expansion.
- Quality control and batch verification of production glass.
Related standards
- ISO 7884-8 - viscosity and transformation temperature references for glass.
- IEC Publication 584 (thermocouple reference tables) and IEC 584-11 (thermocouple types).
- Calibration and certified reference material guidance (REMCO/ISO).
Keywords: ISO 7991:1987, glass thermal expansion, coefficient of mean linear thermal expansion, dilatometer, push-rod dilatometer, thermocouple, annealing, glass testing.
