ISO 7991:1987

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.