ISO 7884-3:1987

Overview

ISO 7884-3:1987 - "Glass - Viscosity and viscometric fixed points - Part 3: Determination of viscosity by fibre elongation viscometer" specifies a laboratory method to determine the dynamic viscosity of glass by measuring the elongation of a drawn glass fibre under a defined uniaxial stress. The procedure also supports checking the viscosity–temperature relationship and the effect of thermal history on viscosity. The standard defines the theoretical basis, required apparatus, measurement practice and necessary corrections.

Key topics and technical requirements

• Measurement principle: viscosity is derived from the timed elongation of a cylindrical glass fibre under a constant gravitational load, assuming Newtonian behaviour and negligible elastic response.
• Viscosity range: the method is intended primarily for high viscosities (the document addresses values up into the 10^8–10^13 dPa·s region), covering the softening and glass-transition domain important for viscometric fixed points.
• Apparatus and controls:
  • Electrically heated furnaces (working up to ~900 °C) with accurate time–temperature programming and stability (temperature constancy to ±1 °C; gradients along the specimen ≤0.2 °C/cm).
  • Thermocouples: low-inertia types (IEC type S or K) with calibration and regular checks; measurement junctions positioned close to the specimen.
  • Hanging/support geometry: fused‑silica supports or platinic rods to hold the upper ball and loading linkage; loading pieces (brass weights) as dead-weights.
  • Elongation measurement: markers, travelling microscope or transducers with minimum sensitivity ~0.02 mm; fibre diameter measured by micrometer (resolution ~0.005 mm).
• Sample preparation: flame-working or electric furnace melting; fibres with homogeneous, bubble‑free material and uniform cross-section.
• Corrections & checks: account for dead-weight distribution and surface-tension forces; use equations provided in the standard to calculate viscosity from measured elongation and time.

Applications and typical users

• Use cases:
  • Determining dynamic viscosity for research, process control and R&D on glass formulations.
  • Establishing viscosity–temperature curves and assessing the influence of thermal history (annealing, cooling rates).
  • Supporting characterization of viscometric fixed points, softening and transition behaviour.
• Typical users:
  • Glass technologists, materials scientists, laboratory metrologists, quality-control teams in glass production, and standards or calibration laboratories.

Related standards and references

• ISO 7884 series (other parts cover alternative viscometry methods and fixed points: beam bending, softening point, annealing/strain points, etc.).
• IEC thermocouple standards (IEC 584 series) for thermocouple types and tables referenced by ISO 7884-3.

Keywords: ISO 7884-3, glass viscosity, fibre elongation viscometer, viscometry, viscosity–temperature relationship, viscometric fixed points, glass testing, thermocouples, laboratory procedure.