ISO 7884-5:1987

Overview

ISO 7884-5:1987 - "Glass - Viscosity and viscometric fixed points - Part 5: Determination of working point by sinking bar viscometer" specifies a standardized method to determine the working point of glass using a sinking-bar viscometer. The working point is defined as the temperature at which a glass melt reaches an equilibrium viscosity of 10^4 dPa·s. The method is optimized for the low-viscosity range of glass processing (about 10^3 to 10^4.5 dPa·s) and relates the bar’s sinking rate in a melt to the melt’s dynamic viscosity.

Key topics and technical requirements

• Purpose: Rapid, reproducible determination of the glass working point for viscosity–temperature characterization.
• Principle: A narrow vertical metal rod (sinking bar) sinks under its own weight into the melt; the sinking rate is used to calculate dynamic viscosity from a calibrated relation.
• Specified geometry and materials:
  • Sinking bar: platinum–rhodium alloy, diameter 0.5 mm, mass 0.902 g (length ≈ 240–245 mm), hemispherical ends.
  • Depth of sink: nominal I = 20 mm for routine measurements.
  • Crucible: inner diameter ≥ 23 mm, depth 30–40 mm; platinum or non-porous refractory recommended.
  • Furnace: electrically heated tube furnace to ~1 200 °C, with ±3 °C uniform zone over the crucible.
• Temperature measurement: Type S (platinum–10% rhodium/platinum) thermocouples recommended; measurement thermocouple should dip ~1 mm into the melt.
• Calibration: Use a reference glass of similar composition at known viscosity (10^4 dPa·s) to determine the bar constant K; repeat after apparatus changes.
• Precision: repeatability ≈ 3 °C (same operator/apparatus); reproducibility ≈ 8 °C (different operators/apparatus).
• Practical procedure: hold bar above melt, detect contact (ammeter), release, time sinking to 20 mm (stopwatch 0.1 s), interpolate if necessary over ≤30 °C interval.

Applications and users

• Practical applications:
  • Determining the working point for process control in glass forming and shaping.
  • Rapid characterization of viscosity–temperature relations during glass R&D.
  • Quality control and batch comparisons in production environments.
  • Calibration and validation of viscometric equipment.
• Typical users:
  • Glass manufacturers, process engineers, laboratory technicians, materials scientists, and instrument manufacturers for viscometers and high-temperature furnaces.

Related standards

• ISO 7884 series (Parts 1–8) - comprehensive methods and principles for glass viscosity and viscometric fixed points (Part 1: principles; Part 2: rotation viscometers; Part 3: fibre elongation; Part 4: beam bending; Part 6: softening point; Part 7 & 8: annealing/strain and transformation temperatures).
• IEC Publication 584-1 - thermocouple type specifications (Type S) referenced for temperature measurement.

Keywords: ISO 7884-5, sinking bar viscometer, working point, glass viscosity, viscometric fixed points, dynamic viscosity, glass processing.