Overview
ISO 7884-2:1987 defines a laboratory method for determining the dynamic viscosity of glass using rotation viscometers. It is Part 2 of the ISO 7884 series on glass viscosity and viscometric fixed points and covers continuous viscosity measurements and flow-curve (shear-rate) tests. Viscosity is obtained from the measured torque and rotational frequency together with a calculated or calibrated flow field coefficient and an instrument constant. The standard describes two rotation viscometer geometries (Couette and Searle) and includes practical annexes with flow-field formulae (Annex A) and an example viscometer (Annex B).
Key topics and technical requirements
- Measurement principle: Viscosity determined from torque, rotational frequency and the flow-field coefficient or instrument constant; supports measurements at varying shear stresses to produce flow curves.
- Viscometer types:
- Couette type - revolving crucible, stationary plunger.
- Searle type - crucible at rest, revolving plunger.
- Calibration: Instruments must be calibrated with viscometric standard liquids or certified reference glasses (see ISO 7884-1) to determine instrument constant and correct for mechanical losses (friction, bearings, air drag).
- Apparatus requirements:
- Electrically heated tube furnace (normally up to 1 400 °C; special cases up to 1 600 °C).
- Thermocouples (type S or B), calibrated and low thermal inertia.
- Crucibles (ceramic or noble metal) with sample volumes typically 20–250 cm3.
- Noble-metal plungers (platinum/rhodium alloys), shaft and torque-measuring device (torsion wire/spring/balance).
- Devices for centring, concentricity checks (≤1/100 plunger diameter), and thermal stability.
- Measurement control: Temperature stability to ±2 °C and gradient ≤1 °C/cm; rotational frequency typically ≤8 s−1 (inertia effects considered above 1 s−1); torque and frequency ranges chosen to give reliable instrument response.
- Sample handling: Melted glass free of bubbles; careful loading and immersion to avoid contamination or foam; crucible protection recommended for fragile ceramics.
Applications and who uses it
- Quality control and process laboratories in the glass industry (melting, refining, forming).
- Research laboratories studying viscosity–temperature relationships and non‑Newtonian behavior of glass melts.
- Instrument manufacturers and calibration labs for rotation viscometers.
- Standardization bodies and testing services validating viscometric fixed points and certified reference glasses.
ISO 7884-2 is essential when accurate, reproducible rotational-viscometer measurements are required to characterize glass melt viscosity for production control, process modelling and material development.
Related standards
- ISO 7884-1:1987 - reference glasses and calibration examples for viscometry.
- Other parts of ISO 7884 series (fibre elongation viscometers, beam bending, sinking bar, softening/annealing points, dilatometric transformation temperatures).
Open preview (PDF)Glass — Viscosity and viscometric fixed points — Part 2: Determination of viscosity by rotation viscometers
