Monitoring & metering
VISCOSITY MONITORING IN QUARTZ GLASS MELTING
Precise monitoring of viscosity during the melting process is crucial for quality assurance in quartz glass production. In this project, a Sensor Technology TorqSense torque transducer (SGR541) was used to enable real-time conclusions to be drawn about material homogeneity, purity and the progress of material bonding.
I
n modern glass technology, precise control of melting conditions is of fundamental importance: the viscosity of a melt in particular has a decisive influence on its further processing, quality and homogeneity. In the laboratory, viscosities are often determined using rotational viscometers: a spindle is rotated at a constant speed in the sample, the required torque is measured and the viscous resistance – i.e. the viscosity – is determined. In Sensor Technology’s customer’s project, this concerns the melting of quartz glass. Viscosity is not only a measure of fluidity but also allows conclusions to be drawn about the internal state of the melt – for example, cavities (air bubbles) or incomplete material bonding. Focusing on precision glass components to improve the manufacturing process for quartz glass in terms of quality and process reliability. The challenge was that the glass is first melted at high temperatures and then has to reach a defined viscosity zone for further processing. Only in this range can it be guaranteed that the melt is homogeneous (no air bubbles), the connection or fusion of different material batches has taken place completely, the transition to the shaping or quenching/pre-forming process starts reliably. The objective was to reliably monitor the viscosity of the quartz glass melt in order to detect deviations in the melting process at an early stage and to establish process-reliable feedback for control and quality assurance.
The viscosity of molten glass plays a key role in processing, as it determines the flowability and thus
40
the moulding behaviour, bubble removal, degassing and homogenisation. In the case of quartz glass, the conditions are particularly demanding with: high temperatures and viscosities having a strong influence on the viscosity curve.
The TorqSense SGR541 was chosen because of its great robustness against extreme process conditions (temperatures, thermal shocks, possible vibrations) in the melting range of quartz glass with separate sensor unit and electronics. Calibration was performed using reference samples or model melts in order to map the torque viscosity relationship in detail in the specific process setup.
The sensor was integrated into the process - typically between the motor/agitator (or melt mixing system) and the impeller (or melt stirrer) - and connected to the process control system in real time. Since torque transducers can be sensitive to lateral forces, double bearings should be used to avoid transverse forces. The relevant target values for viscosity were defined together with the customer: e.g., a range from x to y Pa·s at temperature T, at which the melt is considered homogeneous and no bubbles are mobile. Reference curves were created using laboratory or pilot melts with known compositions and viscosity values. This allowed the sensor output to be mapped to absolute or relative viscosity (see Basics: “Torque Viscosity” at constant speed conditions). The sensor continuously provided torque data from which the viscosity was derived. Trend analysis enabled early detection of deviations – for example,
if the viscosity did not fall within the target range, which could indicate trapped bubbles or incomplete mixing of the material batches.
As soon as the measured viscosity deviated from the target curve, measures were taken immediately: adjustment of the temperature, extension of the stirring time, addition of degassing (refining), or even return of the batch for reprocessing.
The introduction of monitoring using SGR541 had the following positive effects:
The melt could be continuously monitored for homogeneity and freedom from bubbles. Deviations were detected early and corrected.
The data obtained (torque/viscosity curves) provided valuable insights into the behaviour of the quartz glass melt, enabling future process optimizations.
This method is also used successfully in other applications - e.g., plaster mixtures, coal slurries, and liquids containing magnetic particles - where conventional viscometers fail and viscosity monitoring is required. In industries such as cosmetics and pharmaceuticals - e.g., in the manufacture of shampoos or medical solutions - it is also important to monitor viscosity during mixing. This allows you to determine when the process is complete and the desired consistency has been achieved.
Sensor Technology
www.sensors.co.uk April 2026 Instrumentation Monthly
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72
