Glass Melt Redox Sensor

glass_sensor_feeder_rhinoThe glass melt redox sensor was specially developed for the in-line measurement of the oxygen activity of the glass melt in an industrial glass melting furnace. In recent years it is more and more realized that the oxygen activity (or redox) of the glass melt affects both efficiency of the industrial glass melting process and product quality. The redox state of the glass melt controls important process and product properties such as:

Using the redox sensor, the glass melting process will become more transparent to the furnace operator and will provide him valuable information for fast correction and finding the optimal production settings.

Simple & robust

Glass Melt Redox - glas system 340x247The measuring system comprises a jacket holder unit (1), a water-cooled jacket (2), a measuring lance (3), a replaceable redox sensor (4), and an oxygen sensor interface (5). This relatively simple and robust construction allows a large freedom in the choice of the measuring location. It is an ideal tool for in-line measurement in feeder, working end or fore-hearth.

The sensor is easily connected to the contact block on the measuring lance. The water-cooled jacket is positioned in the furnace so that the lower end is just above the glass melt level. The redox sensor extends partially out of the water-cooled jacket with its measuring tip immersed in the glass melt.

Measuring system concept

box of sensors 320x245

The ceramic redox sensor has a limited lifetime, as it is partially immersed in a hot and corrosive glass melt. For this reason a measuring concept of sensor exchange after a certain operating time has been chosen. Using a water-cooled jacket, the replaceable redox sensor can be kept small and of a standard dimension, irrespective of the measuring location. This makes the measuring system flexible and relatively less expensive. Sensor lifetime depends on glass melt type, flow velocity and temperature. Lifetime indication: approximately 6 weeks in a container glass melt at 1200 oC. The sensor can be ordered in sets of 5 sensors per box:

Low investment

Glass Melt Redox - feeder sensor 340x320The cost of the complete measuring system is low because of the simple construction with a replaceable sensor. Apart from continuous redox monitoring for process control, it is therefore also an ideal tool for process optimisation programs or research projects, e.g., during a colour conversion, batch component change or an increase of the recycling cullet share. Moreover, following the instructions in the manual, the system can be easily installed by the personnel of the plant. This may be ideal for confidential production processes or research projects.

Glass colours

Glass sensor redox colours 550x355

The colour of the glass product depends very often on the oxidation state of the melt. Commercial container glass colours range from very reduced amber to very oxidized UVA green.

In the present situation, the redox state of the glass is expressed as the iron ratio Fe2+/Fetot. The Fe2+ , Fe3+ or total Fe of the cold glass are analysed by means of a spectrofotometer or by wet chemical analysis. As the oxygen activity (or redox state) of the glass melt determines the valency state of the multivalent ions in the glass melt such as iron, a strong relation exists between the oxygen activity of the glass melt and the iron ratio in the (cold) glass. A more reduced glass contains more Fe2+ and a more oxidised glass contains more Fe3+.

Redox monitoring and control

Glass Melt Redox - sensor lab analysis 340x365There is strong correlation between the daily iron ratio analysis on the cold glass product and the log pO2 indicated by the redox sensor in the feeder channel. The big advantage of an in-line redox sensor compared to a daily optical redox check is the continuous availability of the the glass melt redox, enabling much faster control actions. The furnace operator feels more confident with in-line redox monitoring, especially during night time and during the week-end, when the lab facilities are not readily available.

Container glass furnaces melting high shares of recycling cullet usually have large redox variations due to more or less polluted cullet batches. By using the glass melt redox sensor, polluted cullet batches charged to the furnace can be easily identified (feedback to cullet supplier!). Necessary batch number corrections (sulfate/coke additions) can be carried out quicker than with only a daily spectrofotometric measurement from the laboratory. Due to better redox control a green glass producer was able to increase the cullet share from 87 to 92%, which is equivalent to an anual saving of 80.000 Euro on raw material cost.

Heat transfer and redox state

Glass sensor redox heat transfer 550x380The redox sensor reveals correlations between the oxidation state of the melt and vital process and product properties. This will make the melting process much more transparant to the furnace operator.

A good example is the correlation between the bottom temperatures in the melting tank (emerald green container glass) and the measured redox state of the glass melt in the feeder. The in-line sensor revealed that the redox state of the glass melt affects the radiant heat transfer from the burner flames into the glass melt in the melting tank. The figure shows that a more reduced (emerald green) melt resulted in higher bottom temperatures, probably related to the broad absorption band of ferrous iron in the near infra red. A better heat transfer in the melting tank will immediatley result in a lower energy consumption, but also reduces the risk of a cold bottom flow with high seed count.

Seed count and redox state

Glass Melt Redox Sensor - seed count 340x300It was found that seed count numbers decrease in emerald green glass when melting more reduced. Emerald green can be melted in a relativily large redox range due to the stability of the Cr3+ ion, responsible for the emerald colour. For low seed count numbers and optimal heat transfer, emerald green is therefore preferably melted in the more reduced range. However, one may riks the formation of amber cords and blisters when melting too reduced, especially when melting a high share of recycling cullet. In that case in-line redox monitoring is essential for precise redox control.

Olive and antique green

olivegreen colours 576x1014


Olive and antique green or dark green, are very sensitive to redox changes as these colours are the result of a delicate balance between an amber and green colour component. These colours move easily out of specs as a result of a too oxidized (shift into green) or too reduced melt (shift into amber).   Already small changes in pull, weathering condition of the recycling cullet or air-fuel firing ratio may cause colour instabilities. An in-line redox sensor will be very helpful to the furnace operator to keep the colour within the given specification. For these amber-green colours the sensor's mV must usually be kept within the very small range of  +/- 5 mV around set point to maintain the correct colour specification.

Moreover, the antique or dark green colour is very sensitive to over-reduction. When the melt becomes too reduced, the amber colour component may disappear again. This means that the amber colour component in the dark green may quickly weaken either due to a too oxidised or due to a too reduced melting condition. An in-line redox sensor continously monitors the redox state and the furnace operator knows immediately whether he should  increase of decrease the batch redox number. This prevents an unfortunate correction in the wrong direction, resuting in even more reject.

Signal converter

Read-Ox provides a DIN rail mountable oxygen sensor interface (READOX IOSI-01). This signal converter was specially developed for Read-Ox' oxygen sensors. The glass melt redox sensor gives out an oxygen cell mV-signal and a thermocouple mV-signal, which are converted by the IOSI-01 interface to three analog 4..20 mA outputs. The standard settings are:

  • I-out1: Glass melt temperature: 4..20 mA = 0..1500C
  • I-out2: Oxygen cell EMF: 4..20 mA = 0..1000 mV
  • I-out3: Log (pO2): 4..20 mA = -12..0 (-)


The output calculations and ranges may also be (re)programmed through the IOSI's USB port by using the specially develloped PC software package ImOxyConfig. In this way the end user can configure the IOSI-01 according to his specific wishes. For a detailed specification of the oxygen sensor interface please click IOSI-01

Recently, tests are carried out with the iron ratio of the cold glass I-out3, instead of log(pO2). read more...