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Tin Melt Oxygen Sensor

The Read-Ox tin melt oxygen sensor is specifically designed for the in-line measurement of oxygen activity in the melt of the tin bath of a float glass production line.

Properties & benefits:
  • Continuous monitoring of the tin melt oxygen and temperature
  • Easy installation with dimensions similar to standard thermocouples
  • Solid reference, no need for reference gas supply
  • Reduction of top surface defects and  bottom surface defects caused by oxygen
  • More efficient and cost-effective use of hydrogen

 

Oxygen Related Defects

One of the major concerns in the operation of the tin bath is to prevent oxidation of the tin melt. The bath is protected by a positive pressure protective atmosphere, typically 1 to 10% hydrogen in nitrogen. However, oxygen can infiltrate by air leaks in the bath perimeter, diffusion from the glass ribbon or as impurities in the protective gas.  Even trace amounts of oxygen in tin (just a few ppm) can cause surface defects, recuding production efficiency and product value.

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Oxygen solubility in the tin bath is highly temperature-dependent. When the saturation threshold at a given temperature is exceeded, tin dioxide (SnO₂) forms on the surface of the molten tin, resulting in dross accumulation, which occurs primarily in the cold end section where solubility is lowest. At elevated temperatures in the hot end section, volatile tin monoxide (SnO) readily evaporates from the bath, leading to the formation of Sn and SnO₂ deposits on cooler overhead equipment and roof structures.

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High oxygen levels in tin melt and consequently higher SnO2 and SnO levels on the tin surface and in the atmosphere, may lead may lead to bottom surface defects such as bloom and tin pickup, and top surface defects such as top tin, top speck and crater drip. All these oxygen related defects negeatively affects production efficiency and product value. Moreover, with the devellopment of high added value coating, a minimal distortion of the surface will lead to quality problems, making oxygen control even more important.

Sensor Features

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Much like a thermocouple, the tin melt oxygen sensor comprises a terminal head (1), a stainless steel protection tube (2), and an alumina housing tube (3). The specialized measuring tip (4) is immersed in the tin melt and engineered for optimal performance under the specific conditions present in the bath.

The oxygen cell is fabricated from a proprietary zirconia ceramic developed in-house, offering enhanced resistance to chemical degradation caused by prolonged exposure to the corrosive tin melt—without sacrificing oxygen sensitivity. This design ensures stable and reliable signal output over extended operating periods. The elongated alumina housing tube further mitigates thermal shock and shields the oxygen cell from mechanical impact during installation.

Sensor longevity is influenced by local conditions within each bay, including temperature profiles, melt flow dynamics, and concentrations of oxygen and sulfur.

Three sensor variants are available, each tailored to a specific zone within the tin bath:

  • Hot end sensor: A reinforced design for high-temperature resilience (application range: 650–1000 °C)
  • Mid bath sensor: Standard configuration for use in and around the shoulder section (application range: 650–850 °C)
  • Cold end sensor: Equipped with a highly sensitive oxygen cell for lower temperature environments (application range: 550–650 °C)

Easy Installation

installation tin melt sensor 557x443The geometry of the tin melt oxygen sensor closely resembles that of a standard thermocouple, enabling straightforward replacement of existing thermocouples throughout the tin bath. Its measuring tip is immersed directly in the tin melt, mirroring the configuration of conventional tin thermocouples. A built-in K-type thermocouple assumes responsibility for temperature measurement at the designated location.

Thanks to its internal solid-state reference, the zirconia oxygen cell requires no external reference gas flushing. The generously sized aluminum terminal head offers ample space for easy connection of signal cables, both the thermocouple millivolt signal and the oxygen cell millivolt signal, to the IOSI signal converter

Potential Savings

Currently, surface defects in float glass production are identified through optical scanning of the glass sheet at the end of the production line. When such defects are caused by elevated oxygen levels in the tin bath, restoring defect-free production can take several hours or even days of intensified venting with increased hydrogen input. This slow recovery is primarily due to the large oxygen buffer (an average tin bath contains approximately 150 tonnes of molten tin) and the relatively low reaction rates, which result from the limited exposed tin surface area. Moreover, during this prolonged recovery under a hydrogen-rich atmosphere, tin oxide condensates on the superstructure may also be reduced and detach, potentially causing top surface defects on the glass sheet.

Continuous oxygen monitoring within the tin bath, ideally at multiple points along its length, can help prevent such adverse scenarios. A rising oxygen level can be detected early, before it leads to defect formation. Timely intervention minimizes production losses and reduces the excessive hydrogen consumption required to restore the bath to optimal conditions.

Furthermore, the impact of bath openings during shutdowns, startups, and maintenance activities on oxygen levels in the tin can be closely tracked. Monitoring the return to acceptable oxygen levels after extended exposure allows for precise control and avoids unnecessary hydrogen usage.

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Signal converter

Read-Ox offers a DIN rail-mountable oxygen sensor interface (IOSI-02), purpose-built for seamless integration with Read-Ox oxygen sensors. The tin melt oxygen sensor outputs two millivolt signals: one from the oxygen cell and one from a built-in type K thermocouple. These signals are processed by the IOSI-02 and converted into three analog 4–20 mA outputs, enabling straightforward integration into industrial control systems. The default output configuration is as follows:

  • I-out1: Tin melt temperature: 4–20 mA corresponds to 0 to 1200 (ºC)
  • I-out2: Log (pO2) of the tin melt: 4–20 mA corresponds to -40 to -10 (-)
  • I-out3: Log (CO) of the tin melt : 4–20 mA corresponds to -3 to +3 (-)

Here, CO the oxygen concentration in the tin melt, expressed in parts per million (ppm), equivalent to milligrams of oxygen per kilogram of tin melt, within a range of 0.001 to 1000 ppm.

The output calculations and signal ranges of the IOSI-02 can also be customized via its USB port using the dedicated IOSI02config PC application. This software allows end users to tailor the IOSI-02 interface to their specific operational requirements, ensuring optimal integration and performance. For a comprehensive overview of the IOSI-02 oxygen sensor interface, including technical specifications and configuration options, please  click here...

 

Read-Ox provides a custom-engineered Teflon extension cable, rated for continuous operation at temperatures up to 250 °C. The cable features four conductors, each color-coded to match the contact blocks within the sensor head, ensuring straightforward and reliable connection to the IOSI-02 interface. This design simplifies installation and minimizes the risk of wiring errors in high-temperature environments.