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Tin Melt Oxygen Sensor

The Read-Ox tin melt oxygen sensor was specially developed for the in-line measurement of the oxygen activity of the tin melt in the tin bath in a float glass production line.

Properties & benefits:
  • Continuous monitoring of the oxygen and temperature of the tin melt
  • Easy installation, similar dimensions as regular thermocouple
  • Solid reference, no reference gas supply needed
  • Reduced oxygen related top surface defects and bottom surface defects
  • More effective and economic use of hydrogen

 

Oxygen related defects

One of the major concerns in the operation of the tin bath is to prevent the oxidation of the tin melt. For this purpose the bath is provided with a positive pressure protective atmosphere consisting of a mixture of 1-10% hydrogen in nitrogen (forming gas). Oxygen may enter the furnace by air leaks in the bath perimeter, by diffusion from the glass sheet or as impurity in the forming gas. Oxygen levels in tin exceeding only a few ppm may already lead to various top and bottom surface defects, negatively affecting the production efficiency and/or the value of the final glass product.

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The oxygen solubility in the tin bath is strongly dependent on the temperature. When the oxygen saturation level at a certain temperature is exceeded, tin dioxide (SnO2) will be formed on the surface of the tin melt (so-called dross), mainly occurring in the cold end section as oxygen solubility is low. Additionally, at the elevated temperatures in the hot end section, volatile tin monoxide (SnO) may easily evaporate from the bath forming Sn and SnO2 deposits on the colder overhead equipment and roof sections.

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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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Similar to a thermocouple, the tin melt oxygen sensor consists of a terminal head (1), stainless steel protection tube (2) and an alumina housing tube (3). The special measuring tip (4) is in contact with the tin melt and has an optimal design for the specific circumstances in the tin bath.

The oxygen cell is made of a special in-house produced zirconia ceramic, making the sensor more resistant to chemical attack by the corrosive tin melt over time, without compromising on the oxygen sensitivity. This ensures a reliable sensor signal over an extended period of time. Additionally, the extended alumina housing tube moderates thermal shock and protects the oxygen cell against mechanical shock during installation.

The exact lifetime of the sensor depends on the local circumstance in the particular bay, such as temperature, tin melt flows, oxygen and sulphur levels.

Three types of tin melt oxygen sensors are supplied, each type optimized for the a specific section of the bath:

  • hot end sensor: a more robust sensor resisting high temperatures in hot end (application range 650 to 1000ºC)
  • mid bath sensor: standard sensor for in and around the shoulder section (application range 650 to 850ºC)
  • cold end sensor: with sensitive oxygen cell for relatively low temperatures (application range 550 to 650ºC)

Easy installation

installation tin melt sensor 557x443The geometry of the tin melt oxygen sensor is similar to that of a regular thermocouple. This allows simple replacement of the existing thermocouples along the entire tin bath length. The measuring tip of the tin melt oxygen sensor is in contact with the tin melt, similar to a tin thermocouple. The built-in K-type t/c of the oxygen sensor takes over the temperature measurement at the specific measuring location. Due to its internal solid reference, flushing of the zirconia cell with a reference gas is not needed. The large aluminium terminal head provides much space for convenient connection of the signal cables (t/c mV-signal and oxygen cell mV-signal) to the IOSI signal converter

Potential savings

Presently, glass surface defects are detected by optical scanning of the glass sheet at the end of the float glass production line. If the defect is caused by a too high oxygen level in the tin bath, it will take many hours or even days of increased venting at a higher hydrogen level before resuming defect free production. The slow bath recovery is caused by a large oxygen buffer (an average tin bath contains about 150 tonnes of liquid tin) and relatively low reaction rates due to a limited free tin surface area in the float furnace. Additionally, during the slow recovery of the bath by a hydrogen rich atmosphere, the tin oxide condensates on the superstructure may also be reduced and may come loose, resulting in top surface defects on the glass sheet.

Continuous monitoring of the oxygen level in the tin bath, preferably at several locations along the tin bath, may prevent such unfavourable situation. An increasing oxygen level is detected in an earlier stage, before it may lead to oxygen related defects. Early detection prevents or limits production (value) loss and saves the large extra amounts of hydrogen needed to reduce the bath again.

Additionally, the effect of bath openings during shut down and start up and various maintainance activities on the oxygen levels in tin can be monitored. Return to acceptable oxygen levels after an extended bath opening can be precisely followed and prevents the use of too much hydrogen.

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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 tin melt oxygen sensor gives out an oxygen cell mV-signal and a thermocouple type K mV-signal, which are converted by the IOSI-01 interface to three analog 4..20 mA outputs. The standard settings are:

  • I-out1: Tin melt temperature: 4..20 mA = 0..1200 (ºC)
  • I-out2: Log (pO2) of the tin melt: 4..20 mA = -40..-10 (-)
  • I-out3: Log (CO) of the tin melt : 4..20 mA = -3..+3 (-)
    Where CO = oxygen concentration of the tin melt in ppm (mg oxygen/kg tin melt).

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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 here...