GST

Galvanising/Galvanneal Strip Thermometer

The Land GST has been specifically developed to provide continuous and accurate measurement of both temperature and emissivity on coated steel strip during the galvanneal reaction.

Key Features

  • Accurate non contact temperature measurement on steel during the galvannealing process.
  • Automatic emissivity compensation for variations in the reaction zone.
  • Continuous emissivity readout for indication of extent of reaction.
  • Reliable, drift free operation with a minimum of maintenance.

Key Benefits

  • By maintaining control of the reaction zone, GST accomodates rapid changes of line speed and furnace power with changes of substrate or coating weight.
  • The furnace is optimised to reduce heating costs, maximise throughput and avoid excessive over-reaction which would resultin powdering or flaking of the coating during subseqent forming operations.
  • Land GST systems ensure the production of consistent, high quality, premium-coated steel, demanded by the automotive industry.
  • Without the close control provided by GST systems, manufacturers produce lower grade coated steels which cannot be sold to customers requiring premium-grade products.
Gallery
This system allows close control of the reaction and position of the reaction zone, thus giving much tighter control of product quality. It allows for rapid changes of line speed and furnace power with changes of substrate or coating weight, in order to maintain control of the reaction zone.

The Problem

Following the brief immersion of the steel strip in the zinc bath, the iron-zinc diffusion reaction is initiated at the metallic interface.

Subsequent reheating maintains and accelerates this reaction so that after a few seconds iron particles start to appear at the surface.

Just prior to this the emissivity is very low, being that of the molten zinc, but the emergence of the iron creates areas with a solid, crystalline and microscopically rough surface. Over a short period of time (approx. five seconds) the emissivity value increases about fourfold and the non greyness factor (ratio of emissivities at two close wavelengths) also changes markedly.

This renders conventional single wavelength or ratio thermometer systems very inaccurate. The position of the reaction zone within the galvannealing furnace will depend critically on temperature, line speed, coating weight, alloy type, strip width and gauge. This makes control of the zone position difficult to achieve without very accurate knowledge of the temperature and the extent of the reaction (i.e. surface emissivity).

The Solution

The GST utilises an extremely accurate infrared radiation thermometer coupled to an intelligent signal processor containing a unique emissivity compensation algorithm. Once set, the system can recognise and compensate automatically for the emissivity changes as they occur.

Measured target emissivity, as well as temperature, is output by the processor to provide an indication of the extent of the galvanneal reaction.

Up to four GST or System 4 thermometers can be connected simultaneously to an LMG GS processor.

Mathematical comparisons between the thermometer readings can also be made using the LMG GS optional Maths Function Card. The LMG GS processor provides digital indication of temperature and current outputs for connection to the galvanneal furnace control system.

A new feature of the system is APCOS (Application Processor Configuration and Optimisation Software) which provides the ability to optimise each GST thermometer individually. This further improves the measurement accuracy at each location.

Optimisation

As a result of experience gained from many site trials and installations, it has been found that slight variations are required in the temperature calculation coefficient values, in order to maximise the system accuracy at each measurement point.

A unique feature of the GST thermometer is the ability to optimise each unit individually. This may then improve the measurement accuracy at each location.

Optimisation requires a small number of temperature comparisons to be made on a representative sample of product types, against the GST Surface Reference Probe, which features a unique emissivity enhancing optic.

Optimisation is performed using APCOS (Application Processor Configuration and Optimisation Software), supplied as standard with the LMG GS processor. APCOS processes data and submits an optimised solution for the temperature calculation.

Temperatures
Low Temperature (C): 350 High Temperature (C): 550
Technical Details
Thermometer type: GST
Processor type: LMG GS
Measurement range (specified): 350 to 550°C/662 to 1022°F
Display range: 300 to 600°C/600 to 1100°F
Response time: 1s to 98%
Target size: 89mm/3.5 in at 2m/78.0in
Resolution: 2°C/4°F
Absolute accuracy: ±5°C/9°F after optimization with APCOS (in application)
System output Temperature: 0 to 20mA or 4 to 20mA (normally non-isolated, isolated option available; nominal load resistance 500 ohms, max load including wiring 600ohms)
Emissivity: 0.1 to 1.0V (output impedance 1Kohm)
Alarm relays: Contact rating 50V a.c./ d.c. at 0.5A max
Output update time: 10ms
Power supply: 100/110/120/220/240V a.c., ±10%, 50/60 Hz, ±10%, 20VA nominal
Signal processing functions
Average: 63% time constant adjustable 0 to 500s
Peak picker: Acquisition time to 98% 1.1s, decay rate adjustable 0 to 400°/s: on and off delays 20ms adjustable; delay threshold temp. set in 1° steps
Ambient temperature limits
Thermometer: 5 to 45°C/40 to 110°F
Processor: 5 to 50°C/40 to 140°F
Weight
Thermometer: 0.8 kg/1.76lb approx
Processor: 3.6 kg/7.93lb approx
EMC: EN 50-082-2 (immunity), EN 50-081-1 (emission), IEC1010-1 (safety)
Download Datasheet
GST

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