Temperature measurement techniques

In the ferrous foundries ladle temperature varies above1700 C.

At this temperature continuous measurement is difficult. Therefore dip type immersion pyrometer technique is widely used.

Immersion pyrometer consist of metal housing, Electronic Measurement Circuitry (EMC), metal lance, Thermocouple connected to the circuitry by means of compensating cables which passes through the lance.

When thermocouple dip into the molten metal, it gives milli volts output. These milli volts are carried to the EMC, where the signal (mv) is amplified and processed to give direct digital output. The processing of the signal is done by various techniques. Peak latching, Plateau Detection, Continuous tracking are some of these techniques.

Peak Latching:

As the name indicates the technique involves latching of the peak temperature and displaying it. In an induction furnace due to its nature hot spots are generated that moves randomly across the molten metal. Generally the temperature at the hot spot is always higher than the molten metal temperature. If the Thermocouple comes into contact with the hot spot then this reading is latched by the EMC and displayed. Thus temperature at the hot spot is not correct representation of the entire bath temperature. Therefore peak latching is not world wide accepted for use in measuring molten metal temperature in induction furnaces.

Continuous tracking:

In this technique temperature is displayed continuously as it senses. So you do not get stable display of temperature. So it is difficult to know, when to remove lance. Temperature recording is fully dependent on operator skill and it can not separate out hot spot temperature and true temperature. So it is not suitable for ferrous foundries.

Plateau detection technique:

In this technique transient response of sensor and system is ignored. Steady state response is monitored and confirmed by taking several samples. Once it fits in its detection criteria the temperature is displayed. The operator can remove the lance to prevent it from melting in the molten metal bath. This technique displays correct temperature and ignores incorrect readings by giving error messages. Thus there is no possibility of hot spot measurement in this technique. This operation is completed in 1-3 sec in case of single dip paper tube thermocouple tips and 3-8 sec in case of Multi dip thermocouple tips. It ensures life of receptacle (connector for tip and lance).

Transient response phenomenon is very common term in the field of process instrumentation i. e. how system (including sensor) responds for any sudden change is characterized by this terminology.

In case of Immersion Pyrometer, as we are suddenly inserting thermocouple inside the molten metal bath; its response could be like shown in Fig 1 or Fig 2 as it depends on various factors.

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Normal track mode, time based & PEAK LATCH temperature indicator fails to identify & measure the true temperature.

For molten metal temperature measurement only, PLATEAU DETECTION TECHNIQUE is recommended & accepted worldwide, In case, if there is a HOT SPOT with molten metal in the furnace, the temperature at HOT SPOT is more than the rest of the molten metal.

Normal track mode, time based & PEAK LATCH temperature indicator fails to identify & measure the true temperature.

But in case of PLATEAU DETECTION TECHNIQUE, as soon as thermocouple is dipped into molten metal, first peak temperature is ignored and it ensures 7 constant readings for a period of one second & declares true temperature readings. It ignores false readings due to furnace hot spot, noise, and transient response & once it detects steady state temperature i.e. flat temperature it will be displayed.

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