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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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