FIELD OF THE INVENTION
The present invention is directed to developing a continuous temperature measuring device
for Induction furnaces. More particularly, the invention relates to a contact type
thermocouple based continuous temperature measuring device for liquid metal bath
temperature measurement in air induction furnace for desired precise control of the bath
parameters in a particular heat. Advantageously, the continuous temperature measurement
device of the present invention is capable of providing information about the temperature
trend in the furnace. Importantly, in order to avoid the undesired influence of induced emf
from the Induction Coil of furnace in the thermocouple wires, a capacitor/resistance circuit
has been introduced across the thermocouple wires in the continuous temperature
measuring device of the invention. A filter circuit connected across the thermocouple wire
comprising a capacitor of appropriate value and a resistor connected across the capacitor to
take care of the leaking current. It is also ascertained that the filter circuit thus provided
cuts off the AC component completely and that the temperature shown is constant. The
thermocouple set up is introduced into the molten pool at zero time and the indicated
temperature measured for every five seconds. The response time is 45 seconds for 100%
value and 22 seconds for 95% final value of temperature measured. The continuous
temperature measuring device is capable of measuring temperature in molten steel up to
1650°C continuously in Induction Furnace environments with effective probe life of 3.0 to
4.0 hours. The device of the present invention is thus adapted to continuous temperature
measurement of liquid metal bath in air induction furnaces with desired precision and thus
favoring wide scale application of such device in steel industry/foundry. The induction
furnace operator can take timely action, with the knowledge of the continuous temperature
profile of the melt, taking temperature related decisions on exact time, thus improving the
quality of the ingots cast minimizing the metal loss in the ladle.
BACKGROUND ART
It is known in the art of steel production through Induction furnace route that for heat
making in Air Induction Furnace, liquid bath temperature is the main process parameter to
be precisely controlled to ensure the quality of cast product. The standard system for bath
temperature measurement in Induction Furnaces involves intermittent dip type
measurement carried out by manually immersing a disposable probe. This method is a
reliable and tested type. However, this method gives instant temperature at a particular
time but not continuous. Thus the dip measurements need to be carried out essentially at a
regular frequency, or else rapid temperature changes in the furnace may go unnoticed.
Operator has to depend on the last readings made for doing the different activities in the
furnace to control the charge characteristics.
Thermocouple based indirect temperature measurement is known in the art wherein Seebec
effect is utilized to convert thermal gradient between two junctions to induced emf to
indicate any unknown temperature. A conventional thermocouple wire connection consists
of making two junctions out of two dissimilar metals. One of the junctions is kept at the
reference temperature termed as cold junction (normally at 0°C, maintained by keeping a
mixture of water and ice), while the other junction is kept at the required temperature.
Standard charts (data) are available to show the emf generated verses the temperature
difference, for various type of dissimilar metal combinations. Different types of known
thermocouple wire combinations are available applicable for a temperature range of 250°C
to 2000°C.
However, for Industrial application, a thermocouple consists of two wires, dissimilar in
chemical composition, welded together at one end forming a close circuit, while the other
ends are connected to a temperature indicating instrument. The welded end is called the hot
junction and the free ends that are connected to a junction head, are known as cold ends. It
is also known in the art that this cold junction needs a compensation correction without
which accurate temperature sensing is not possible.
The cold junction temperature is usually calibrated against an ice-point temperature and so
the standard table for thermocouples is based on a 0°C reference temperature. But many
instruments are designed to operate with thermocouples being provided with terminals for
direct connection to the thermocouple or extension cable conductors, thus needing no
separate reference junction. So temperature is corrected in such cases by simply adding the
room temperature to the measured value of temperature. This is conventionally called the
cold junction compensation.
Usually the thermocouple is attached to the indicating device by a special wire known as the
compensating or extension cable. In extension cable, wires of nominally the same
conductors are used as the thermocouple itself. They are recommended for best accuracy.
In compensating cables, wires of relatively low cost alloy conductor materials are used

whose net thermoelectric coefficients are similar to those of the thermocouple. They are less
precise, but cheaper.
In practice, in the induction furnace, a conductive charge is placed within a helical coil of
copper. The alternating current applied to the coil produces a varying magnetic field, which
is concentrated within the helical coil. This magnetic field passing through the charge
induces secondary current in the charge piece. The current circulating in the charge causes
it to heat up. The same magnetic flux also creates an 'emf' in any thermocouple that is
introduced into the Induction furnace for temperature measurement. This will introduce
errors in the measurement. Thus a thermocouple can not be used unless corrective
measures are taken.
There has thus been a need in the art for developing a continuous temperature
measurement device involving contact type thermocouple used to serve the purpose of
providing information about the temperature trend in the furnace accurately avoiding error
due to induced 'emf' so that the operator can take timely action, with full knowledge of the
continuous temperature profile of the melt. Such device would provide the operator with
greater confidence at the time of taking temperature related decisions, leaving no scope for
any unwanted temperature variation go unnoticed, and thus improving the quality of the
ingot cast and minimizing the metal loss in the ladle.
OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to developing a device for
continuous temperature measurement of liquid steel bath in air induction furnace involving
contact type thermocouple in order to precisely measure the temperature online in order to
control the steel making process and to achieve desired steel/cast ingot quality.
It is another object of the present invention directed to developing a device for continuous
temperature measurement of melt in air induction furnace for steel making wherein 'S' -
type thermocouple with wire combination of Pt- Rh (10%) + Pt, covered with alumina beads is
selected from available types for use in said device.

Another object of the present invention is directed to developing a device for continuous
temperature measurement of bath in air induction furnace wherein operator can take timely
action and temperature related decision for steel making with the knowledge of the
continuous temperature profile of the melt.
A further object of the present invention is directed to developing a device for continuous
temperature measurement of bath in air induction furnace such that there is no
discontinuity in temperature recording and displaying so that no undesired temperature
variation of melt go unnoticed which is detrimental to steel quality/end properties.
A still further object of the present invention is directed to developing a device for
continuous temperature measurement of bath for steel making in air induction furnace
wherein operator can be confident about improving the quality of the ingot cast and
minimizing the metal loss in the ladle.
A still further object of the present invention is directed to developing a device for
continuous temperature measurement of molten bath for steel making in air induction
furnace wherein the effect of induced emf due to the induction heating of the furnace
incorporating error in temperature measurement is eliminated and a filter circuit across the
thermocouple wire is introduced so that the AC component is completely suppressed
avoiding current leaking and thus favouring precise, accurate continuous temperature
measurement free of fluctuations.
A still further object of the present invention is directed to developing a device for
continuous temperature measurement of bath in air induction furnace wherein said device
comprised an oscilloscope and a temperature indicator connected to the contact type
thermocouple wire in order to analyze the signal output on continuous basis and indicate the
measured steady state temperature.
A still further object of the present invention is directed to developing a device for
continuous temperature measurement of bath in air induction furnace wherein the
temperature is measured with the induction power on as well as off conditions to ensure
that the steady state temperatures measured is constant in either condition indicating
reliability of the device.

A still further object of the present invention is directed to developing a device for
continuous temperature measurement of bath in air induction furnace wherein the response
time to reach the steady state temperature is reasonably small with respect to the final
temperature value, ensuring prompt and timely recording of process temperature to
meeting desired quality criteria of steel.
A still further object of the present invention is directed to developing a device for
continuous temperature measurement of molten bath for steel making in air induction
furnace wherein the temperature sensing probe involving contact type thermocouple is
having a operating life of 3 to 4 hours favouring continuous temperature measurement
without frequent interruptions.
A still further object of the present invention is directed to developing a device for
continuous temperature measurement of molten bath for steel making in air induction
furnace wherein said device is adapted to measure temperature in molten steel up to 1650 °
C continuously in Induction Furnace environments.
SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to a continuous temperature measuring
device for Induction Furnaces, comprising
thermocouple means providing hot and cold junctions. The hot junction of thermocouple
fitted into a crystallized alumina tube whereas the cold junctions are connected to the
measuring device;
said hot junction along with the crystallized alumina tube of the continuous temperature
measuring unit adapted to be dipped in the melt within air induction furnace for desired
continuous temperature measurement;
a filter circuit comprising a capacitor and a resistance connected in parallel across the
thermocouple wires;

an oscilloscope connected across the thermocouple wires for desired signal analysis and a
temperature indicator connected in parallel to the resistance for desired continuous display
of measured temperature.
Another aspect of the present invention is directed to a continuous temperature measuring
device for Induction Furnaces, wherein said thermocouple means comprises a contact type
thermocouple with attached compensating/extension cables covered with twin hole alumina
beads preferably a standard S-type thermocouple having wire combination of Pt - Rh (10%)
+ Pt, adapted to measure temperature in the range of 0°C to above 1650°C.
A further aspect of the present invention is directed to a continuous temperature measuring
device for Induction Furnaces, wherein in said filter circuit the value of capacitor is in the
range of 10,000 to 50,000 uF and the value of resistance is in the range of 1.0 to 5.0 kQ.
A still further aspect of the present invention is directed to a continuous temperature
measuring device for Induction Furnaces wherein, the filter circuit used in said device is
adapted to neutralize the effect of AC component of the induced current in the measuring
device and completely suppress the AC peaks of the signal.
According to yet another aspect of the present invention directed to a continuous
temperature measuring device for Induction Furnaces wherein, said device is adapted to
measure temperatures continuously for 3.0 to 4.0 hours.
A still further aspect of the present invention is directed to a continuous temperature
measuring device for Induction Furnaces wherein, the device is adapted for response time
22 seconds for 95% final value, whereas the response time is 45 seconds for 100% final
value.
A still further aspect of the present invention is directed to a continuous temperature
measuring device for Induction Furnaces wherein, said device is adapted to measure
temperature in molten steel up to 1650°C continuously in Induction Furnace environments.
The present invention and its objects and advantages are described in greater details with
reference to the following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure la: is the schematic view showing a 'S-Type' thermo couple used in the device of the
present invention.
Figure lb: is the schematic view of the twin hole alumina bead for covering the thermo
couple wire.
Figure lc: is the schematic recrystallized alumina tube for housing the S-type thermocouple
wire covered with alumina bead for use in the device of the invention.
Figure Id: is the schematic illustration of the continuous temperature measuring unit for
use in the device of the present invention.
Figure 2: is the schematic illustration of the circuit diagram of the continuous temperature
measuring unit showing the connection details of capacitor and resistance across the
thermocouple wire, the oscillator and the temperature indicator in said circuit.
Figure 3: is the schematic illustration of the continuous temperature measuring unit/device
of the invention placed inside the air induction furnace with hot junction of the
thermocouple kept at the bottom inside of the recrystallised alumina tube dipped in the
melt.
Figure 4: is the schematic illustration of a cross-sectional view of the Air Induction Furnace.
Figure 5: is the graphical plot of the temperature versus time to record the response time to
reach a final value of steady state temperature.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING DRAWINGS
The present invention relates to contact type thermocouple based continuous temperature
measuring device for liquid metal bath temperature measurement in air induction furnace
for desired precise online temperature measurement and control of the bath parameters in
a particular heat.
It is known for recording temperature in Industrial application, a thermocouple consisting of
two wires is used, which are dissimilar in chemical composition, welded together at one end
forming a close circuit, while the other ends are connected to an indicating instrument. The
welded end is called the hot junction and the free ends that are connected to a junction
head, are known as cold ends. For this cold junction compensation is required to be
provided.
The hot junction temperature is calibrated conventionally against an ice-point temperature
(cold junction) and so the standard table for thermocouples is based on a 0°C reference
temperature. However, many of the standard instruments designed to operate with
thermocouples are provided with terminals for direct connection to the thermocouple or
extension cable conductors, without the need for a separate reference junction. So
temperature is corrected by simply adding the room temperature to the measured value of
temperature. This is known as cold junction compensation.
Usually the thermocouple is attached to the indicating device by a special wire known as the
compensating or extension cable. In extension cable, wires of nominally the same
conductors are used as the thermocouple itself. These are recommended for best accuracy.
In compensating cables, wires of relatively low cost alloy conductor materials are used
whose net thermoelectric coefficients are similar to those of the thermocouple. These are
less precise, but cheaper.
The relationship between the temperature difference and the output voltage of a
thermocouple is nonlinear and is given by a polynomial interpolation.

The coefficients an are given for n from 0 to between 5 and 9. For achieving accurate
measurement this equation is usually implemented in a digital controller or a lookup table.
A variety of thermocouples are available, suitable for measuring different temperatures
mentioned in the following table 1. Type B, S, R, & K thermocouple are used extensively in
the steel and iron industry to monitor temperature throughout the steel making process.

The Continuous temperature measuring device of the invention wherein a 'S' - type
thermocouple having wire combination of Pt - Rh (10%) and Pt with compensating cable
connection has been selectively used, suitable for carrying out temperature measurements
in the desired temperature range in air induction furnace, as shown in the accompanying
Figure1a and which is covered with twin hole alumina beads as illustrated in the
accompanying Figure1b. The hot junction of thermocouple is fitted into a crystallized
alumina tube of Figure 1c whereas the cold ends are connected with the measuring device.
A capacitor and a resistance are connected in parallel across the thermocouple wires. An
oscilloscope and a temperature indicator are also connected in parallel to the resistance.
The circuit diagram for the continuous temperature measuring unit is shown in the
accompanying Figure 2. The temperature measuring unit is placed inside the recrystallised
alumina tube of Figure lc for deployment in furnace for actual temperature measurement
as illustrated in accompanying Figure 1d. The Induction furnace consists of a refractory
container, capable of holding the molten bath, which is surrounded by a water cooled helical
copper coil connected to a source of alternating current. A simplified cross sectional view of
an air induction furnace is shown in the accompanying Figure 3, with melt or liquid steel
charge inside. The whole arrangement for continuous temperature measuring device dipped
in liquid metal along with induction furnace has been shown in the accompanying Figure 4.
The experimental set up for continuous temperature measuring device according to the
present invention as shown in the Figure 4 is utilized to conduct series of experiments in the
5kg Induction Furnace using cast iron as charge. To avoid the influence of induced emf from
the Induction Coil in the thermocouple wires, a capacitor-resistance based filter circuit has
been introduced across the thermocouple wires as shown in Figure 2. The signal out put in
each case is analyzed using an oscilloscope. As soon as the furnace is switched on, heating
of the charge is started. The oscilloscope showing AC sinusoidal curve is tuned. The AC peak
is filtered by means of different alternative values of capacitors selectively used. Finally, a
capacitor of appropriate value in the range of 10,000 to 50,000 µF is found suitable. A
resistor of a certain value in the range of 1.0 to 5.0 kO is connected across the capacitor to
take care of the leaking current. The temperature indicator started showing the temperature
of the furnace without any fluctuations. The temperature of the furnace is raised to different
levels during the experimentation.
It is necessary to ascertain that the filter circuit provided is able to cut off the AC
component completely and that the temperature shown is constant. For this purpose, the
temperature across the thermocouples has been measured under the following two
conditions:
i) Induction power is 'ON': the filter circuit is included
ii) Induction power is 'Switched Off', to cut off the AC signal source. The
filter circuit is "Switched ON".
It has been observed that the temperature difference between the above two cases is
always zero. The temperatures recorded are presented in the following table 2.

To ensure whether the circuit is working satisfactorily, data is recorded when the induction
power is on, and temperature is measured under two conditions: i) with the filter circuit
included across the thermocouple wires, and ii) without the filter circuit across the
thermocouple wires. It is observed that a difference in temperature of about 40°C is
recorded for the above two conditions. The results obtained are shown in Table - 3.

During the first set of experiment, the probe was immersed in the liquid cast iron for 45
minutes at 1415°C max. The temperature was further raised to 1595°C and the probe was
held at this temperature 30 minutes. Similarly, in the second and third sets of experiments
the same probe was used for lhr. 45 minutes and 45 minutes respectively. Thus, the total
probe life was measured 3 hrs. 45 minutes.
The thermocouple set up for continuous measurement of melt temperature in air induction
furnace introduced into the molten pool at zero time and the temperature indicated was
measured for every five Seconds. Data collected is presented in Table - 4. The final steady
state temperature was 1415°C. This was achieved in 45 seconds. The response time is
taken as 45 seconds for 100% value. The corresponding data have been plotted graphically
as shown in accompanying Figure 5. From the graph the response time for 95% final value
is 22 seconds.

The above experimental observations indicate that the device of the invention is adapted to
be scaled up to the actual production situation with higher furnace capacities with desired
accuracy and fast response for online measurement of bath temperature.
It is thus possible by way of the present invention to providing a device for continuous
temperature measurement of melt/liquid steel bath in air induction furnace. More
particularly, the device of the invention makes use of contact type thermocouple for such
online temperature measurement of melt in air arc furnace wherein a filter circuit
comprising a capacitor and a resistor used to effectively suppress the AC peak to ascertain
avoiding the measuring inaccuracy due to induced emf or leaking current, at different
temperature range with reasonably fast response time to attain the steady state
temperature. The device of the invention is thus capable of providing the operator of the
furnace an effective means to constantly measure and monitor the melt temperature with
desired precision in air arc furnace favoring prompt time dependent decision making with
confidence so that the end quality/composition of steel could be maintained, ensuring good
quality of cast ingot, less rejection/metal loss in the ladle. The device of the invention is a
low cost yet accurate means for continuous measurement of melt in air induction furnace
free of any possibility of unwanted temperature variation during steel making process going
unattended and thus favoring wide scale application in steel industry with significant cost
advantage.
We Claim:
1. A continuous temperature measuring device for Induction Furnaces comprising,
thermocouple, means providing hot and cold junctions;
hot junction of thermocouple fitted into a crystallized alumina tube whereas the cold
junctions are connected to the measuring device;
the said hot junction along with the crystallized tube of the continuous temperature
measuring unit adapted to be dipped in the melt within air induction furnace for desired
continuous temperature measurement;
a filter circuit comprising a capacitor and a resistance connected in parallel across the
thermocouple wires;
an oscilloscope connected across the thermocouple wires for desired signal analysis and a
temperature indicator connected in parallel to the resistance for desired continuous display
of measured temperature.
2. A continuous temperature measuring device for Induction Furnaces as claimed in claim 1,
wherein said thermocouple means comprises a contact type thermocouple with attached
compensating/extension cables covered with twin hole alumina beads preferably a standard
S-type thermocouple having wire combination of Pt - Rh (10%) + Pt, adapted to measure
temperature in the range of 0°C to above 1650°C.
3. A continuous temperature measuring device for Induction Furnaces as claimed in claims 1
or 2, wherein in said filter circuit the value of capacitor is in the range of 10,000 to 50,000
µF and the value of resistance is in the range of 1.0 to 5.0 kO.
4. A continuous temperature measuring device for Induction Furnaces as claimed in anyone
of claims 1 to 3 wherein, the filter circuit used in said device is adapted to neutralize the
effect of AC component of the induced current in the measuring device and completely
suppress the AC peaks of the signal.
5. A continuous temperature measuring device for Induction Furnaces as claimed in anyone
of claims 1 to 4 wherein, said device is adapted to measure temperatures continuously for
3.0 to 4.0 hours.
6. A continuous temperature measuring device for Induction Furnaces as claimed in anyone
of claims 1 to 5 wherein, the device is adapted for response time 22 seconds for 95% final
value, whereas the response time is 45 seconds for 100% final value.
7. A continuous temperature measuring device for Induction Furnaces as claimed in anyone
of claims 1 to 6 wherein, said device is adapted to measure temperature in molten steel up
to 1650 °C continuously in Induction Furnace environments.
8. A continuous temperature measuring device for Induction Furnaces substantially as
herein described with reference to the accompanying non limiting illustrative drawings.

The invention disclosed a continuous temperature measuring device for Induction furnaces.
More particularly, it relates to a contact type thermocouple based continuous temperature
measuring device for liquid metal bath temperature measurement in air induction furnace.
It is capable of providing information about the temperature trend in the furnace. A
capacitor/resistance filter circuit is introduced across the thermocouple wires to ascertain
that the AC component is completely cut off and that the temperature shown is constant.
The response time is 45 seconds for 100% value and 22 seconds for 95% final value of
temperature measured. The device is capable of measuring temperature in molten steel up
to 1650°C continuously in Induction Furnace environments with effective probe life of 3.0 to
4.0 hours. The device is having prospect of wide scale application providing the knowledge
of the continuous temperature profile of the melt, assisting temperature related decisions on
exact time, thus improving the quality of the ingots cast minimizing the metal loss in the
ladle.