Carbon Sensor and Indicator
The RASKIN Carbon Sensor (Oxygen Probe) type ST is a state
of the art product manufactured using patented sensors of Ceramic
Oxide Fabricators and exotic space age materials. These Probes
are designed to withstand high temperatures and highly corrosive
carburishing atmospheres for long periods, including frequent
burn outs experienced in carburising process.
RASKIN carbon sensor is designed to work in conjunction with Raskin
C Potential monitor/control systems which provides a direct read
out in sensor millivolts (mV) & temperature, °C (where
TC is porvided). This sensor mV, in conjuction with process temperature
is then interpreted in terms of atmosphere carbon potential. RASKIN
provides suitable tables for this interpretation. Based on the
metallurgical knowledge heat treatment times, steel composition
of work load, etc. this can be directly related to the surface
carbon and case depth in the work load, etc.
monitor and/or control of these furnace atmospheres has been made
by measuring either the dew point or CO2 content, of the furnace
atmosphere. Both of these entail an external measuring system,
with its associated sampling system and all the problems an external
sampling system can bring - frequent maintenance, due to filter
/ pump cleaning, etc.
measurement approach by Raskin through carbon sensor completely
eliminates these problems. Further more there are no equilibrium
shift problems as would be encountered in an external sampling
system where the gas requires to be cooled. Carbon sensor gives
"IN-SITU" read out of the gas conditions prevailing
within the furnace; at the process temperature.
of routine cleaning & maintenance has proved of an immense
advantage where labour rates are constantly rising and skilled
maintenance staff are in short supply. Additionally instantaneous
response to atmosphere changes permits close control to be achieved
even with even simpler control elements.
Various forms of controls possible are ON/OFF, time proportional,
3 term, etc. in conjunction with solenoid valve in the enriching
medium. Necessary additional elements are provided for boost /
diffuse control with timers, recorders, etc. as well as regular
purge elements for probe.
measurement technique virtually eliminates maintenance as normally
required in extraction type of systems (Dew Point, CO2, etc.).
Further errors due to equilibrium shift encountered while sample
cooling, leakages, etc., are totally eliminated.
Precise and rapid measure of the atmosphere conditions can be
obtained in the process in proximity to the work load and at
the process temperature. A measurement accuracy of ±
0.05% C & a response time of less than 1 second permit simple
control systems to be used. Distance between the probe location
& instrument location is not a limitation as compared to
sampling systems where increasing distance increases response
sensor control systems enable greatly enhanced quality control
to be achieved in carburising, carbo-nitriding, neutral hardening,
etc. It is estimated that carbon potential control to atleast
± 0.05% C can be repeatedly obtained throughout the sensor
The system accuracy is maintained throughout the sensor life.
Simple calibration procedure and in-built checks in the system
enable to monitor probe health.
carbon sensors are repairable and its peak efficiency can be
maintained through refurbishing/reconditioning periodically.
reworks results in increased furnace availability, faster furnace
reconditioning and more rapid recovery to control following
furnace loading, with carbon sensor control.
atmosphere control can be obtained by the use of the dedicated
time proportional / 3 term controllers, for each measurement
point - and at no greater cost than for conventional multipoint
systems. Furthermore the system can readily be linked to a programme
controller to facilitate the selection & control of a variety
of pre-determined boost / diffuse, carburise / diffuse or boost
/ soak cycles.
more efficient utilisation of endo-thermic gas, enrichment gas,
etc. results in appreciable energy savings.
sensor control systems can be applied for the measurement /
control of nitrogen based and other types of furnace reacted
atmospheres; when conventional instruments (eg. CO measurement)
a multipoint CO2 / Dew point system, any problem with analyser
results in total shut down of all the analysis points, whereas
only corresponding point gets affected in case of problem with
Carbon Potential Relationship :
It has been well established that the theoretical equilibrium
C-Potential of an atmosphere normally provides a good initial guide
to the required control value for any carburising or carbo-nitriding
process. However, the ultimate operating condition is usually determined
by means of surface carbon measurements in shim stock or sample
pieces because the treatment time and work composition usually dictate
a small deviation from the theoretical control value. RASKIN provides
the necessary date tables of probe mV, temperature and
C-Potential relationships, for different feed-stocks.
The Carbon Sensor in principle is a high temperature oxygen concentration
cell constructed from a stabilised Zirconium oxide pellet eutectically
fused to a super refractory tube. To this are attached internal
and external electrodes & thermocouple. Zirconium Oxide acts
as a solid electrolyte which has the property of conducting electricity
by means of O2 ions (probe construction See Fig). The outer electrode
is exposed to the measured gas and inner electrode to reference
air (O2 content 20.9%). When the sensor temperature is within the
recommended range, a potential difference will be set up between
the two electrodes dependent on the ratio of oxygen partial pressures
at the two electrodes.
If the oxygen partial pressure at one end is known then a measurement
of this potential difference will enable the oxygen partial pressure
at the other electrode to be determined.
practical purposes, a minimum operating temperature of 700°C
upto a maximum operating temperature of 1050°C, permits measurement
and control of all endothermic atmospheres encountered in heat treatment
BASED CP CONTROLLER
0-100 Deg. C, 0-1999 mV & 0-9999K ohms, digital.
- one from oxygen probe and the 2nd from thermocouple. Based
on these inputs, signals are processed and % C calculations
+/- 5%, 50 Hz stabilised spikes free, with proper earthing.
wheel set points range 0-1.5%
C/0-1999 mV, time proportional control action through action
and pause timers.
solid state, zero cross over pulsed output either 0 or 230
V (110V) on/off, for resistive load upto 20W (solenoid valve
/ auxillary contactor)
outside operating range and high impedence alarm. Also fixed
pre set temperature alarm normally set at 850 Deg C. Solid
state pulsed output.
% C or O2 mV units, through selector switch.
DC linear isolated corresponding to
0-1.5%C/1000-1200mV into a load of upto 400 ohms
288 W X
144H x 400D mm with panel cutout 282x138mm
It is strongly
recommended to locate this instrument in a dust free ventilated
environment. Further power supply should be made from a CVT/spikes