CP CONTROL SYSTEMS

Raskin Carbon Sensor and Indicator

Introduction:
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.

Application :
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.

Traditionally, 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.

"IN-SITU" 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.

Elimination 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.

The Advantages :

  1. IN-SITU 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.
  2. A 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 time.
  3. Carbon 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 life.
  4. The system accuracy is maintained throughout the sensor life. Simple calibration procedure and in-built checks in the system enable to monitor probe health.
  5. RASKIN carbon sensors are repairable and its peak efficiency can be maintained through refurbishing/reconditioning periodically.
  6. Reduced reworks results in increased furnace availability, faster furnace reconditioning and more rapid recovery to control following furnace loading, with carbon sensor control.
  7. Improved 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.
  8. The more efficient utilisation of endo-thermic gas, enrichment gas, etc. results in appreciable energy savings.
  9. Carbon 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) are inadequate.
  10. In 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 sensor.

Theoretical 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.

Carbon Sensor :
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.

For 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 processes.

 

MICROPROCESSOR BASED CP CONTROLLER

Specifications :

Indication

:

0-1.5% C, 0-100 Deg. C, 0-1999 mV & 0-9999K ohms, digital.

Input
:

2mV inputs - one from oxygen probe and the 2nd from thermocouple. Based on these inputs, signals are processed and % C calculations made.

Supply
:

110/230V +/- 5%, 50 Hz stabilised spikes free, with proper earthing.

Control action
:

3 thumb wheel set points range 0-1.5%
C/0-1999 mV, time proportional control action through action and pause timers.

Output
:

Through solid state, zero cross over pulsed output either 0 or 230 V (110V) on/off, for resistive load upto 20W (solenoid valve / auxillary contactor)

Alarms
:

Probe temperature outside operating range and high impedence alarm. Also fixed pre set temperature alarm normally set at 850 Deg C. Solid state pulsed output.

Control mode
:

Either in % C or O2 mV units, through selector switch.

Current output
:

4-20mA DC linear isolated corresponding to
0-1.5%C/1000-1200mV into a load of upto 400 ohms

Dimensions
:

288 W X 144H x 400D mm with panel cutout 282x138mm

Location
:

It is strongly recommended to locate this instrument in a dust free ventilated environment. Further power supply should be made from a CVT/spikes buster.

 

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