CA1102877A - Oxygen monitoring circuit - Google Patents
Oxygen monitoring circuitInfo
- Publication number
- CA1102877A CA1102877A CA321,846A CA321846A CA1102877A CA 1102877 A CA1102877 A CA 1102877A CA 321846 A CA321846 A CA 321846A CA 1102877 A CA1102877 A CA 1102877A
- Authority
- CA
- Canada
- Prior art keywords
- probe
- oxygen
- output
- circuit
- high impedance
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
- G01K1/00—Details of thermometers not specially adapted for particular types of thermometer
- G01K1/02—Means for indicating or recording specially adapted for thermometers
- G01K1/04—Scales
- G01K1/045—Scales temperature indication combined with the indication of another variable
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/416—Systems
- G01N27/417—Systems using cells, i.e. more than one cell and probes with solid electrolytes
- G01N27/4175—Calibrating or checking the analyser
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
- G01N33/20—Metals
- G01N33/205—Metals in liquid state, e.g. molten metals
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- Chemical & Material Sciences (AREA)
- Health & Medical Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- General Physics & Mathematics (AREA)
- Physics & Mathematics (AREA)
- Pathology (AREA)
- Analytical Chemistry (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- Immunology (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- Molecular Biology (AREA)
- Engineering & Computer Science (AREA)
- Food Science & Technology (AREA)
- Medicinal Chemistry (AREA)
- Measuring Oxygen Concentration In Cells (AREA)
Abstract
OXYGEN MONITORING CIRCUIT An oxygen monitoring circuit is disclosed. The circuit comprises a high impedance follow and hold circuit adapted for connection to an oxygen probe, a readout unit connected to the high impedance follow and hold circuit, a programmer normally connecting the oxygen probe to the high impedance follow and hold circuit but momentarily disconnecting the oxygen probe from the high impedance follow and hold circuit for a relatively short time interval and loading it with a resistor having an impedance relatively higher than the normal impedance of the probe, and a probe failure indicator connected to the outputs of the oxygen probe and of the high impedance follow and hold circuit for detecting any significant change in impedance of the probe and resistor combination as a result of loading and for operating an alarm when the impedance of the probe exceeds a predetermined value.
Description
~2~7 This invention relates to an oxygen probe system for monitoring the oxygen content and temperature of molten metal and, more particularly, to a monitoring circuit which can detect aging of the oxygen probe and ~arn an operator of impending pro-be failure.
It is well known to use probes based on galvanic or electromotive cells employing solid electrolytes for measuring the oxygen content of molten metal, such as copper. Some of these probes do not stand the operative conditions of the molten metal more than a few seconds and are discarded and replaced by new probes after each measurement. However, probes for conti-nuously measuring the oxygen content of molten metal are also known. One example of such probes is the one disclosed in Cana-dian Patent No. 990,352 issued June 1st, 1976. The oxygen probe disclosed in the above patent consists of a solid electrolyte tube encased in a stainless steel tube and is capable of with-standing the operative conditions of the molten metal for a sub-stantial length of time but, nevertheless, has a limited life time.
It is the object of the present invention to provide an oxygen monitoring circuit which includes means for detecting aging of the oxygen probe and warning an operator of impending probe failure as the probe approaches the end of its normal life time.
The oxygen monitoring circuit, in accordance with the invention, comprises a high impedance follow and hold circuit adapted for connection to an oxygen probe, a readout unit connec-ted to the high impedance follow and hold circuit, a programmer .~
~ - 2 ~ Z~77 connecting the oxygen probe to the high impedance follow and hold circuit but momentarily disconnecting the oxygen probe from said high impedance follow and hold circuit for a relatively short time interval and loading it with a resistor having an im-pedance relatively higher than that of the probe, and a probe failure indicator connected to the output of the oxygen probe and of the high impedance follow and hold circuit for detecting any significant change in impedance of the probe and resistor combination as a result of loading and for operating an alarm when the impedance of the probe exceeds a predetermined value.
The high impedance follow and hold circuit preferably comprises a high impedance voltage follower adapted to hold the last measured voltage of the probe representing the oxygen con-tent of the molten metal during the time when the oxygen probe is disconnected from the high impedance follow and hold circuit.
The programmer preferably includes a timing circuit normally providing an output voltage of a predetermined value but momentarily switching such voltage to zero during the rela-tively short time interval when the probe is loaded with the re-sistor, a first relay which is switched on by the timing circuit when its output is high for operating a set of normally open contacts to connect the oxygen probe to the high impedance fol-low and hold circuit, and which is switched off by the timing circuit when its output is zero to disconnect the oxygen probe from the high impedance follow and hold circuit, and a second relay which is switched on by the timing circuit when its output is zero for connecting the resistiye load to the oxygen probe.
Means are also preferably provided for delaying the operation of - 3 ~ 28~
the second relay for a short time interval after the output of the timing circuit is switched to zero so-as to make sure that the high impedance follow and hold circuit is disconnected from the oxygen probe before loading the probe with the resistor.
The probe failure indicator may comprise a high impe-dance voltage follower connected to the oxygen probe and a volta-ge comparator connected to the output of the voltage follower and the output of the follow and hold circuit for comparing the loa-ded and unloaded probe voltages, a switching device connected to the output of the voltage comparator, and an indicator device connected to the switching device for providing an alarm when the loaded probe voltage falIs below a predetermined fraction of the unloaded probe voltage.
A high-low oxygen limit indicator may also be connected to the output of the high impedance follow and hold circuit for indicating when high or low ppm oxygen limits are reached.
The output voltage of oxygen probes is normally a loga-rithmic function of the oxygen content of the molten metal. The-refore, an antilog conversion circuit is preferably connected between the output of the follow and hold circuit and the read-out device for converting the normal logarithmic output of the oxygen probe to a linear output which can be more easily shown on the readout device. A variable voltage reference source is also preferably provided for periodic checking of the antilog conversion circuit.
The oxygen probe monitoring circuit may also be provi-ded with a thermocouple probe immersed in the molten metal, a thermocouple signal conditioner connected to the thermocouple _ - 4 ~ Z~
probe, and means for connecting the output of the thermocouple probe to the readout device.
The invention will now be disclosed, by way of example, with reference to the accompanying drawings in which:
Figure 1 illustrates a block diagram of a monitoring circuit in accordance with the invention;
Figures 2 and 3 illustrate a circuit diagram of the monitoring circuit shown in schematic form in Figure l;
Figure 4 illustrates the timing cycle of the program-mer shown in Figure 2; and Figure 5 illustrates the output of a probe which may be used in conjunction with the monitoring circuit in accordance with the invention.
Referring to the drawings, Figure 1 is a block diagram of an embodiment of a monitoring circuit in accordance with the invention shown in combination with a diagrammatic illustration of an oxygen probe 10 immersed in molten metal 12. The probe system is also provided with a thermocouple probe 14 the outside shell of which also acts as a reference electrode for the oxygen probe 10.
The output of the oxygen probe 10 is applied to a high impedance follow and hold circuit 16 which is controlled by a programmer 18. As it will be clearly disclosed later, the high impedance follow and hold circuit 16 is normally connected to the output of the oxygen probe but is disconnected at regular intervals by the programmer 18 for momentarily testing the probe.
During testing of the probe, the last reading made by the probe is held by the follow and hold circuit until the pro~e is con-~ ~ 5 ~ ~ 1~` Z8 ~
nected back to the follow and hold clrcuit.
The output of the oxygen probe is also applied to aprobe failure indicator 20 which provides an alarm when the pro-be is about to fail. The probe failure indicator is responsive to the programmer 18 during testing of the probe for detecting an increase in impedance of the probe as an indication of impen-ding probe failure.
The output of the high impedance follow and hold cir-cuit is applied to a high and low oxygen limit indicator 22 and, more importantly, to an antilog conversion circuit 24 which con-verts the normal logarithmic output of the probe to a linear output which is applied to a recorder 26. A voltage reference 28 is also provided for testing the antilog conversion circuit.
The output of the thermocouple probe 14 is applied to a thermocouple signal conditioner 30 the output of which is ap-plied to recorder 26. Recorder 26 is advantageously a two pen type for indicating and recording both the oxygen content and the temperature of the molten metal. Of course, two separate readout devices could be used.
Figures 2 and 3 illustrate a circuit diagram of the monitoring circuit shown in schematic form in Figure 1. The output of the probe is fed to the non-inverting input terminal of an operational amplifier OPI through a resistor Rl and nor-mally open contacts RLl-l of a relay RLl to be disclosed later.
Operational amplifier OPl is connected as a voltage follower and has a high input impedance. Resistor Rl has a negligible impe-dance value with respect to the probe impedance. A zener diode Dl is connected across the non-inverting terminal of the opera-`~` - 6 ~ 2~
tional amplifier and ground to protect the operational amplifier in case of overvoltage. A capacitor Cl is connected between the non-inverting term;~nal of the operational amplifier and ground to hold the voltage applied to the non-inverting terminal of the operational amplifier when contacts RLl-l are open during tes-ting of the probe as it will be more clearly disclosed later.
The programmer 18 includes, as main components, a con-ventional timer TM operated by a 12V reference source, transis-tors TR1 and TR2 and relays RLl and RL2. A suitable timer inte-grated circuit may be Model No. NE 555 made by SIGNETICS. The timing cycle of the timer is shown in Figure 4 of the drawings and is controlled by resistors R2 and R3 and a capacitor C2 for-ming part of the timer. When the output TP of the timer is high (12V in this case), transistor TR1 is biased into conduction through series resistor R4 and parallel connected resistor R5 and capacitor C3. Relay RLl which is connected in series with the collector emitter electrodes of transistor TRl across the 12V reference source is energized to close its contacts RLl-l.
The output of the probe is thus applied to the non-inverting in-put terminal of the operational amplifier OPl. During that time, the high potential appearing at the output TP of the timer is also applied to the base of transistor TR2 through resistor R6.
However, transistor TR2 is maintained non-conductive by a capa-citor C4 connected to the base of the transistor and charged to the voltage of the 12V reference source through a resistor R7.
Relay RL2 which is connected in series with the emitter and col-lector electrodes of transistor TR2 across the 12V reference source is not energized and contacts RL2-1 of relay RL2 remain ~ 7 ~ ~1~2~
open.
When the output TP of the timer-goes down to zero, ca-pacitor C3 is instantaneously discharged through a diode D2 con-nected across resistor R4 and transistor TRl is cut-off. Relay RLl is released and contacts RLl l are opened. At the same time, capacitor C4 discharges through resistor R6 and, after a slight delay, transistor TR2 becomes conductive. Relay RL2 is energi-zed and contacts RL2-1 of relay RL2 are closed to connect a re-sistor R8 across the oxygen probe. The impedance of resistor R8 is selected high enough as compared to the normal impedance of the probe so that it will not influence significantly the rea-ding of the probe when the probe is good, but will when the probe is about to fail and its impedance approaches that of the resis-tor R8. The slight delay introduced into the operation of relay RL2 after release of relay RLl insures that the output of the probe is disconnected from the input of the operational amplifier OPl before loading of the probe so that, if there is any influen-ce on the output of the probe by such loading, it will not appear at the input of the recorder 26.
When the output TP of the timer goes back to high, transistor TR2 is instantaneously rendered non-conductive by a diode D3 connected across resistor R6 which charges up quickly capacitor C4 and relay RL2 is released, opening its contacts RL2-1. After a short delay, as determined by the charging time of capacitor C3, transistor TRl is rendered conductive again and relay RLl is energized to close contacts RLl-l and connect the output of the probe to the high impedance follow and hold cir-cuit.
- 8 ~ 7 The output of the probe is also applied to the non-inverting input terminal of an opera~ional amplifier OP2 of the probe failure indicator. The operational amplifier OP2 is con-nected as a voltage follower. The output of the operational am-plifier is applied to the inverting input terminal of a voltage comparator VCl. A predetermined offset potential, which may be about 70% of the non-loaded value of the output of the high im-pedance follow and hold circuit as determined by the relative values of resistors R9 and R10, is applied to the non-inverting input terminal of the voltage comparator VCl. The output of the voltage comparator VCl is applied to the base of a transistor TR3 through a resistor Rll. A resistor R12 is connected between the emitter and base of transistor TR3. A relay RL3 is connec-ted in series with the emitter and collector electrodes of tran-sistor TR3 across the 12V reference source. When loading of the probe with resistor R8 changes the output of the probe by more than, say 30%, due to a significant increase in the impedance of the probe caused by aging, the voltage comparator provides an output which biases transistor TR3 into conduction. Relay RL3 and a pilot light PLl connected across relay RL3 are energized.
Contacts RL3-~ of relay RL3 are closed to lock the pilot light across the 12V reference until release button RB is operated.
The thermocouple signal conditioner 30 is a conventio-nal device such as Model No. 4100-1298 made by Action Ins-truments Co. It is used to linearize and expand the temperature scale in the range of 18QQ-230QFt ~hich is a practical tempera-ture range for copper casting. Such a temperature w~ll be indi-cated on a scale of the readout device 26 and recorded by one of
It is well known to use probes based on galvanic or electromotive cells employing solid electrolytes for measuring the oxygen content of molten metal, such as copper. Some of these probes do not stand the operative conditions of the molten metal more than a few seconds and are discarded and replaced by new probes after each measurement. However, probes for conti-nuously measuring the oxygen content of molten metal are also known. One example of such probes is the one disclosed in Cana-dian Patent No. 990,352 issued June 1st, 1976. The oxygen probe disclosed in the above patent consists of a solid electrolyte tube encased in a stainless steel tube and is capable of with-standing the operative conditions of the molten metal for a sub-stantial length of time but, nevertheless, has a limited life time.
It is the object of the present invention to provide an oxygen monitoring circuit which includes means for detecting aging of the oxygen probe and warning an operator of impending probe failure as the probe approaches the end of its normal life time.
The oxygen monitoring circuit, in accordance with the invention, comprises a high impedance follow and hold circuit adapted for connection to an oxygen probe, a readout unit connec-ted to the high impedance follow and hold circuit, a programmer .~
~ - 2 ~ Z~77 connecting the oxygen probe to the high impedance follow and hold circuit but momentarily disconnecting the oxygen probe from said high impedance follow and hold circuit for a relatively short time interval and loading it with a resistor having an im-pedance relatively higher than that of the probe, and a probe failure indicator connected to the output of the oxygen probe and of the high impedance follow and hold circuit for detecting any significant change in impedance of the probe and resistor combination as a result of loading and for operating an alarm when the impedance of the probe exceeds a predetermined value.
The high impedance follow and hold circuit preferably comprises a high impedance voltage follower adapted to hold the last measured voltage of the probe representing the oxygen con-tent of the molten metal during the time when the oxygen probe is disconnected from the high impedance follow and hold circuit.
The programmer preferably includes a timing circuit normally providing an output voltage of a predetermined value but momentarily switching such voltage to zero during the rela-tively short time interval when the probe is loaded with the re-sistor, a first relay which is switched on by the timing circuit when its output is high for operating a set of normally open contacts to connect the oxygen probe to the high impedance fol-low and hold circuit, and which is switched off by the timing circuit when its output is zero to disconnect the oxygen probe from the high impedance follow and hold circuit, and a second relay which is switched on by the timing circuit when its output is zero for connecting the resistiye load to the oxygen probe.
Means are also preferably provided for delaying the operation of - 3 ~ 28~
the second relay for a short time interval after the output of the timing circuit is switched to zero so-as to make sure that the high impedance follow and hold circuit is disconnected from the oxygen probe before loading the probe with the resistor.
The probe failure indicator may comprise a high impe-dance voltage follower connected to the oxygen probe and a volta-ge comparator connected to the output of the voltage follower and the output of the follow and hold circuit for comparing the loa-ded and unloaded probe voltages, a switching device connected to the output of the voltage comparator, and an indicator device connected to the switching device for providing an alarm when the loaded probe voltage falIs below a predetermined fraction of the unloaded probe voltage.
A high-low oxygen limit indicator may also be connected to the output of the high impedance follow and hold circuit for indicating when high or low ppm oxygen limits are reached.
The output voltage of oxygen probes is normally a loga-rithmic function of the oxygen content of the molten metal. The-refore, an antilog conversion circuit is preferably connected between the output of the follow and hold circuit and the read-out device for converting the normal logarithmic output of the oxygen probe to a linear output which can be more easily shown on the readout device. A variable voltage reference source is also preferably provided for periodic checking of the antilog conversion circuit.
The oxygen probe monitoring circuit may also be provi-ded with a thermocouple probe immersed in the molten metal, a thermocouple signal conditioner connected to the thermocouple _ - 4 ~ Z~
probe, and means for connecting the output of the thermocouple probe to the readout device.
The invention will now be disclosed, by way of example, with reference to the accompanying drawings in which:
Figure 1 illustrates a block diagram of a monitoring circuit in accordance with the invention;
Figures 2 and 3 illustrate a circuit diagram of the monitoring circuit shown in schematic form in Figure l;
Figure 4 illustrates the timing cycle of the program-mer shown in Figure 2; and Figure 5 illustrates the output of a probe which may be used in conjunction with the monitoring circuit in accordance with the invention.
Referring to the drawings, Figure 1 is a block diagram of an embodiment of a monitoring circuit in accordance with the invention shown in combination with a diagrammatic illustration of an oxygen probe 10 immersed in molten metal 12. The probe system is also provided with a thermocouple probe 14 the outside shell of which also acts as a reference electrode for the oxygen probe 10.
The output of the oxygen probe 10 is applied to a high impedance follow and hold circuit 16 which is controlled by a programmer 18. As it will be clearly disclosed later, the high impedance follow and hold circuit 16 is normally connected to the output of the oxygen probe but is disconnected at regular intervals by the programmer 18 for momentarily testing the probe.
During testing of the probe, the last reading made by the probe is held by the follow and hold circuit until the pro~e is con-~ ~ 5 ~ ~ 1~` Z8 ~
nected back to the follow and hold clrcuit.
The output of the oxygen probe is also applied to aprobe failure indicator 20 which provides an alarm when the pro-be is about to fail. The probe failure indicator is responsive to the programmer 18 during testing of the probe for detecting an increase in impedance of the probe as an indication of impen-ding probe failure.
The output of the high impedance follow and hold cir-cuit is applied to a high and low oxygen limit indicator 22 and, more importantly, to an antilog conversion circuit 24 which con-verts the normal logarithmic output of the probe to a linear output which is applied to a recorder 26. A voltage reference 28 is also provided for testing the antilog conversion circuit.
The output of the thermocouple probe 14 is applied to a thermocouple signal conditioner 30 the output of which is ap-plied to recorder 26. Recorder 26 is advantageously a two pen type for indicating and recording both the oxygen content and the temperature of the molten metal. Of course, two separate readout devices could be used.
Figures 2 and 3 illustrate a circuit diagram of the monitoring circuit shown in schematic form in Figure 1. The output of the probe is fed to the non-inverting input terminal of an operational amplifier OPI through a resistor Rl and nor-mally open contacts RLl-l of a relay RLl to be disclosed later.
Operational amplifier OPl is connected as a voltage follower and has a high input impedance. Resistor Rl has a negligible impe-dance value with respect to the probe impedance. A zener diode Dl is connected across the non-inverting terminal of the opera-`~` - 6 ~ 2~
tional amplifier and ground to protect the operational amplifier in case of overvoltage. A capacitor Cl is connected between the non-inverting term;~nal of the operational amplifier and ground to hold the voltage applied to the non-inverting terminal of the operational amplifier when contacts RLl-l are open during tes-ting of the probe as it will be more clearly disclosed later.
The programmer 18 includes, as main components, a con-ventional timer TM operated by a 12V reference source, transis-tors TR1 and TR2 and relays RLl and RL2. A suitable timer inte-grated circuit may be Model No. NE 555 made by SIGNETICS. The timing cycle of the timer is shown in Figure 4 of the drawings and is controlled by resistors R2 and R3 and a capacitor C2 for-ming part of the timer. When the output TP of the timer is high (12V in this case), transistor TR1 is biased into conduction through series resistor R4 and parallel connected resistor R5 and capacitor C3. Relay RLl which is connected in series with the collector emitter electrodes of transistor TRl across the 12V reference source is energized to close its contacts RLl-l.
The output of the probe is thus applied to the non-inverting in-put terminal of the operational amplifier OPl. During that time, the high potential appearing at the output TP of the timer is also applied to the base of transistor TR2 through resistor R6.
However, transistor TR2 is maintained non-conductive by a capa-citor C4 connected to the base of the transistor and charged to the voltage of the 12V reference source through a resistor R7.
Relay RL2 which is connected in series with the emitter and col-lector electrodes of transistor TR2 across the 12V reference source is not energized and contacts RL2-1 of relay RL2 remain ~ 7 ~ ~1~2~
open.
When the output TP of the timer-goes down to zero, ca-pacitor C3 is instantaneously discharged through a diode D2 con-nected across resistor R4 and transistor TRl is cut-off. Relay RLl is released and contacts RLl l are opened. At the same time, capacitor C4 discharges through resistor R6 and, after a slight delay, transistor TR2 becomes conductive. Relay RL2 is energi-zed and contacts RL2-1 of relay RL2 are closed to connect a re-sistor R8 across the oxygen probe. The impedance of resistor R8 is selected high enough as compared to the normal impedance of the probe so that it will not influence significantly the rea-ding of the probe when the probe is good, but will when the probe is about to fail and its impedance approaches that of the resis-tor R8. The slight delay introduced into the operation of relay RL2 after release of relay RLl insures that the output of the probe is disconnected from the input of the operational amplifier OPl before loading of the probe so that, if there is any influen-ce on the output of the probe by such loading, it will not appear at the input of the recorder 26.
When the output TP of the timer goes back to high, transistor TR2 is instantaneously rendered non-conductive by a diode D3 connected across resistor R6 which charges up quickly capacitor C4 and relay RL2 is released, opening its contacts RL2-1. After a short delay, as determined by the charging time of capacitor C3, transistor TRl is rendered conductive again and relay RLl is energized to close contacts RLl-l and connect the output of the probe to the high impedance follow and hold cir-cuit.
- 8 ~ 7 The output of the probe is also applied to the non-inverting input terminal of an opera~ional amplifier OP2 of the probe failure indicator. The operational amplifier OP2 is con-nected as a voltage follower. The output of the operational am-plifier is applied to the inverting input terminal of a voltage comparator VCl. A predetermined offset potential, which may be about 70% of the non-loaded value of the output of the high im-pedance follow and hold circuit as determined by the relative values of resistors R9 and R10, is applied to the non-inverting input terminal of the voltage comparator VCl. The output of the voltage comparator VCl is applied to the base of a transistor TR3 through a resistor Rll. A resistor R12 is connected between the emitter and base of transistor TR3. A relay RL3 is connec-ted in series with the emitter and collector electrodes of tran-sistor TR3 across the 12V reference source. When loading of the probe with resistor R8 changes the output of the probe by more than, say 30%, due to a significant increase in the impedance of the probe caused by aging, the voltage comparator provides an output which biases transistor TR3 into conduction. Relay RL3 and a pilot light PLl connected across relay RL3 are energized.
Contacts RL3-~ of relay RL3 are closed to lock the pilot light across the 12V reference until release button RB is operated.
The thermocouple signal conditioner 30 is a conventio-nal device such as Model No. 4100-1298 made by Action Ins-truments Co. It is used to linearize and expand the temperature scale in the range of 18QQ-230QFt ~hich is a practical tempera-ture range for copper casting. Such a temperature w~ll be indi-cated on a scale of the readout device 26 and recorded by one of
2~
the pens of the recorder.
Figure 3 illustrates a circuit diagram of the voltage reference source 28 shown in Figure 1. Potentiometers R20 and R22 are connected across a 15V reference source and used to pre-set upscale set points for the 0-25Q and 0-500 ppm oxygen ranges, respectively. Potentiometers R24 and R26 are also connected to the 15V reference source and used to preset downscale set points for the 0-250 and 0-500 ppm oxygen ranges, respectively. The above set points are used for periodic checking of the antilog conversion circuit. A convenient value for a downscale set point might be 546 mV corresponding to 50 ppm oxygen in the mol-ten metal as measured by an oxygen probe as disclosed in the above mentioned Canadian Patent No. 990,352 the output of which is shown in Figure 5. Similarly, a convenient value for an up-scale set point may be 475 mV corresponding to 200 ppm oxygen.
The chosen set points are selected by switch SWl. Switches SW2, SW3 and SW4 are all ganged together for the selection of the 0-250 or the 0-500 ppm oxygen range.
Figure 3 also illustrates a circuit diagram of a sui-table high-low oxygen limit indicator shown at 22 in Figure 1.
The output of the probe is applied to the inverting input termi-nal of a voltage comparator VC2 and to the non-inverting input terminal of a voltage comparator VC3. The high limit for volta-ge comparator VC2 is set by a potentiometer R28 connected across the 15V reference source whereas the low limit for voltage compa-rator VC3 is set by a potentiometer R30 also connected to the 15V
reference source. The output of voltage comparator VC2 is con-nected to the base of a transistor TR4 through a resistor R32.
- 10 - ~ 287~
A resistor R33 is also connected between the emitter and b,ase of transistor TR4. A pilot light PL2 is connected in series with the emitter and collector electrodes of transistor TR4 across a 12V reference source, ~henever t~e upper oxygen limit is excee-ded, voltage comparator VC2 provides an output current to the base of transistor TR4 through resistor R32 to render the tran-sistor conductive and operate the pilot light PL2.
The output of voltage comparator VC3 is applied to the base of a transistor TR5 through a resistor R34. A resistor R35 is also connected between the emitter and base of transistor TR5.
A pilot light PL3 is connected in series with the emitter and collector electrodes of transistor TR5 across the 12V reference source. Whenever the lower oxygen limit is reached, voltage com-parator VC3 provides an output current to the base of transistor TR5 through resistor R34 to render transistor TR5 conductive and operate the pilot light PL3. The set points of resistors R28 and R30 may be checked on the readout device through switch SW5.
Figure 3 also discloses the circuit diagram of a suita-ble antilog conversion circuit which is shown schematically at 24 in Figure 1. In this embodiment, a Teledyne Phillick No.
4351 logarithmic amplifier has been used although other antilog modules could be used. In this amplifier, designated by refe-rence LA, the input decade selected goes from 0.1 to l.OV. In order to match a decade of say 50-500 ppm oxygen in the molten metal (which gives a corresponding voltage value of 546-430 mV
at the output of a probe as disclosed in the above mentioned Ca-nadian Patent No. 220,352 and shown in Figure 61 an operational amplifier OP3 is used to amplify the output of the probe and to 2~7~
match it to the decade input of the antilog module. Theoutput of the probe as it appears at the output of the high impedance fol-low and hold circuit is fed to the inverting input terminal of operational amplifier OP3 t~rough a resistor R36. Avariable re-sistor R37 is connected across the invertinginput terminal and the output terminal of the operational amplifier OP3 to provide adjustment for the required amplification. Acapacitor C5 is con-nected across resistor R37for filtering transients. Apotentiome-ter R38 is connected acrossa -15V reference source and has its variable tap connected to the inverting terminalof the operatio-nal amplifier through aresistor R39 toprovide the requiredoffset for theoperational amplifier. Theantilog module LAis also provi-ded with atrimming resistor R40which is connectedbetween the +15V
referencesource and the -15Vreference source tomake the final ad-justments.
When the above logarithmic amplifier is operated in the input range of 0.1 to lV, its linear output voltage varies from -10 to -79.4 mV. The recorder used in the present invention ope-rates in the range of 0-1000 mV and it is thus required to ampli-fy the output voltage of the logarithmic amplifier by a predeter-mined factor so that the maximum output going to the recorder will be 1000 mV. This amplification is done by an operational amplifier OP4. Resistors R41 and R42 are selected in the ratio of 1:2 and are selectively engaged by switch SW4, depending on the ppm oxygen range being measured, to connect the output of the logarithmic amplifier LA to the inverting input terminal of the operational amplifier OP4. Operational amplifier oP4 is al-so used to adjust the offset so that the output of the operatio-nal amplifier will be 0-lV for a measured oxygen range of 0-250 -- 12 ~ 2~77 or 0-500 ppm. This offset is regulated by a potentiometer R43 which is connected between ground and the-+lSV reference source and has its variable tap connected to the non-inverting input terminal of the operational amplifier through a resistor R44. A
variable resistor R45 is connected across the inverting input terminal and the output terminal of the operational amplifier OP4 in order to provide means for final gain adjustment. Capacitors C6-C9 of incremental capacitance value are connected across the resistor R45 for providing an averaging effect and filtering of the transients passing through the instrument. The proper capa-citor is selected by selector switch SW5.
As it will be obviously seen from the above descrip-tion, the oxygen probe is loaded at regular intervals with re-sistor R8. If any significant increase in the impedance of the probe occurs, this will be detected and shown by the probe failu-re indicator as an indication of impending probe failure. During loading of the probe, the previous ppm oxygen level is held by the high impedance follow and hold circuit. The loading time in-terval is very short (i.e. about 1 second) as compared to the intervals between loadings (i.e. about five minutes).
The logarithmic output of the probe is converted to a linear function by the antilog conversion circuit so that it may be easy to read on a linear scale readout device.
The monitoring circuit is also conveniently provided with a high-low oxygen limit indicator as well as an adjustable reference source for calibrating the antilog conversion circuit.
Although the invention has been disclosed with referen-ce to a preferred embodiment~ it is to be understood that it is ~ - 13 -2~77 not limited to such embodiment. For example, other types of high impedance follow and hold circuit, readout unit, programmer and probe failure ind;cator could be used.
~ ,.
the pens of the recorder.
Figure 3 illustrates a circuit diagram of the voltage reference source 28 shown in Figure 1. Potentiometers R20 and R22 are connected across a 15V reference source and used to pre-set upscale set points for the 0-25Q and 0-500 ppm oxygen ranges, respectively. Potentiometers R24 and R26 are also connected to the 15V reference source and used to preset downscale set points for the 0-250 and 0-500 ppm oxygen ranges, respectively. The above set points are used for periodic checking of the antilog conversion circuit. A convenient value for a downscale set point might be 546 mV corresponding to 50 ppm oxygen in the mol-ten metal as measured by an oxygen probe as disclosed in the above mentioned Canadian Patent No. 990,352 the output of which is shown in Figure 5. Similarly, a convenient value for an up-scale set point may be 475 mV corresponding to 200 ppm oxygen.
The chosen set points are selected by switch SWl. Switches SW2, SW3 and SW4 are all ganged together for the selection of the 0-250 or the 0-500 ppm oxygen range.
Figure 3 also illustrates a circuit diagram of a sui-table high-low oxygen limit indicator shown at 22 in Figure 1.
The output of the probe is applied to the inverting input termi-nal of a voltage comparator VC2 and to the non-inverting input terminal of a voltage comparator VC3. The high limit for volta-ge comparator VC2 is set by a potentiometer R28 connected across the 15V reference source whereas the low limit for voltage compa-rator VC3 is set by a potentiometer R30 also connected to the 15V
reference source. The output of voltage comparator VC2 is con-nected to the base of a transistor TR4 through a resistor R32.
- 10 - ~ 287~
A resistor R33 is also connected between the emitter and b,ase of transistor TR4. A pilot light PL2 is connected in series with the emitter and collector electrodes of transistor TR4 across a 12V reference source, ~henever t~e upper oxygen limit is excee-ded, voltage comparator VC2 provides an output current to the base of transistor TR4 through resistor R32 to render the tran-sistor conductive and operate the pilot light PL2.
The output of voltage comparator VC3 is applied to the base of a transistor TR5 through a resistor R34. A resistor R35 is also connected between the emitter and base of transistor TR5.
A pilot light PL3 is connected in series with the emitter and collector electrodes of transistor TR5 across the 12V reference source. Whenever the lower oxygen limit is reached, voltage com-parator VC3 provides an output current to the base of transistor TR5 through resistor R34 to render transistor TR5 conductive and operate the pilot light PL3. The set points of resistors R28 and R30 may be checked on the readout device through switch SW5.
Figure 3 also discloses the circuit diagram of a suita-ble antilog conversion circuit which is shown schematically at 24 in Figure 1. In this embodiment, a Teledyne Phillick No.
4351 logarithmic amplifier has been used although other antilog modules could be used. In this amplifier, designated by refe-rence LA, the input decade selected goes from 0.1 to l.OV. In order to match a decade of say 50-500 ppm oxygen in the molten metal (which gives a corresponding voltage value of 546-430 mV
at the output of a probe as disclosed in the above mentioned Ca-nadian Patent No. 220,352 and shown in Figure 61 an operational amplifier OP3 is used to amplify the output of the probe and to 2~7~
match it to the decade input of the antilog module. Theoutput of the probe as it appears at the output of the high impedance fol-low and hold circuit is fed to the inverting input terminal of operational amplifier OP3 t~rough a resistor R36. Avariable re-sistor R37 is connected across the invertinginput terminal and the output terminal of the operational amplifier OP3 to provide adjustment for the required amplification. Acapacitor C5 is con-nected across resistor R37for filtering transients. Apotentiome-ter R38 is connected acrossa -15V reference source and has its variable tap connected to the inverting terminalof the operatio-nal amplifier through aresistor R39 toprovide the requiredoffset for theoperational amplifier. Theantilog module LAis also provi-ded with atrimming resistor R40which is connectedbetween the +15V
referencesource and the -15Vreference source tomake the final ad-justments.
When the above logarithmic amplifier is operated in the input range of 0.1 to lV, its linear output voltage varies from -10 to -79.4 mV. The recorder used in the present invention ope-rates in the range of 0-1000 mV and it is thus required to ampli-fy the output voltage of the logarithmic amplifier by a predeter-mined factor so that the maximum output going to the recorder will be 1000 mV. This amplification is done by an operational amplifier OP4. Resistors R41 and R42 are selected in the ratio of 1:2 and are selectively engaged by switch SW4, depending on the ppm oxygen range being measured, to connect the output of the logarithmic amplifier LA to the inverting input terminal of the operational amplifier OP4. Operational amplifier oP4 is al-so used to adjust the offset so that the output of the operatio-nal amplifier will be 0-lV for a measured oxygen range of 0-250 -- 12 ~ 2~77 or 0-500 ppm. This offset is regulated by a potentiometer R43 which is connected between ground and the-+lSV reference source and has its variable tap connected to the non-inverting input terminal of the operational amplifier through a resistor R44. A
variable resistor R45 is connected across the inverting input terminal and the output terminal of the operational amplifier OP4 in order to provide means for final gain adjustment. Capacitors C6-C9 of incremental capacitance value are connected across the resistor R45 for providing an averaging effect and filtering of the transients passing through the instrument. The proper capa-citor is selected by selector switch SW5.
As it will be obviously seen from the above descrip-tion, the oxygen probe is loaded at regular intervals with re-sistor R8. If any significant increase in the impedance of the probe occurs, this will be detected and shown by the probe failu-re indicator as an indication of impending probe failure. During loading of the probe, the previous ppm oxygen level is held by the high impedance follow and hold circuit. The loading time in-terval is very short (i.e. about 1 second) as compared to the intervals between loadings (i.e. about five minutes).
The logarithmic output of the probe is converted to a linear function by the antilog conversion circuit so that it may be easy to read on a linear scale readout device.
The monitoring circuit is also conveniently provided with a high-low oxygen limit indicator as well as an adjustable reference source for calibrating the antilog conversion circuit.
Although the invention has been disclosed with referen-ce to a preferred embodiment~ it is to be understood that it is ~ - 13 -2~77 not limited to such embodiment. For example, other types of high impedance follow and hold circuit, readout unit, programmer and probe failure ind;cator could be used.
~ ,.
Claims (10)
1. An oxygen monitoring circuit adapted for connec-tion to an oxygen probe immersed in molten metal comprising:
a) a high impedance follow and hold circuit adapted for connection to the oxygen probe;
b) a readout unit connected to the high impedance follow and hold circuit;
c) a programmer normally connecting said oxygen probe to said high impedance follow and hold circuit but momentarily disconnecting the oxygen probe from said high impedance follow and hold circuit for a relatively short time interval and loa-ding it with a resistor having an impedance relatively higher than the normal impedance of the probe, and d) a probe failure indicator connected to the output of said oxygen probe and to the output of said high impedance follow and hold circuit for detecting any significant change in impedance of the probe and resistor combination as a result of loading and for operating an alarm when the impedance of the probe exceeds a predetermined value.
a) a high impedance follow and hold circuit adapted for connection to the oxygen probe;
b) a readout unit connected to the high impedance follow and hold circuit;
c) a programmer normally connecting said oxygen probe to said high impedance follow and hold circuit but momentarily disconnecting the oxygen probe from said high impedance follow and hold circuit for a relatively short time interval and loa-ding it with a resistor having an impedance relatively higher than the normal impedance of the probe, and d) a probe failure indicator connected to the output of said oxygen probe and to the output of said high impedance follow and hold circuit for detecting any significant change in impedance of the probe and resistor combination as a result of loading and for operating an alarm when the impedance of the probe exceeds a predetermined value.
2. An oxygen monitoring circuit as defined in claim 1, wherein said high impedance follow and hold circuit comprises a high impedance voltage follower adapted to hold the last mea-sured output voltage of the oxygen probe corresponding to the oxygen level in the molten metal during the time that the oxygen probe is disconnected from the high impedance follow and hold circuit.
3. An oxygen probe monitoring circuit as defined in claim 1, wherein said programmer includes a timing circuit nor-mally providing an output voltage of predetermined value but mo-mentarily switching said voltage to zero during the relatively short time interval when the probe is loaded with said resistor, a first relay which is switched on by said timing circuit when its output is high for operating a set of normally open contacts to connect said oxygen probe to said high impedance follow and hold circuit and which is switched off by said timing circuit when its output is zero to disconnect the oxygen probe from the high impedance follow and hold circuit, and a second relay which is switched on by said timing circuit when its output is zero for connecting said high impedance load to the oxygen probe.
4. An oxygen monitoring circuit as defined in claim 3, further comprising means for delaying the operation of said second relay for a short time interval after the output of said timing circuit has switched to zero so as to make sure that the high impedance follow and hold circuit is disconnected from the oxygen probe before loading the probe with said resistor.
5. An oxygen monitoring circuit as defined in claim 1, wherein said probe failure indicator comprises a high impedan-ce voltage follower connected to said oxygen probe, a voltage comparator connected to the output of said voltage follower and to the output of the follow and hold circuit for comparing the loaded and unloaded probe voltages, a switching device connected to the output of said voltage comparator, and an indicator devi-ce connected to said switching device for providing an alarm when the loaded probe voltage falls below a predetermined frac-tion of the unloaded probe voltage.
6. An oxygen monitoring circuit as defined in claim 1, further comprising a high-low oxygen limit indicator connec-ted to the output of said high impedance follow and hold circuit for indicating when high or low ppm oxygen limits are reached.
7. An oxygen monitoring circuit as defined in claim 1, wherein the output voltage of the oxygen probe is a logarith-mic function of the oxygen content of the molten metal, and fur-ther comprising an antilog conversion circuit interconnecting said high impedance follow and hold circuit to said readout de-vice for converting the normal logarithmic output of the oxygen probe to a linear output.
8. An oxygen monitoring circuit as defined in claim 7, further comprising a variable voltage reference source for testing said antilog conversion circuit.
9. An oxygen monitoring circuit as defined in claim 1, further comprising a thermocouple probe immersed in the molten metal, a thermocouple signal conditioner connected to said ther-mocouple probe, and means for connecting the output of said ther-mocouple conditioner to said readout device.
10. An oxygen monitoring circuit as defined in claim 9, wherein said readout device is a two pen recorder for recor-ding both temperature and the oxygen content of the molten metal.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA321,846A CA1102877A (en) | 1979-02-16 | 1979-02-16 | Oxygen monitoring circuit |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA321,846A CA1102877A (en) | 1979-02-16 | 1979-02-16 | Oxygen monitoring circuit |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1102877A true CA1102877A (en) | 1981-06-09 |
Family
ID=4113580
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA321,846A Expired CA1102877A (en) | 1979-02-16 | 1979-02-16 | Oxygen monitoring circuit |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1102877A (en) |
-
1979
- 1979-02-16 CA CA321,846A patent/CA1102877A/en not_active Expired
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