CA1148218A - Apparatus for oxygen partial pressure measurement - Google Patents
Apparatus for oxygen partial pressure measurementInfo
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- CA1148218A CA1148218A CA000361917A CA361917A CA1148218A CA 1148218 A CA1148218 A CA 1148218A CA 000361917 A CA000361917 A CA 000361917A CA 361917 A CA361917 A CA 361917A CA 1148218 A CA1148218 A CA 1148218A
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- 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
- G01N27/49—Systems involving the determination of the current at a single specific value, or small range of values, of applied voltage for producing selective measurement of one or more particular ionic species
Abstract
ABSTRACT OF THE DISCLOSURE
Disclosed is apparatus and method for continuously com-pensating for electrode drift during the measurement of the par-tial pressure of oxygen by the net charge transport technique.
The apparatus derives a correction factor from variations in wave-forms representing charge returned from an electrochemical cell after successive interrogating voltage pulses. These waveforms are independent of oxygen partial pressure hut dependent on elec-trode parameters thereby permitting drift to be monitored and oxygen partial pressure measurements to be corrected.
Disclosed is apparatus and method for continuously com-pensating for electrode drift during the measurement of the par-tial pressure of oxygen by the net charge transport technique.
The apparatus derives a correction factor from variations in wave-forms representing charge returned from an electrochemical cell after successive interrogating voltage pulses. These waveforms are independent of oxygen partial pressure hut dependent on elec-trode parameters thereby permitting drift to be monitored and oxygen partial pressure measurements to be corrected.
Description
This invention relates to the measurement of the par- -tial pressure oE oxygen and more particularly to apparatus and me-thod for continuously compensating for oxygen electrode drift.
Typical oxygen sensors consist of an anode and a cath-ode immersed in an electrolyte. The electrodes and electrolyte are contained within a membrane which blocks passage of the elec-trolyte but which allows molecular oxygen to pass through freely.
One important oxygen sensor measures the oxygen which perfuses through the skin. In operation, such a transcutaneous sensor is placed against skin, for e~ample, the wrist, which has ~een heated to cause hyperemia within the underlying capillaries. The in- i creased ~lood flow elevates the capillary ~lood oxygen partial pressure to a level approaching that of arterial ~lood, Thus, the oxygen which perfuses from the capillaries through the heated skin gives an indication of the oxygen partial pressure of arter-ial blood, Successive electrical voltage pulses are applied across the electrodes of such oxygen sensors thereby causing current to flow via three mechanisms. The first mechanism is the ion-elec-tron transfer within the electrolyte. The second is the current flow associated with charging the so-calIed double layer at the electrode electrolyte interface, This dou~le layer may thus ~e thought of as a~ting as an electrical capacitor. The third mech-anism, the one of interest, is current flow associated with the reduction of molecular oxygen. Thus, only part of total charge transferred to the cell during a voltage pulse is a function of the concentration of o~ygen within the electrolyte. After the pulse, charge is returned from the cell, the amount of charge so returned being nearly independent of the oxygen partial pressure.
1 The charge returned from the cell arises primarily from discharge of the double layer. Because the charge returned from the cell is nearly independent of oxygen concentration whereas the charge delivered to the cell is so dependent, the di-Eerence is propor-tional to the partial pressure of oxygen (Po2) in solution. The use of this difference in inferring P2 is known as the net charge transfer technique. This difference between the charge delivered to and returned from the cell, however, is still su~-ject to the very serious problem of oxygen electrode drift.
Charges in the amount o~ charge returned from the cell from pulse to pulse thus indicate changes in the electrodes themselves which give rise to drift, since the charge so returned in nearly inde-pendent of the quantity to be measured--the partial pressure of oxygen in solution.
The drift or "aging" associated with oxygen electrodes has several origins. One cause of drift is the precipltatio~ of insoluble salts on the electrode surfaces which reduce their effective area. ~nother cause is the attraction of large protein molecules to the cathode. Although considera~,le effect has ~een
Typical oxygen sensors consist of an anode and a cath-ode immersed in an electrolyte. The electrodes and electrolyte are contained within a membrane which blocks passage of the elec-trolyte but which allows molecular oxygen to pass through freely.
One important oxygen sensor measures the oxygen which perfuses through the skin. In operation, such a transcutaneous sensor is placed against skin, for e~ample, the wrist, which has ~een heated to cause hyperemia within the underlying capillaries. The in- i creased ~lood flow elevates the capillary ~lood oxygen partial pressure to a level approaching that of arterial ~lood, Thus, the oxygen which perfuses from the capillaries through the heated skin gives an indication of the oxygen partial pressure of arter-ial blood, Successive electrical voltage pulses are applied across the electrodes of such oxygen sensors thereby causing current to flow via three mechanisms. The first mechanism is the ion-elec-tron transfer within the electrolyte. The second is the current flow associated with charging the so-calIed double layer at the electrode electrolyte interface, This dou~le layer may thus ~e thought of as a~ting as an electrical capacitor. The third mech-anism, the one of interest, is current flow associated with the reduction of molecular oxygen. Thus, only part of total charge transferred to the cell during a voltage pulse is a function of the concentration of o~ygen within the electrolyte. After the pulse, charge is returned from the cell, the amount of charge so returned being nearly independent of the oxygen partial pressure.
1 The charge returned from the cell arises primarily from discharge of the double layer. Because the charge returned from the cell is nearly independent of oxygen concentration whereas the charge delivered to the cell is so dependent, the di-Eerence is propor-tional to the partial pressure of oxygen (Po2) in solution. The use of this difference in inferring P2 is known as the net charge transfer technique. This difference between the charge delivered to and returned from the cell, however, is still su~-ject to the very serious problem of oxygen electrode drift.
Charges in the amount o~ charge returned from the cell from pulse to pulse thus indicate changes in the electrodes themselves which give rise to drift, since the charge so returned in nearly inde-pendent of the quantity to be measured--the partial pressure of oxygen in solution.
The drift or "aging" associated with oxygen electrodes has several origins. One cause of drift is the precipltatio~ of insoluble salts on the electrode surfaces which reduce their effective area. ~nother cause is the attraction of large protein molecules to the cathode. Although considera~,le effect has ~een
2~ devoted to minimizing drift, its elimination has not ~een achieved.
Heretofore, such electrode drift has necessitated fre~uent instru-ment calibration and recali~ration, greatly reducing the utility of measuring the partial pressure o oxygen using an electrochem-ical cell.
~ t is an object of the present invention, therefore, to provide apparatus and method for continuously compensating for electrode drift in the net charge transport technique for de-ter-mining oxygen partial pressure.
SUMMARY OF THE INVENTION
.... _ .. . .
The apparatus disclosed herein compensates for electrode z~
1 drift in the net charge transport technique for determining the partial pressure of oxygen in solu-tion, This technique comprises providing an electrochemical cell having an anode and a cathode immersed in an electrolyte and disposed for contact with oxygen~
Successive voltage pulses are applied across the cell causing current to flow. E'or each pulse, the difference ~etween the a-mount of charge delivered to the cell during the pulse and the amount of charge returned from the cell after the pulse is deter-mined. This charge difference indicates an uncorrected value of 1~ the oxygen partial pressure.
The apparatus disclosed herein compensates for electrode drift hy multiplying the charge difference ~y a function of a correction factor derived from the variation ~etween a first transient waveform representing the charge returned from the cell as a function of time after a first pulse and a second transient waveform representing the charge returned.from the cell as a function of time after a succeeding pulse.
In a preferred embodiment, the correction factor i5 the percent change equal to the quotient of the amount of charge re-2~ turned from the cell after a first pulse minus the amount ofcharge returned from the cell after a succeeding pulse divided ~y the amount of charge returned from the cell after the first pulse.
The function of the correction factor ~hich multiplies the charge difference to give the corrected value.of oxygen partial pressure is one plus the correction factor. In this em~odiment, the a-mount of charge deliYered to the cell during one of the pulse is determined ~y integrating the current waveform for the duration of the pulse and the amount of charge returned from the cell after the pulse is determined ~y integrating the current waveform from
Heretofore, such electrode drift has necessitated fre~uent instru-ment calibration and recali~ration, greatly reducing the utility of measuring the partial pressure o oxygen using an electrochem-ical cell.
~ t is an object of the present invention, therefore, to provide apparatus and method for continuously compensating for electrode drift in the net charge transport technique for de-ter-mining oxygen partial pressure.
SUMMARY OF THE INVENTION
.... _ .. . .
The apparatus disclosed herein compensates for electrode z~
1 drift in the net charge transport technique for determining the partial pressure of oxygen in solu-tion, This technique comprises providing an electrochemical cell having an anode and a cathode immersed in an electrolyte and disposed for contact with oxygen~
Successive voltage pulses are applied across the cell causing current to flow. E'or each pulse, the difference ~etween the a-mount of charge delivered to the cell during the pulse and the amount of charge returned from the cell after the pulse is deter-mined. This charge difference indicates an uncorrected value of 1~ the oxygen partial pressure.
The apparatus disclosed herein compensates for electrode drift hy multiplying the charge difference ~y a function of a correction factor derived from the variation ~etween a first transient waveform representing the charge returned from the cell as a function of time after a first pulse and a second transient waveform representing the charge returned.from the cell as a function of time after a succeeding pulse.
In a preferred embodiment, the correction factor i5 the percent change equal to the quotient of the amount of charge re-2~ turned from the cell after a first pulse minus the amount ofcharge returned from the cell after a succeeding pulse divided ~y the amount of charge returned from the cell after the first pulse.
The function of the correction factor ~hich multiplies the charge difference to give the corrected value.of oxygen partial pressure is one plus the correction factor. In this em~odiment, the a-mount of charge deliYered to the cell during one of the pulse is determined ~y integrating the current waveform for the duration of the pulse and the amount of charge returned from the cell after the pulse is determined ~y integrating the current waveform from
3~ the end of the pulse for a time equal to that of the first inte-gration, 1 BRIEF DES~RIPTION O~ T~IE DRAWINGS
The invention disclosed herein may ~e ~etter under-stood with reference to the following drawing of which: -Fig. 1 is a conceptual representation of a transcuta-neous oxygen sensor;
Fig. 2 is a graphical representation of a polarizing pulse produced by the electronic instrumentation of Fig. l;
Fig. 3 is a graphical representation of the ~aveform of current through the electrochemical cell;
1~ Fig. 4 is a block diagram of the oxygen sensing system - according to the present invention; and Fig, 5 is a block diagram representing an analog imple-mentation of the present invention, DESCRIPTI~N OF THE PREFER~ED EMBODIMENTS
.
Figure 1 depicts the concept of detecting the partial pressure of o~ygen transcutaneously~ Oxygen sensor 10 is~disposed so that membrane 11 is in co~tact with skin 12, The membrane 11 allows oxygen which has perfused through the skin 12 to enter the sensor chamber 13. A suitable mem~rane is hydroxypropyl meth-acrylate~ Within the c~amber 13 are an anode 14 and a cathode15 immersed in an electrolyte such as a buffered potassium chlor-ide solution. A suita~le cathode is made of gold and a suitable anode is maae of-a silver-silver chloride composition. It is thus seen that the oxygen sensor 10 is an electrochemical ce~l.
Also included in typical sensors ~ut not shown here~ are a heater and a thermistor for measuring temperature. The heater heats the skin to aid oxygen perfusion. A suitable temperature is approx-imately 43C.
Still referring to Fig. 1, the sensor 10 is under the control of an electronic instrumentation package 16. The instru-mentation package 16 has essentially three functions relating 1 specifically to the sensiny of the partial pressure of molecular oxygen. The first function is to control the temperature of the sensor to a constant value by a heating resistor-thermistor com-bination. The second function is to apply across the anode 1~
ancl cathode 15 square wave polarizing voltage pulses Vp as repre-sented in Fig, 2. The third function is to monitor and operate upon the waveforms representing the current flow through the cell lO ~oth during and after the polarizing pulse to produce a cor-rected value of the oxygen partial pressure. Figure 3 shows representative current flow waveforms in response to the polar-izing pulses Vp of Fig. 2.
Now specifically referring to Figs, 2 and 3, two repre-sentative voltage pulses Vp are shown having non-zero values only between times 0 and tl and t3 and t4. When one such pulse is applied across an electrochemical cell disposed for contact with molecular oxygen in solution, a charging current I flows through the circuit representing the charge being delivered to the cell.
At time tl when the pulse ~p has become zero, the cell returns charge to the external circuit giving rise to the discharge cur-rent wa~eform shown between tl and t2, The charge delivered ~othe cell by the pulse Vp is, therefore, given hy ¦ I dt ;
the charge returned from the cell after Vp becomes zero is given by ~ I dt tl ~L~
1 where t2 = 2tl.
The net charge, which is proportional to the partial pressure of oxygen in solu-tion, Po2~ is thus (tl t2 P2 ~ ) I dt - ~ I dt .
tl As discussed hereinbefore, this measured value is su~-ject to error due to electrode drift resulting, for example, from electrode contamination which reduces its effective area. This drift can be detected ~y comparing the discharge waveforms (that is, the waveforms produced ~y charge being returned from the cell~ produced ~y successive pulses, As pointed out above, the discharge waveforms are substantially independent of the P2 level; in the a~sence of drift, the discharge waveform from tl to t2 would ~e virtually identical to the discharge waveform from t4 to t5 even if the P2 level had changed in the time ~etween the two pulses. If there is variation in the discharge waveforms between tl and t2 and t4 and t5, then there has ~een electrode drift. A correction factor derived from the variation in the discharge waveforms is employed to modify the measured value o~
Po2. A convenient variation ~etween discharge wavéforms for der-iving a correction factor is the percent change in the amount of charge returned ~y the cell for successive pulses. That is, a preferred correc~ion factor C may ~e expressed as I dt - ~ I dt t t4 C . ,, ~2 ) I dt tl With this correction factor, the corrected value of P2 at the time of the current pulse becomes P2 corrected ~ ~ol Idt ~ t~ 2Idt] ~l+C]
Fig. 4 is a block diagram of the overall eléctronic system according to the present invention which implements the correction techniques discussed above in conjunction with Figs. 2 and 3.
The control electronics comprises five major modules and two separate power units, A microcomputer, Texas Instruments 9900, includes a central processing unit 20 and an input-output ~I/O) interface 21 of conventianal design. The central process-ing unit 20 is a Texas Instruments 100/M ~oard. Alpha numeric display 22 is a self-scan display having a maximum capa~ility of 64 characters ~l6 characters by 4 lines), Input to the system is accomplished by means of a hex key~oard 23 having,a 16 key in-put pad, An electrode ~oard 24 carries the analog circuitry for interfacing with the oxygen sensing electrodes, In this case two sensing electrodes can ~e accommodated~ A conventional unisolated 1 power supply 25 is used to power allelectronic5 e~cept for the electrode ~oard 2~, The electrode ~oard 2~ is powered by a UL
approved isolated power supply 26 which has optical isolation on all digital lines to insure patient isolation. The overall elec-tronics system o~ Fig, ~ has ~een designed to exceed the require-ments of publishecl human patient safety standards currently in practice, The electrode board 24 of Fig, 4 applies the s~uare wave polarizing voltage pulses across the anode and cathode of the oxygen sensor or sensors and monitors the waveforms represent-ing the current flow through the circuit, This is done under the control of the central processing unit 20.
Specifically~ the microcomputer comprising the central processing unit 20 stores the overall program for operation o~
the oxysen measuring system, Several conventional su~routines implement the electrode drift correction procedure. With each polarizing pulse~ a S~MPLE routine samples ~oth the polarizing and depolarizing or discharge current waveforms and puts one hundred digitized values into BUFFER. An integration subroutine next calculates from the data stored in BUFFER the areas under the charging or polarizing waveform and under the depolarizing or discharging waveform! These areas represent the net charge trans-ferred to and returned fxom the electrochemical cell in response to a polarizing pulse, The difference in these two areas is pro-portional to the uncorrected value of the partial pressure of oxygen. This measured value of po~ along with the area under the discharging waveform corresponding to the present pulse next enters the correction subroutine, Already stored in this subroutine is the discharging waveform area from an earlier pulse which serves as a reference, The correc-tion algorit~m does two things. First, if the area under the discharging waveform for the present pulse .
1 differs from the corresponding area for the reference pulse by 10~ or more, this fact is displayed to the operator by display 22. If the dif~erence is less than 10%, this percentage change forms the preferred correction Eactor since any change in dis-charging waveform area results from electrode drift because the charge returned from the cell is nearly independent of oxygen partial pressure, The uncorrected value of oxygen partial pres-sure is then multiplied ~y this correction factor plus one to give the corrected measured value of the partial pressure of oxygen. Although the variation ~etween the amount of charge re-turned by the cell for successive pulses gives rise to the pre-ferred correction factor, it is to ~e understood that other indi-cators of waveform variation are contemplated to ~e ~ithin the scope of this invention, Although it is preferred that the computation of the corrected value of oxygen partial pressure be performed digitally as described a~ove, the invention disclosed herein may also ~e practiced using con~entional analog techni~ues.
Fig, 5 shows one such analog implementation, The signal from the oxygen electrode is first integrated ~y an integrator 30 to produce the uncorrected value of the oxygen partial pres-sure:
t(l (2 P2 = ~ I dt -) I dt , /
to ~1 where a voltage polarizing pulse is applied across the electrode during the interval from to to tl, A second integrator 31 inte-~ rates the signal from the electrodes only for the period after the pulse, tl to t2, when charge is being returned to the external 1 circuit. This value is held in sample and hold element 32. For the next polarizing pulse, the integrator 31 produces the charge returned to the external circuit for that next pulse. These two values representing the amount of charge returned to the external circuit after successive voltage pulses are subtracted in a sub-tractor 33. This difference is then divided by the first value stored in the element 32 by a dividing element 34, The output from the dividing element 34 is the correction factor C as de-fined hereinbefore, The output from the dividing element 34 multiplies the output from the integrator 30 in a multiplier 35.
The output from the multiplier 35 is then added to the output of the integrator 10 by an adder 36 to give the corrected value for the oxygen partial pressure.
It is thus seen that the invention disclosed herein pro~ides novel apparatus for continuously compensating for elec-trode drift in the net charge transport technique for determining the partial pressure of oxygen, -lQ-
The invention disclosed herein may ~e ~etter under-stood with reference to the following drawing of which: -Fig. 1 is a conceptual representation of a transcuta-neous oxygen sensor;
Fig. 2 is a graphical representation of a polarizing pulse produced by the electronic instrumentation of Fig. l;
Fig. 3 is a graphical representation of the ~aveform of current through the electrochemical cell;
1~ Fig. 4 is a block diagram of the oxygen sensing system - according to the present invention; and Fig, 5 is a block diagram representing an analog imple-mentation of the present invention, DESCRIPTI~N OF THE PREFER~ED EMBODIMENTS
.
Figure 1 depicts the concept of detecting the partial pressure of o~ygen transcutaneously~ Oxygen sensor 10 is~disposed so that membrane 11 is in co~tact with skin 12, The membrane 11 allows oxygen which has perfused through the skin 12 to enter the sensor chamber 13. A suitable mem~rane is hydroxypropyl meth-acrylate~ Within the c~amber 13 are an anode 14 and a cathode15 immersed in an electrolyte such as a buffered potassium chlor-ide solution. A suita~le cathode is made of gold and a suitable anode is maae of-a silver-silver chloride composition. It is thus seen that the oxygen sensor 10 is an electrochemical ce~l.
Also included in typical sensors ~ut not shown here~ are a heater and a thermistor for measuring temperature. The heater heats the skin to aid oxygen perfusion. A suitable temperature is approx-imately 43C.
Still referring to Fig. 1, the sensor 10 is under the control of an electronic instrumentation package 16. The instru-mentation package 16 has essentially three functions relating 1 specifically to the sensiny of the partial pressure of molecular oxygen. The first function is to control the temperature of the sensor to a constant value by a heating resistor-thermistor com-bination. The second function is to apply across the anode 1~
ancl cathode 15 square wave polarizing voltage pulses Vp as repre-sented in Fig, 2. The third function is to monitor and operate upon the waveforms representing the current flow through the cell lO ~oth during and after the polarizing pulse to produce a cor-rected value of the oxygen partial pressure. Figure 3 shows representative current flow waveforms in response to the polar-izing pulses Vp of Fig. 2.
Now specifically referring to Figs, 2 and 3, two repre-sentative voltage pulses Vp are shown having non-zero values only between times 0 and tl and t3 and t4. When one such pulse is applied across an electrochemical cell disposed for contact with molecular oxygen in solution, a charging current I flows through the circuit representing the charge being delivered to the cell.
At time tl when the pulse ~p has become zero, the cell returns charge to the external circuit giving rise to the discharge cur-rent wa~eform shown between tl and t2, The charge delivered ~othe cell by the pulse Vp is, therefore, given hy ¦ I dt ;
the charge returned from the cell after Vp becomes zero is given by ~ I dt tl ~L~
1 where t2 = 2tl.
The net charge, which is proportional to the partial pressure of oxygen in solu-tion, Po2~ is thus (tl t2 P2 ~ ) I dt - ~ I dt .
tl As discussed hereinbefore, this measured value is su~-ject to error due to electrode drift resulting, for example, from electrode contamination which reduces its effective area. This drift can be detected ~y comparing the discharge waveforms (that is, the waveforms produced ~y charge being returned from the cell~ produced ~y successive pulses, As pointed out above, the discharge waveforms are substantially independent of the P2 level; in the a~sence of drift, the discharge waveform from tl to t2 would ~e virtually identical to the discharge waveform from t4 to t5 even if the P2 level had changed in the time ~etween the two pulses. If there is variation in the discharge waveforms between tl and t2 and t4 and t5, then there has ~een electrode drift. A correction factor derived from the variation in the discharge waveforms is employed to modify the measured value o~
Po2. A convenient variation ~etween discharge wavéforms for der-iving a correction factor is the percent change in the amount of charge returned ~y the cell for successive pulses. That is, a preferred correc~ion factor C may ~e expressed as I dt - ~ I dt t t4 C . ,, ~2 ) I dt tl With this correction factor, the corrected value of P2 at the time of the current pulse becomes P2 corrected ~ ~ol Idt ~ t~ 2Idt] ~l+C]
Fig. 4 is a block diagram of the overall eléctronic system according to the present invention which implements the correction techniques discussed above in conjunction with Figs. 2 and 3.
The control electronics comprises five major modules and two separate power units, A microcomputer, Texas Instruments 9900, includes a central processing unit 20 and an input-output ~I/O) interface 21 of conventianal design. The central process-ing unit 20 is a Texas Instruments 100/M ~oard. Alpha numeric display 22 is a self-scan display having a maximum capa~ility of 64 characters ~l6 characters by 4 lines), Input to the system is accomplished by means of a hex key~oard 23 having,a 16 key in-put pad, An electrode ~oard 24 carries the analog circuitry for interfacing with the oxygen sensing electrodes, In this case two sensing electrodes can ~e accommodated~ A conventional unisolated 1 power supply 25 is used to power allelectronic5 e~cept for the electrode ~oard 2~, The electrode ~oard 2~ is powered by a UL
approved isolated power supply 26 which has optical isolation on all digital lines to insure patient isolation. The overall elec-tronics system o~ Fig, ~ has ~een designed to exceed the require-ments of publishecl human patient safety standards currently in practice, The electrode board 24 of Fig, 4 applies the s~uare wave polarizing voltage pulses across the anode and cathode of the oxygen sensor or sensors and monitors the waveforms represent-ing the current flow through the circuit, This is done under the control of the central processing unit 20.
Specifically~ the microcomputer comprising the central processing unit 20 stores the overall program for operation o~
the oxysen measuring system, Several conventional su~routines implement the electrode drift correction procedure. With each polarizing pulse~ a S~MPLE routine samples ~oth the polarizing and depolarizing or discharge current waveforms and puts one hundred digitized values into BUFFER. An integration subroutine next calculates from the data stored in BUFFER the areas under the charging or polarizing waveform and under the depolarizing or discharging waveform! These areas represent the net charge trans-ferred to and returned fxom the electrochemical cell in response to a polarizing pulse, The difference in these two areas is pro-portional to the uncorrected value of the partial pressure of oxygen. This measured value of po~ along with the area under the discharging waveform corresponding to the present pulse next enters the correction subroutine, Already stored in this subroutine is the discharging waveform area from an earlier pulse which serves as a reference, The correc-tion algorit~m does two things. First, if the area under the discharging waveform for the present pulse .
1 differs from the corresponding area for the reference pulse by 10~ or more, this fact is displayed to the operator by display 22. If the dif~erence is less than 10%, this percentage change forms the preferred correction Eactor since any change in dis-charging waveform area results from electrode drift because the charge returned from the cell is nearly independent of oxygen partial pressure, The uncorrected value of oxygen partial pres-sure is then multiplied ~y this correction factor plus one to give the corrected measured value of the partial pressure of oxygen. Although the variation ~etween the amount of charge re-turned by the cell for successive pulses gives rise to the pre-ferred correction factor, it is to ~e understood that other indi-cators of waveform variation are contemplated to ~e ~ithin the scope of this invention, Although it is preferred that the computation of the corrected value of oxygen partial pressure be performed digitally as described a~ove, the invention disclosed herein may also ~e practiced using con~entional analog techni~ues.
Fig, 5 shows one such analog implementation, The signal from the oxygen electrode is first integrated ~y an integrator 30 to produce the uncorrected value of the oxygen partial pres-sure:
t(l (2 P2 = ~ I dt -) I dt , /
to ~1 where a voltage polarizing pulse is applied across the electrode during the interval from to to tl, A second integrator 31 inte-~ rates the signal from the electrodes only for the period after the pulse, tl to t2, when charge is being returned to the external 1 circuit. This value is held in sample and hold element 32. For the next polarizing pulse, the integrator 31 produces the charge returned to the external circuit for that next pulse. These two values representing the amount of charge returned to the external circuit after successive voltage pulses are subtracted in a sub-tractor 33. This difference is then divided by the first value stored in the element 32 by a dividing element 34, The output from the dividing element 34 is the correction factor C as de-fined hereinbefore, The output from the dividing element 34 multiplies the output from the integrator 30 in a multiplier 35.
The output from the multiplier 35 is then added to the output of the integrator 10 by an adder 36 to give the corrected value for the oxygen partial pressure.
It is thus seen that the invention disclosed herein pro~ides novel apparatus for continuously compensating for elec-trode drift in the net charge transport technique for determining the partial pressure of oxygen, -lQ-
Claims (8)
1. Apparatus for the measurement of the partial pressure oxgyen level comprising:
1) an electrochemical cell having an anode and a cath-ode immersed in an electrolyte, said cell disposed for contact with oxygen;
2) means for applying successive voltage pulses across said cell;
3) means for determining for each of said pulses the difference between the amount of charge delivered to said cell during one of said pulses and the amount of charge returned from said cell after said one of said pulses, said difference indicat-ing an uncorrected value of said oxygen partial pressure; and 4) means for multiplying said difference by a function of a correction factor derived from variation between a first transient waveform representing the charge returned from said cell as a function of time after a first of said pulses and a second transient waveform representing the charge returned from said cell as a function of time after a succeeding one of said pulses.
1) an electrochemical cell having an anode and a cath-ode immersed in an electrolyte, said cell disposed for contact with oxygen;
2) means for applying successive voltage pulses across said cell;
3) means for determining for each of said pulses the difference between the amount of charge delivered to said cell during one of said pulses and the amount of charge returned from said cell after said one of said pulses, said difference indicat-ing an uncorrected value of said oxygen partial pressure; and 4) means for multiplying said difference by a function of a correction factor derived from variation between a first transient waveform representing the charge returned from said cell as a function of time after a first of said pulses and a second transient waveform representing the charge returned from said cell as a function of time after a succeeding one of said pulses.
2. The apparatus of claim 1 wherein said correction factor comprises the percent change equal to the quotient of the amount of charge returned from said cell after a first of said pulses minus the amount of charge returned from said cell after a succeed-ing one of said pulses divided by the amount of charge returned from said cell after said first pulse.
3. The apparatus of claim 2 wherein said function of said correction factor comprises one plus said correction factor.
4. The apparatus of claim 1 wherein said amount of charge delivered to said cell during one of said pulses is determined by integrating with respect to time the charging current waveform for a time equal to the duration of said pulse, and wherein said amount of charge returned from said cell after said one of said pulses is determined by integrating with respect to time the discharging current waveform for a time equal to the duration of said pulse.
5. In the continuous determination of the partial pressure of molecular oxygen in a medium comprising the steps of:
1) providing an electrochemical cell comprising an anode and a cathode immersed in an electrolyte and disposed for contact with said oxygen;
2) applying successive voltage pulses across said cell;
3) determining for each of said pulses the difference between the amount of charge delivered to said cell during one of said pulses and the amount of change returned from said cell after said one of said pulses, said difference indicating said partial pressure;
the method for correcting said difference to compensate for drift associated with said cell, said method comprising multiplying said difference by a function of a correc-tion factor derived from variation between a first transient wave-form representing the charge returned from said cell as a function of time after a first of said pulses and a second transient wave-form representing the charge returned from said cell as a function of time after a succeeding one of said pulses.
6. The method of claim 5 wherein said correction factor comprises the percent change equal to the quotient of the amount
1) providing an electrochemical cell comprising an anode and a cathode immersed in an electrolyte and disposed for contact with said oxygen;
2) applying successive voltage pulses across said cell;
3) determining for each of said pulses the difference between the amount of charge delivered to said cell during one of said pulses and the amount of change returned from said cell after said one of said pulses, said difference indicating said partial pressure;
the method for correcting said difference to compensate for drift associated with said cell, said method comprising multiplying said difference by a function of a correc-tion factor derived from variation between a first transient wave-form representing the charge returned from said cell as a function of time after a first of said pulses and a second transient wave-form representing the charge returned from said cell as a function of time after a succeeding one of said pulses.
6. The method of claim 5 wherein said correction factor comprises the percent change equal to the quotient of the amount
Claim 6 cont.
of charge returned from said cell after a first of said pulses minus the amount of charge returned from said cell after a suc-ceeding one of said pulses divided by the amount of charge re-turned from said cell after said first pulse.
of charge returned from said cell after a first of said pulses minus the amount of charge returned from said cell after a suc-ceeding one of said pulses divided by the amount of charge re-turned from said cell after said first pulse.
7. The method of claim 6 wherein said function of said correction factor comprises one plus said correction factor.
8. The method of claim 5 wherein said amount of charge delivered to said cell during one of said pulses is determined by integrating the current waveform for a time equal to the duration of said pulse, and wherein said amount of charge returned from said cell after said one of said pulses is determined by inte-grating the current waveform for a time equal to the duration of said pulse.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US080,257 | 1979-10-01 | ||
| US06/080,257 US4269684A (en) | 1979-10-01 | 1979-10-01 | Apparatus for oxygen partial pressure measurement |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1148218A true CA1148218A (en) | 1983-06-14 |
Family
ID=22156224
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000361917A Expired CA1148218A (en) | 1979-10-01 | 1980-10-01 | Apparatus for oxygen partial pressure measurement |
Country Status (8)
| Country | Link |
|---|---|
| US (1) | US4269684A (en) |
| JP (1) | JPS56100352A (en) |
| CA (1) | CA1148218A (en) |
| DE (1) | DE3036824A1 (en) |
| FR (1) | FR2466770A1 (en) |
| GB (1) | GB2059597B (en) |
| NL (1) | NL8005381A (en) |
| SE (1) | SE439548B (en) |
Families Citing this family (17)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3267802D1 (en) * | 1981-09-04 | 1986-01-16 | Hoffmann La Roche | Method and apparatus for the calibration of sensors |
| US4534356A (en) * | 1982-07-30 | 1985-08-13 | Diamond Shamrock Chemicals Company | Solid state transcutaneous blood gas sensors |
| US4536274A (en) | 1983-04-18 | 1985-08-20 | Diamond Shamrock Chemicals Company | pH and CO2 sensing device and method of making the same |
| US4459180A (en) * | 1983-05-16 | 1984-07-10 | General Signal Corporation | Method and means for compensating for IR voltage drop in electrochemical cells |
| US4457808A (en) * | 1983-05-16 | 1984-07-03 | General Signal Corporation | Method and means for recalibrating electrochemical cells in situ |
| NL8303792A (en) * | 1983-11-03 | 1985-06-03 | Cordis Europ | Apparatus provided with an measuring circuit based on an ISFET; ISFET SUITABLE FOR USE IN THE MEASURING CIRCUIT AND METHOD FOR MANUFACTURING AN ISFET TO BE USED IN THE MEASURING CIRCUIT |
| DE3429583A1 (en) * | 1984-08-10 | 1986-02-20 | Siemens AG, 1000 Berlin und 8000 München | METHOD FOR ELECTROCHEMICALLY DETERMINING THE OXYGEN CONCENTRATION |
| DE3506731A1 (en) * | 1985-02-26 | 1986-08-28 | Drägerwerk AG, 2400 Lübeck | METHOD AND DEVICE FOR CALIBRATING MEASURING VALUES FOR DETERMINING GAS CONCENTRATIONS |
| US4741343A (en) * | 1985-05-06 | 1988-05-03 | Massachusetts Institute Of Technology | Method and apparatus for measuring oxygen partial pressure and temperature in living tissue |
| US5007988A (en) * | 1987-09-08 | 1991-04-16 | Westinghouse Electric Corp. | Method for determining the concentration of a plurality of combustible gases in a stream |
| JPH03123847A (en) * | 1989-10-06 | 1991-05-27 | Fujitsu Ltd | Method for driving oxygen electrode |
| WO1991016624A1 (en) * | 1990-04-26 | 1991-10-31 | Hartmann & Braun Aktiengesellschaft | Process for determining gas concentrations and gas sensor with solid electrolyte |
| US5059908A (en) * | 1990-05-31 | 1991-10-22 | Capital Controls Company, Inc. | Amperimetric measurement with cell electrode deplating |
| SE9703958D0 (en) * | 1997-10-29 | 1997-10-29 | Pacesetter Ab | Method and device for determination of concentration |
| SE9703957D0 (en) | 1997-10-29 | 1997-10-29 | Pacesetter Ab | Method and device for sensing |
| US20090009039A1 (en) * | 2007-07-06 | 2009-01-08 | Beach Box Llc | Outdoor Storage Devices and Methods for Manufacturing the Same |
| US11099151B2 (en) | 2017-05-05 | 2021-08-24 | neoSense Technologies AB | Oxygen sensor |
Family Cites Families (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2766423A (en) * | 1951-09-08 | 1956-10-09 | Nat Res Dev | Polarographs |
| US3420764A (en) * | 1964-03-16 | 1969-01-07 | North American Rockwell | Electrochemical analysis system |
| US3678375A (en) * | 1969-09-24 | 1972-07-18 | Abbott Lab | Electronic apparatus for accurately monitoring and recording the partial pressure of a gas |
| US3794575A (en) * | 1972-10-30 | 1974-02-26 | Gen Electric | Oxygen sensor |
| US4048041A (en) * | 1976-05-24 | 1977-09-13 | The United States Of America As Represented By The Secretary Of The Army | Electronic system for providing specificity in an electrochemical analytical device |
| DE2632931C3 (en) * | 1976-07-22 | 1980-05-08 | Max-Planck-Gesellschaft Zur Foerderung Der Wissenschaften E.V., 3400 Goettingen | Polarographic oxygen measurement |
-
1979
- 1979-10-01 US US06/080,257 patent/US4269684A/en not_active Expired - Lifetime
-
1980
- 1980-09-23 GB GB8030674A patent/GB2059597B/en not_active Expired
- 1980-09-23 SE SE8006634A patent/SE439548B/en not_active IP Right Cessation
- 1980-09-29 NL NL8005381A patent/NL8005381A/en not_active Application Discontinuation
- 1980-09-30 DE DE19803036824 patent/DE3036824A1/en not_active Withdrawn
- 1980-09-30 JP JP13666880A patent/JPS56100352A/en active Pending
- 1980-09-30 FR FR8020932A patent/FR2466770A1/en active Granted
- 1980-10-01 CA CA000361917A patent/CA1148218A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| GB2059597B (en) | 1983-10-19 |
| US4269684A (en) | 1981-05-26 |
| NL8005381A (en) | 1981-04-03 |
| JPS56100352A (en) | 1981-08-12 |
| DE3036824A1 (en) | 1981-04-16 |
| FR2466770B1 (en) | 1984-12-21 |
| SE8006634L (en) | 1981-04-02 |
| SE439548B (en) | 1985-06-17 |
| GB2059597A (en) | 1981-04-23 |
| FR2466770A1 (en) | 1981-04-10 |
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