Smart determination of dissolved oxygen probe operating bias

Details Smart determination of dissolved oxygen probe operating bias Smart determination of dissolved oxygen probe operating bias

2001-12-19

2004-07-13

Olsen, Kaj K. (Department: 1753)

Electrolysis: processes, compositions used therein, and methods

Electrolytic analysis or testing

For oxygen or oxygen containing compound

C204S401000, C204S415000

Reexamination Certificate

active

06761817

ABSTRACT:

TECHNICAL FIELD
The present invention generally relates to electrochemical sensing techniques and devices thereof. The present invention also relates to techniques and devices thereof for measuring the concentration of gases in fluids. The present invention additionally relates to oxygen probes for measuring dissolved oxygen in fluids.
BACKGROUND OF THE INVENTION
The concentration of dissolved oxygen (DO) in water-based systems is important either to assure adequate oxygen level in natural waters or to assure low dissolved oxygen in processes where dissolved oxygen can be expected to be corrosive. Dissolved oxygen is generally measured with an amperometric electrochemical sensor, whose signal is proportional to oxygen partial pressure. Because dissolved oxygen concentration is directly proportional to oxygen partial pressure at constant temperature, with a secondary measurement of water temperature, dissolved oxygen concentration in suitable units of measurement can be readily determined.
In a conventional oxygen probe, a thin gas-permeable membrane is utilized to isolate the water sample and the electrochemical cell. The electrochemical cell consists of at least two electrodes, one of which can be located internal to the thin gas-permeable membrane. This electrode is controlled by suitable means at a relatively negative potential compared to the second electrode. This electrode is often referred to as a cathode. Two or more internal electrodes can be immersed in an ionically conducting electrolyte. In operation, oxygen diffuses through the membrane from the sample side to the cathode, where it is electrochemically reduced to water. Hence, the oxygen partial pressure is zero at the membrane cathode interface and the difference in partial pressure across the membrane determines the oxygen flux. Measurement of probe current is directly proportional to membrane flux and oxygen partial pressure; with temperature measurement, probe current is suitably temperature compensated to allow for dissolved oxygen concentration computation and display.
One implementation of a dissolved oxygen probe is described in U.S. Pat. No. 4,076,596 to Connery et al., which is incorporated herein by reference. Connery et al. describes an apparatus for electrolytically determining a species in a fluid, including a method of use thereof.
Connery et al. generally describe an electrolytic cell for measuring the concentration of a species, such as oxygen. Depositing closed-spaced interleaved inert electrode surfaces on the surface of an insulating substrate and covering the electrode surfaces with a thin film of electrolyte and permeable membrane can construct the electrolytic cell. The electrolyte can be selected so the species being measured is generated at one electrode surface and consumed at the other with no net reaction in the electrolyte. Alternatively, closely winding two thin electrode wires about a cylindrical base and covering it with an electrolyte and a membrane may form a cylindrical configuration.
Dissolved oxygen can thus be measured in a liquid or fluid based on an amperometric sensor or probe in which oxygen gas from a measurement sample initially diffuses through a gas permeable membrane. Oxygen diffuses through the membrane into an electrolyte and is consumed at an electrode by electrochemical reduction to water at a working electrode, often referred to as a cathode. The chemical reaction that takes place can be represented by the following chemical formulation of equation (1):
O
2
+4H
+
+4
e→
2H
2
O  (1)
The driving force for oxygen diffusion through the membrane can be calculated by determining the difference in partial pressures across the membrane. In addition to that described in equation (1), a conventional dissolved oxygen (“DO”) probe can employ a chemically inert counter electrode, as described in U.S. Pat. No. 4,076,596. Such an oxygen probe, including later applications thereof, employ configurations in which the electrical current results from the reaction of equation (1) above passing through a companion counter electrode (or anode). This current is equal in magnitude to equation (1) but is of the opposite sign; and, hence, the reverse chemical reaction occurs at this counter electrode. The chemical reaction that can occur at this counter electrode can be represented by the following formulation of equation (2):
2H
2
O→O
2
+4H
+
+4
e
  (2)
It is readily apparent that the sum of equations (1) and (2) does not correspond to any net chemical reaction. Because a net reaction does not result, reagents are not consumed. This method substantially reduces possible contributions of parameters that influence permeability, such as measurement sample stirring and membrane fouling. The electron flow illustrated by equations (1) and (2) above is thus directly proportional to oxygen partial pressure, which in turn is directly proportional to the oxygen concentration.
FIG. 1
depicts a prior art graph
10
illustrating normal dissolved oxygen probe operation at a controlled potential. Graph
10
illustrates half of the electrochemical fingerprint of oxygen dissolved in a conductive electrolyte within the DO probe. Only positive currents, corresponding to electrochemical reduction reactions are generally illustrated in FIG.
1
. Because the actual probe current is a function of electrode geometry, current values are not displayed on the Y-axis
12
in graph
10
.
FIG. 1
indicates that DO can be reduced to water at potentials more negative than approximately −0.1 V and that a potential window exists wherein current is independent of applied voltage. The curve
14
illustrated in
FIG. 1
has a characteristic sigmoid shape with a plateau region
16
, centered about −0.6 V as indicated by reference numeral
19
. This plateau region
16
corresponds to a limitation of reduction current because oxygen consumption is diffusion limited. As indicated at reference numeral
13
, limiting current is generally proportional to oxygen partial pressure. Line
17
generally in
FIG. 1
generally indicates a lower spec limit, while line
15
generally indicates an upper spec limit.
Because of the current limitation, it should be clear that probe current has little to no dependence on a reference electrode bias voltage as long as the controlled potential is located near the middle of the wave's plateau. At all bias voltages at which oxygen is reducible to water, measured current is generally directly proportional to oxygen partial pressure. At potentials more negative than approximately −0.9 V, sufficient energy is available to electrochemically reduce water to hydrogen gas. The slope
18
of the water reduction is quite steep, reflecting the large relative concentration of water. Thus, no diffusion limiting current is observed, based on graph
10
of FIG.
1
.
FIG. 2
illustrates a prior art schematic diagram illustrating a DO probe
20
in controlled potential mode.
FIG. 2
thus depicts a simplified representation of DO measurement electronics. A working electrode or cathode
23
of the DO probe
20
is generally connected to a transconductance amplifier
22
in which input voltage is maintained at a signal common potential and the amplifier output
29
is generally proportional to the current. Because node
25
is generally connected to a negative input of amplifier
22
, this can result in the formulation E
o
=−R
f
I
in
(&agr; pO
2
) at amplifier output
29
. Amplifier
22
is generally configured electronically in parallel with a resistor
25
, labeled R
f
in FIG.
2
. The remaining amplifiers
24
and
26
indicated in
FIG. 2
are composed of a reference electrode buffer (i.e., amplifier
24
) and a control amplifier (i.e., amplifier
26
). Amplifier
26
is generally connected to a voltage regulator
27
. Amplifier
26
maintains the reference electrode +0.6 V above the signal common potential. Namely, in control, cathode
23
is at −0.6 V compared to the reference electrode voltage, cons

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