U.S. patent application number 14/763252 was filed with the patent office on 2015-12-10 for ph value measuring device comprising in situ calibration means.
This patent application is currently assigned to VEOLIA WATER SOLUTIONS & TECHNOLOGIES SUPPORT. The applicant listed for this patent is VEOLIA WATER SOLUTIONS & TECHNOLOGIES SUPPORT. Invention is credited to Carine BERIET, Yves DE COULON, Cyrille LEMOINE.
Application Number | 20150355134 14/763252 |
Document ID | / |
Family ID | 48771555 |
Filed Date | 2015-12-10 |
United States Patent
Application |
20150355134 |
Kind Code |
A1 |
DE COULON; Yves ; et
al. |
December 10, 2015 |
PH VALUE MEASURING DEVICE COMPRISING IN SITU CALIBRATION MEANS
Abstract
The invention concerns a device for measuring the pH of an
effluent, said device comprising means for measuring an item of
information representative of the pH of said effluent intended to
be brought into contact with said effluent. According to the
invention, such a device further comprises means for modifying the
pH value of said effluent close to said means for measuring.
Inventors: |
DE COULON; Yves;
(Thielle-Wavre, CH) ; BERIET; Carine; (Peseux,
CH) ; LEMOINE; Cyrille; (Sartrouville, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
VEOLIA WATER SOLUTIONS & TECHNOLOGIES SUPPORT |
Saint-Maurice Cedex |
|
FR |
|
|
Assignee: |
VEOLIA WATER SOLUTIONS &
TECHNOLOGIES SUPPORT
SAINT-MAURICE CEDEX
FR
|
Family ID: |
48771555 |
Appl. No.: |
14/763252 |
Filed: |
January 24, 2014 |
PCT Filed: |
January 24, 2014 |
PCT NO: |
PCT/EP2014/051454 |
371 Date: |
July 24, 2015 |
Current U.S.
Class: |
205/787.5 ;
204/416; 204/418; 204/433 |
Current CPC
Class: |
G01N 27/3335 20130101;
G01N 27/414 20130101; G01N 27/302 20130101; G01N 27/4165
20130101 |
International
Class: |
G01N 27/333 20060101
G01N027/333; G01N 27/416 20060101 G01N027/416; G01N 27/30 20060101
G01N027/30 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 25, 2013 |
FR |
1350666 |
Claims
1-15. (canceled)
16. Device for measuring the pH of an effluent, said device
comprising means for measuring a piece of information representing
the pH of said effluent that are to be put into contact with said
effluent, and means for modifying the value of the pH of said
effluent in proximity to said means for measuring, characterized in
that it comprises means for calibrating said device for measuring,
said means for calibrating being configured to calibrate the device
for measuring after modification of said pH of said effluent in
proximity to said means for measuring by said means for
modifying.
17. Device according to claim 16, characterized in that said means
for measuring comprise: an ISFET type transistor comprising a
source and a drain disposed on a substrate, and a gate that is to
be put into contact with said effluent; a reference electrode;
means for generating a constant voltage at the terminals of said
source and said drain; second means for generating a constant
electric current between said source and said drain; means for
measuring a control voltage V.sub.GS at the terminals of said
source and said reference electrode; means for determining the
value of the pH of said effluent as a function of the value of said
control voltage V.sub.GS, the value of the control voltage V.sub.GS
and the value of the pH being related by a formula of the type:
V.sub.GS=C.sub.2pH+E.sup.0, where E.sup.0 and C.sub.2 are
predefined constants; and in that said means for calibrating are
configured to implement steps of calibration during which: they act
on said means for modifying the value of the pH to take it
momentarily to at least one first known value pH.sub.1, then they
act on said means for measuring a control voltage V.sub.GS, to
measure its corresponding value V.sub.GS1; they compute the value
of the constant E.sup.0 according to the values of pH.sub.1 and of
V.sub.GS1.
18. Device according to claim 17, characterized in that said means
for calibrating are configured to implement steps of calibration
during which: they act on said means for modifying the value of the
pH to take it momentarily to a first known value pH.sub.1, then
they act on said means for measuring a control voltage V.sub.GS, to
measure the value V.sub.GS1 corresponding to the first value
pH.sub.1; they act on said means for modifying the value of the pH
to take it momentarily to a second known value pH.sub.2; then they
act on said means for measuring a control voltage V.sub.GS, to
measure the value V.sub.GS2 corresponding to the second value
pH.sub.2; they compute the value of said constants E.sup.0, C.sub.2
according to the known values of pH.sub.1, pH.sub.2, V.sub.GS1 and
V.sub.GS2.
19. Device according to claim 16, characterized in that said means
for modifying the value of the pH comprise an anode and a cathode
to be put into contact with said effluent, and first means for
generating electric current between said anode and said
cathode.
20. Device according to claim 19, characterized in that said means
for modifying the pH comprise command means to implement or not
implement said first means for generating an electric current
21. Device according to claim 20, characterized in that said means
for calibrating are configured to act on said command means to
implement said first means for generating an electric current to
modify the value of the pH during said steps of calibration.
22. Device according to claim 19, characterized in that it
comprises a membrane permeable to the H.sup.+ ions covering and
being in contact with said anode and at least partly said means for
measuring.
23. Device according to claim 22, characterized in that said
membrane covers said gate and said anode, said gate and said anode
being in contact with said membrane.
24. Device according to claim 22, characterized in that said
membrane is made of polymer.
25. Device according to claim 24, characterized in that said
polymer is poly(2-hydroxyethylmethacrylate).
26. Method for measuring the pH of an effluent by means of a device
according to claim 16, characterized in that it comprises: a step
for measuring the pH with said means for measuring; a step for
calibrating comprising at least one step for modifying said pH of
said effluent in proximity to said means for measuring by said
means for modifying, and a step for calibrating said device for
measuring by said means for calibrating.
27. Method according to claim 26, characterized in that said step
for measuring the pH comprises: a step for generating a constant
voltage at the terminals of said source and said drain; a step for
generating a constant electric current at the terminals of said
source and said drain; a step for measuring a control voltage at
the terminals of said source and said reference electrode; a step
for determining the value of said pH as a function of the value of
the control voltage V.sub.GS, the value of the control voltage
V.sub.GS and the value of the pH being related by a formula of the
type: V.sub.GS=C.sub.2pH+E.sup.0, where E.sup.0 and C.sub.2 are
predefined constants; and in that said step for calibrating
comprises: a step for modifying said pH by said means for modifying
the value of the pH to take it momentarily to a first known value
pH.sub.1, then a step for measuring the corresponding control
voltage V.sub.GS1 by said means (31) for measuring a control
voltage; a step of computation by said means for calibrating of
said constant E.sup.0 as a function of the known values pH.sub.1
and V.sub.GS1.
28. Method according to claim 27, characterized in that said step
for calibrating comprises: a step for modifying said pH by said
means for modifying the value of the pH to take it momentarily to a
first known value pH.sub.1, then a step for measuring the control
voltage V.sub.GS1 by said means for measuring a control voltage
V.sub.GS, to measure the value V.sub.GS1 corresponding to the first
value pH.sub.1, then a step for modifying said pH by said means for
modifying the value of the pH to take it momentarily to a second
known value pH.sub.2, a step for measuring the control voltage
V.sub.GS2 by said means for measuring a control voltage V.sub.GS to
measure the value V.sub.GS2 corresponding to the second value
pH.sub.2; a step of computation by said means for calibrating of
the value of said constants E.sup.0, C.sub.2 as a function of the
known values of pH.sub.1, pH.sub.2, V.sub.GS1 and of V.sub.GS2
29. Method according to claim 27, characterized in that said step
or steps for modifying said pH comprise a step for implementing
said first means for generating an electrical current.
30. Method according to claim 26, characterized in that said step
for calibrating is implemented at a predetermined frequency.
31. A method of measuring the pH of an effluent with a pH measuring
device and periodically calibrating the pH measuring device
comprising: engaging the effluent with a probe that forms a part of
the pH measuring device; utilizing the probe to measure a parameter
that is representative of the pH of the effluent and determining
the pH of the effluent as a function of the parameter; and
periodically calibrating the pH measuring device by: (i) adjusting
the pH of the effluent in the vicinity of the probe to a known pH;
(ii) adjusting the function such that the parameter measured by the
probe yields a measured pH that equals the known pH.
Description
1. FIELD OF THE INVENTION
[0001] The field of the invention is that of techniques for
measuring the value of the pH of a liquid effluent.
[0002] More specifically, the invention pertains to the designing
and manufacture of probes and to a method for the continuous
measurement of the value of the pH of a liquid effluent.
2. PRIOR ART
[0003] Potential hydrogen, more commonly called pH, represents the
chemical activity of hydrogen ions in solution. The value of the pH
of a solution reveals its acidity or its basicity.
[0004] The pH is a parameter used in many applications.
[0005] The pH is for example used in water treatment where it is an
indicator, for example of the healthy biological condition of
water. It is also often used as a control parameter when
implementing water treatment methods.
[0006] The pH is also often used in microbiology since its value
governs enzyme reactions and the growth of bacteria. The pH is also
used in the pharmaceutical and medical fields since minute
variations in pH can be symptomatic of serious metabolic
disturbances.
[0007] There are numerous techniques for measuring the value of the
pH of a solution. Among these there are especially:
[0008] pH paper whose color varies when it is put into contact with
a solution according to the pH value of this solution;
[0009] glass electrode probes;
[0010] non-glass electrode probes.
[0011] Only glass electrode probes or non-glass electrode probes
are suited to carrying out a continuous measurement of the pH of a
solution.
[0012] Glass electrode probes are relatively brittle and require
daily or weekly maintenance operations, especially because the
glass electrode contains an electrolyte, which is a consumable.
This drawback can be reduced through the use of electrolyte in the
form of gel but cannot be completely removed. Besides, the storage
of glass electrodes implies compliance with special and
constraining conditions. Indeed, glass electrodes have to be stored
in a potassium chloride solution since dry storage induces
premature ageing.
[0013] Non-glass electrode probes have especially been developed in
order to overcome these drawbacks.
[0014] The invention relates more particularly but not exclusively
to non-glass electrode pH measuring probes.
[0015] As can be seen in FIG. 1, non-glass electrode pH measuring
probes classically comprise an ISFET (Ion-Sensitive Field Effect
Transistor) type transistor and a reference electrode 15.
[0016] The ISFET transistor comprises a substrate 10 generally made
of silicon on which are placed a doped source 11, a doped drain 12
and a gate 13 separated from the source 11 and the drain 12 by an
insulator 14.
[0017] The gate 13 has a layer sensitive to variations in H.sup.+
ion concentration.
[0018] In some variants, the reference electrode 15 can be
constituted by a MOSFET (Metal Oxide Semiconductor Field Effect
Transistor) type transistor.
[0019] The layer that is sensitive to variations in the H.sup.+ ion
concentration as well as the reference electrode 15 are to be
placed in contact with the solution E, the pH of which is to be
measured.
[0020] The source 11 and the drain 12 are connected to a generator
16 of electric voltage and electric current capable of generating a
voltage and an electric current of constant values at their
terminals.
[0021] The reference electrode 15 and the source 11 are connected
to a means, such as a voltmeter, for measuring a so-called control
voltage 17. This voltmeter is capable of measuring a voltage at
their terminals. Inasmuch as the reference electrode is connected
to the contact of the gate, the voltage measured by the voltmeter
is a voltage V.sub.GS of the ISFET transistor across the gate and
the source.
[0022] In order to measure the pH of a solution, it is put into
contact with the gate 13 and the reference electrode 15.
[0023] The current generator and the voltage generator 16 are used
to generate the passage of a constant current and a constant
electric voltage between the source 11 and the drain 12. The values
of this voltage and of this current are stable and high enough to
enable the transistor to be biased.
[0024] The variation in the pH of the solution to be analyzed
induces the variation of its electrochemical potential which
modifies the voltage V.sub.GS of the transistor. The gate-source
voltage V.sub.GS varies linearly according to the pH for a
drain-source current I.sub.DS and a drain-source voltage V.sub.DS
that are constant. The voltage V.sub.GS, called a control voltage,
is then measured at the terminals of the reference electrode 15 and
the source 11. The measurement of this voltage therefore enables
the value of the pH of the solution to be determined
[0025] As compared with glass electrode probes, ISFET electrode
probes are more resistant, easier to store since they can be stored
in a dry state, more precise and faster because they have a very
short response time.
[0026] ISFET electrode probes and more generally probes for
measuring pH can however be further improved.
3. DRAWBACKS OF THE PRIOR ART
[0027] The main drawback of ISFET electrode probes is related to
the fact that a drift is observed over time between the measured
value of the pH and its real value. This drift dictates the regular
recalibration of the probe.
[0028] In order to ensure that the measurement of the pH by the
probe represents reality, the frequency of the recalibration is
generally daily.
[0029] The linear function relating the pH to the voltage V.sub.GS
measured by the ISFET probe is:
V.sub.GS=C.sub.2pH+E.sup.0
where C.sub.2 (the slope) and E.sup.0 (the intercept point) are
constants.
[0030] In the recalibration phases, the probe is dismounted so as
to be placed alternately in solutions having pH values that are
known and different from one another. The comparison of the pH
values measured by the probe with the real values then allows to
correct the value of the slope and/or the intercept point of the pH
curve of the probe in such a way that the pH value measured with
the probe is identical to the real value of the pH of the solution
analyzed.
[0031] These recalibrations therefore require qualified workers,
which entails a cost factor that can be high.
[0032] They require the dismounting of the probe, which can be a
lengthy and irksome task since the probe is not always very
accessible.
[0033] In addition, the recalibration phases dictate the stoppage
of the processes in which the pH is used as a control parameter.
This leads to a loss of productivity. Thus, in certain water
treatment methods, the recalibrations induce a drop in the
production of treated water.
4. GOALS OF THE INVENTION
[0034] The invention is aimed especially at overcoming the
drawbacks of the prior art.
[0035] More specifically, it is a goal of the invention to provide
a technique for measuring the pH, which enables the reduction, in
at least one embodiment, of the frequency of the recalibrations as
compared with the techniques of the prior art.
[0036] It is another goal of the invention to implement a technique
of this kind that makes it possible, in at least one embodiment, to
simplify the recalibration operations.
[0037] In particular, it is a goal of the invention, in at least
one embodiment, to procure a technique of this kind that does not
require the dismounting of the probe to carry out its
recalibration.
[0038] It is another goal of the invention, in at least one
embodiment, to procure a technique of this kind that does not call
for the probe to be put into contact with various solutions having
different known pH values in order to carry out its
recalibration.
[0039] It is yet another goal of the invention to provide a
technique of this kind which, in at least one embodiment, is simple
to implement and/or to store and is reliable and/or robust and/or
precise.
[0040] It is another goal of the invention, in at least one
embodiment, to procure a technique of this kind which does not
necessitate the use of reagent such as a liquid electrolyte or
reagent in the form of gel to carry out the measurement because
this gel would have to be renewed during maintenance, as is the
case with glass electrodes where the reference electrode bathes in
an electrolyte.
5. SUMMARY OF THE INVENTION
[0041] These goals as well as others that shall appear here below
are achieved by means of device for measuring the pH of an
effluent, said device comprising means for measuring a piece of
information representing the pH of said effluent, that are to be
put into contact with said effluent.
[0042] According to the invention, such a device also comprises
means for modifying the value of the pH of said effluent in
proximity to said means for measuring.
[0043] Also according to the invention, such a device in addition
preferably comprises means for calibrating said device for
measuring, said means for calibrating being configured to calibrate
the device for measuring after modification of said pH of said
effluent in proximity to said means for measuring by said means for
modifying.
[0044] Thus, the invention relies on a wholly original approach in
which means are integrated, into a pH measurement probe, to modify
the pH of the effluent locally, i.e. in proximity to the active
part of the probe (elements of the probe in contact with the
effluent at the level at which the measurement is made).
[0045] It is thus possible to locally modify the value of the pH of
the effluent so as to carry out the calibration of the probe
without dismounting it or plunging it into different buffer
solutions of known pH values in order to calibrate the device.
[0046] The technique according to the invention thus makes it
possible to carry out an in situ calibration without dismounting
the pH probe and therefore facilitates the calibration, reduces the
time needed for calibration and reduces its inherent cost.
[0047] According to a preferred embodiment, said means for
measuring comprise:
[0048] an ISFET type transistor comprising a source and a drain
disposed on a substrate, and a gate that is to be put into contact
with said effluent;
[0049] a reference electrode;
[0050] means for generating a constant voltage at the terminals of
said source and said drain;
[0051] second means for generating a constant electric current
between said source and said drain;
[0052] means for measuring a control voltage V.sub.GS at the
terminals of said source and said reference electrode,
[0053] means for determining the value of the pH of said effluent
as a function of the value of said control voltage, the value of
the control voltage V.sub.GS and the value of the pH being
preferably related by a formula of the type:
V.sub.GS=C.sub.2pH+E.sup.0, where E.sup.0 and C.sub.2 are
predefined constants;
said means for calibrating being preferably configured to implement
phases or steps of calibration during which:
[0054] they act on said means for modifying the value of the pH to
take it momentarily to at least one first known value pH.sub.1,
then
[0055] they act on said means for measuring a control voltage
V.sub.GS, to measure its corresponding value V.sub.GS1;
[0056] they compute the value of the constant E.sup.0 according to
the values of pH.sub.1 and of V.sub.GS1.
[0057] Thus, the invention relies in this embodiment on a wholly
original approach in which there is integrated, into a probe for
measuring the pH of a type comprising an ISFET transistor, means to
locally modify the pH of the effluent and means to calibrate, in
situ, the device for measuring.
[0058] To measure the pH of a solution, the reference electrode and
the gate are put into contact with it.
[0059] When the probe is put into contact with the effluent of
which it wishes to measure the pH, the H.sup.+ ions that it
contains modify the electrochemical potential of the solution and
therefore the voltage V.sub.GS of the ISFET.
[0060] Means for generating a constant electric current and a
constant voltage are then implemented to generate a constant
current and a constant voltage at the terminals of the source and
the drain, the values of which are chosen to enable the ISFET
transistor to be biased.
[0061] The control voltage V.sub.GS is then measured at the
terminals of the source and the gate or more specifically of the
reference electrode. The value of this control voltage varies
according to the pH of the effluent. The pH of the effluent is then
determined according to the value of the control voltage.
[0062] So that the probe does not perceive a drift between the
value of pH measured by means of the probe and the real value of
the pH, this probe is regularly calibrated in situ. To this end,
the means for calibrating preferably act on the means for modifying
of the pH to locally carry the value of the pH of the effluent to a
known value. They then command the measurement of V.sub.GS and then
compute the value of E.sup.0 which is the intercept point of the
curve of the voltage V.sub.GS as a function of the pH.
[0063] Said means for modifying the value of the pH preferentially
comprise an anode and a cathode to be put into contact with said
effluent, and first means for generating electric current between
said anode and said cathode.
[0064] The invention in this case relies on a wholly original
approach which consists of the integration, into a pH-measuring
probe of the type comprising an ISFET transistor, of an anode and a
cathode planned to come into contact with the effluent to be
analyzed, and means for generating an electric current at the
terminals of this electrodes.
[0065] So that the probe does not perceive a drift between the
value of pH measured by means of the probe and the real value of
the pH, this probe is regularly calibrated in situ. To this end, an
electric current is applied between the anode and the cathode.
Thus, the production of protons is generated in the effluent in
proximity to the measurement means; in the case of an ISFET, at the
proximity of the active surface of the gate, by oxidation of water
according to the formula
H.sub.2O.fwdarw.2O.sub.2+4H.sup.304e.sup.-. The pH of the effluent
can thus be modified locally in a controlled manner.
[0066] Indeed, by regulating the value of the electric current
between the anode and the cathode, it is possible to control the
value of the pH in proximity to the active part of the measuring
means, in the case of an ISFET in proximity to the active surface
of the gate. It is thus possible successively to place the pH at
one or more different known values in order to calibrate the
probe.
[0067] The technique according to the invention therefore makes it
possible to carry out the calibration of the probe, also called a
recalibration, in situ, i.e. without dismounting it and without
influencing the medium in which the measurement is made. Indeed,
the quantity of H.sup.+ ions generated is small as compared with
the volume of liquid in which the measurement is made.
[0068] The technique of the invention therefore takes part in
facilitating the calibration of a pH measuring probe of the type
comprising an ISFET transistor and accordingly reducing the cost
inherent in this calibration.
[0069] Said device, for example said means for modifying the pH,
preferably comprises command means to implement or not implement
said first means for generating an electric current.
[0070] In this case, said means for calibrating are preferably
configured to act on said command means to implement said first
means for generating an electric current to modify the value of the
pH during said phases of calibration.
[0071] Thus, it is possible that the first means of current
generation will not be implemented to carry out a classic
measurement of pH and then could be implemented to locally modify
the pH before carrying out new measurements of pH in order to
calibrate the device.
[0072] According to a preferred characteristic, a device according
to the invention comprises a membrane permeable to the H.sup.+ ions
covering and being in contact with said anode and at least partly
said means for measuring, in particular the active part of these
means.
[0073] When the probe is put into contact with the effluent for
which the pH is to be measured, the H.sup.+ ions that it contains
spread within the membrane in order to reach an equilibrium of
concentration between the interior and the exterior of the
membrane. The concentration in H.sup.+ ions inside the membrane is
then identical to that of the effluent. The measurement of the pH
is therefore done within the membrane. During the calibration, the
pH is modified only within the membrane, i.e. in a restricted
volume. In particular, the precision of calibration is
improved.
[0074] When the device for measuring is of the type comprising an
ISFET type transistor, and when it comprises a membrane permeable
to H.sup.+ions, this membrane covers said gate and said anode, said
gate and said anode being in contact with said membrane.
[0075] When the probe is put into contact with the effluent for
which the pH is to be measured, the H.sup.+ ions that it contains
spread within the membrane in order to reach an equilibrium of
concentration between the interior and the exterior of the
membrane. The concentration in H.sup.+ ions inside the membrane is
then identical to that of the effluent. A constant electric current
and a constant electric voltage of sufficient value are applied
between the source and the drain in order to bias the transistor. A
control voltage V.sub.GS is then measured at the terminals of the
reference electrode and the source, the value of which varies as a
function of the pH of the effluent. The pH of the effluent is then
determined according to the value of the control voltage.
[0076] During the calibration, an electric current is applied
between the anode and the cathode. Thus, the process generates the
production of protons inside the membrane by oxidation of water
according to the formula H.sub.2O.fwdarw.2O.sub.2+4H.sup.+4e.sup.-.
The pH of the effluent can thus be modified locally, easily and
rapidly in the membrane without interfering with the external
medium. The membrane plays the role of a buffer between the
external medium and the sensor, the electrodes placed beneath this
membrane enabling the pH around the sensor to be modified at
will.
[0077] Implementing the membrane makes it possible to vary the
value of the pH only in the membrane, i.e. in a restricted volume.
The value of the voltage between the anode and the cathode
generates a constant production of protons within the membrane that
is more stable than when the membrane is not used. The reliability
of the device and the precision of the calibration are thus
improved.
[0078] Said membrane is preferably made of polymer such as, for
example, poly(2-hydroxyethylmethacrylate).
[0079] The use of a polymer and especially of
poly(2-hydroxyethylmethacrylate) gives a membrane permeable to the
H.sup.+ ions, the use of which gives efficient results in terms of
control of local variation of pH.
[0080] A device according to the invention preferably comprises
means to calibrate said device from at least one measurement of
said control voltage after an implementation of said first means
for generating an electric current by said command means.
[0081] In some variants, said reference electrode could include a
MOSFET transistor or any other reference pseudo-electrode such as
for example a silver-silver chloride wire, gold wire, etc.
[0082] The MOSFET transistor fulfils the function of the reference
electrode, and the measured control voltage which is proportional
to the pH of the solution analyzed, is the voltage at the terminals
of the gates of the MOSFET and the ISFET. In this case, the MOSFET
is completely encapsulated. Only the gates of the ISFET as well as
the anode and the cathode can be put into contact with the effluent
to be analyzed.
[0083] In the case of the ISFET, the reference electrode will be
designed to be put into contact with the effluent to carry out the
measurement of the pH.
[0084] The invention also relates to a method for measuring the pH
of an effluent by means of a device according to any one of the
variants described here above.
[0085] Such a method comprises:
[0086] a phase or step for measuring the pH with said means for
measuring;
[0087] a step for calibrating comprising, in addition to the
previous step, at least one step for modifying the value of the pH
of said effluent in proximity to said means for measuring with said
means for modifying the value of the of said effluent.
[0088] More specifically, such a method preferably comprises:
[0089] a phase for measuring the pH with said means for
measuring;
[0090] a step for calibrating comprising at least one step for
modifying said pH of said effluent in proximity to said means for
measuring by said means for modifying, and a step for calibrating
said device for measuring by said means for calibrating.
[0091] According to a first preferred embodiment, said step for
measuring the pH comprises:
[0092] a step for generating a constant voltage at the terminals of
said source et said drain;
[0093] a step for generating a constant electric current at the
terminals of said source et said drain;
[0094] a step for measuring control voltage V.sub.GS at the
terminals of said source and said reference electrode;
[0095] a step for determining the value of said pH as a function of
the value of the control voltage V.sub.GS, the value of the control
voltage V.sub.GS and the value of the pH being preferably related
by the formula of the type: V.sub.GS=C.sub.2pH+E.sup.0, where
E.sup.0 and C.sub.2 are predefined constants.
[0096] Said step for calibrating preferably comprises at least:
[0097] a step for modifying said pH by said means for modifying the
value of the pH to take it momentarily to a first known value
pH.sub.1, then
[0098] a step for measuring the corresponding control voltage
V.sub.GS1 by said means for measuring control voltage;
[0099] a step of computation by said means for calibrating of said
constant E.sup.0 as a function of the known values pH.sub.1 and
V.sub.GS1.
[0100] In this case, the calibration consists in modifying the
value of the intercept point of the control voltage V.sub.GS
expressed as a function of pH.
[0101] According to a second embodiment, said step for calibrating
comprises
[0102] a step for modifying said pH by said means for modifying the
value of the pH to take it momentarily to a first known value
pH.sub.1, then
[0103] a step for measuring the control voltage V.sub.GS1 by said
means for measuring a control voltage V.sub.GS, to measure the
value V.sub.GS1 corresponding to the first value pH.sub.1, then
[0104] a step for modifying said pH by said means for modifying the
value of the pH to take it momentarily to a second known value
pH.sub.1,
[0105] a step for measuring the control voltage V.sub.GS2 by said
means for measuring a control voltage V.sub.GS to measure the value
V.sub.GS2 corresponding to the second value pH.sub.2;
[0106] a step of computation by said means for calibrating of the
value of said constants E.sup.0 , C.sub.2 as a function of the
known values of pH.sub.1, pH.sub.2, V.sub.GS1 and of V.sub.GS2.
[0107] In this case, the calibration consists in modifying the
value of the intercept point E.sup.0 and the slope C.sub.2 of the
control voltage V.sub.GS expressed as a function of the pH.
[0108] In variants of the invention, said step for calibrating,
especially said step for modifying the pH, could include a step for
generating a constant electric current between said anode and said
cathode.
[0109] Said step or steps for modifying said pH could include a
step for implementing said first means for generating an electric
current.
[0110] In variants, said step for calibrating could be implemented
at a predetermined frequency, preferably daily. Said step for
measuring could be implemented continuously or not continuously.
The measurement of the pH will naturally be stopped during the
calibration except for the pH measurement needed for the
calibration. The frequency of implementation of the calibration
could be adjustable.
[0111] The invention also concerns an element for measuring the pH
of a device according to any one of the variants explained here
above. Such a element comprises:
[0112] an ISFET type transistor comprising a source and a drain
disposed on a substrate, and a gate to be put into contact with
said effluent;
[0113] a reference electrode;
[0114] means for connecting means for generating a constant voltage
at the terminals of said source and said drain;
[0115] means for connecting second means for generating a constant
electric current at the terminals of said source and said
drain;
[0116] means for connecting means for measuring a control voltage
at the terminals of said source and said reference electrode;
[0117] an anode;
[0118] a cathode;
[0119] means for connecting a first means for generating an
electric current between said anode and said cathode.
6. LIST OF FIGURES
[0120] Other features and advantages of the invention shall appear
more clearly from the following description of a preferred
embodiment, given by way of a simple, illustratory and
non-exhaustive example, and from the appended drawings, of
which:
[0121] FIG. 1 illustrates a probe for measuring the pH according to
the prior art;
[0122] FIG. 2 illustrates a probe for measuring the pH according to
the invention;
[0123] FIGS. 3 and 4 illustrate curves showing the evolution in
time of the real value of the pH of a solution, the value of the pH
measured by means of a probe according to the prior art, and the
value of the pH measured by means of a probe according to the
invention;
[0124] FIG. 5 illustrates the diagram of a part of an electric
circuit of a probe according to the invention, the reference
electrode of which is constituted by a MOSFET transistor.
7. DESCRIPTION OF ONE EMBODIMENT OF THE INVENTION
7.1. Reminder of the General Principle of the Invention
[0125] The general principle of the invention consists of the
integration, into a pH measuring probe, of means for modifying the
pH of the effluent locally, i.e. in proximity to the active part of
the probe (elements of the probe in contact with the effluent at
which the measurement is made).
[0126] It is thus possible to locally modify the value of the pH of
the effluent so as to carry out the calibration of the probe
without dismounting it or plunging it into different buffer
solutions having known pH values.
[0127] The technique according to the invention thus enables a
calibration to be done in situ without dismounting the pH probe and
therefore facilitates the calibration, reduces the time needed for
the calibration and reduces the cost inherent in this
calibration.
[0128] In one variant, the invention consists of the integration,
into a pH measuring probe of a type comprising an ISFET transistor,
of an anode and a cathode to be put into contact with the effluent
to be analyzed and of means for generating an electric current at
the terminals of the anode and the cathode.
[0129] To measure the pH of an effluent, the probe is put into
contact with it, a constant voltage V.sub.DS is applied to the
terminals of the drain and the source and a constant electric
current I.sub.DS is put into circulation across these terminals in
order to bias the transistor. Then, the generation of a voltage
V.sub.GS is observed at the terminals of the source and the gate,
the value of which is proportional to that of the pH of the
effluent. This voltage is measured and then the pH of the effluent
is determined as a function of the value of voltage measured.
[0130] In order to regularly calibrate the probe, an electric
current is created between the anode and the cathode. Thus, the
production of protons is generated in proximity to the active
surface of the gate in order to locally modify the pH of the
effluent.
[0131] In regulating the value of the electric current across the
anode and the cathode, it is possible to control the local value of
the pH. It is thus possible to successively place the pH locally at
one or more different known values and thus carry out the
calibration of the probe.
[0132] The technique of the invention therefore plays a part in
facilitating the calibration of a pH measuring probe of a type
comprising an ISFET transistor and therefore in reducing its
inherent cost.
7.2. Example of One Embodiment of a Probe According to the
Invention
7.2.1. Architecture
[0133] Referring to FIG. 2, we present an embodiment of a device
for measuring pH according to the invention, also called a pH
measuring probe.
[0134] As shown in this FIG. 2, such a probe comprises an ISFET
type transistor. This probe classically comprises a source 21 and a
drain 22 placed on a substrate 23. Classically, the drain and the
source are doped. Their doping could respectively be N type or P
type doping, or vice versa, depending on the type of flow between
the source and the drain. It also classically comprises a gate 24.
The gate 24 is separated from the source 21 and the drain 23 by an
insulator 25 and comprises a surface sensitive to the H.sup.+ ions.
In this embodiment, the gate is made out of Ta.sub.2O.sub.5.
[0135] The probe also comprises a reference electrode 26, which is
connected with the gate contact of the electronic control circuitry
and enables the measurement of the variations in potential at the
contact of the gate 24. It also comprises an anode 27 and a cathode
28.
[0136] In this embodiment, the anode is made of platinum and the
cathode is made of stainless steel. Other suitable materials can
also be used.
[0137] The anode 27 extends all around the gate 24 without being in
contact with it.
[0138] The anode 27 as well as the gate 24 are coated with a
membrane 29 with which they are in contact. This membrane 29 is
permeable to the H.sup.+ ions. It is made out of polymer such as
for example poly(2-hydroxyethylmethacrylate), agarose, polyvinyl
alcohol (PVA), etc. It preferably takes the form of a gel. Its
thickness preferably ranges from 40 to 150 microns. It is
preferably fixedly attached to the anode and to the gate by
covalent bonds.
[0139] This membrane 29 as well the reference electrode 26 and the
cathode 28 are designed to be put into contact with the effluent E,
the pH of which is to be measured.
[0140] The probe comprises means 30 for generating a voltage
V.sub.DS, such as a voltage generator, and a current generator
I.sub.DS, such as an electric current generator, which are
connected to the terminals of the source 21 and the drain 22
through means for connecting provided for this purpose. These means
enable the application of a voltage V.sub.DS of constant value and
an electric current I.sub.DS of constant value between the source
and the drain.
[0141] The probe comprises means for measuring a voltage V.sub.GS
between the gate 24 and the source 21 such as for example a
voltmeter 31. In this embodiment, these means for measuring are
connected to the source and to the reference electrode. They enable
the measurement of V.sub.GS since the gate and the reference
electrodes are both in contact with the effluent to be
analyzed.
[0142] This voltage varies as a function of the pH of the solution
to be analyzed.
[0143] The probe comprises means 32 for generating an electric
current, such as an electric current generator, that are connected
to the terminals of the anode 27 and the cathode 28 through means
for connecting provided for this purpose. These means for
generating current enable the generation of a constant electric
current between the anode and the cathode. This current enables the
generation of a fixed concentration of protons proportional to the
pH.
[0144] The probe comprises a command means for acting on the means
32 for generating current so as to control the intensity of the
current that they deliver. The means for generating current can
thus generate constant currents of different values for
predetermined durations.
[0145] The application of an electric current at the terminals of
the cathode and the anode by means of the generator 32 enables the
generation of the production of H.sup.+ protons in the membrane by
oxidation of water according to the formula:
H.sub.2O=<2O.sub.2+4H.sup.++4e-
and the modifying of pH therein. The concentration in protons
[H.sup.+] generated is proportional to the intensity i of the
current imposed between the anode and the cathode:
[H.sup.+]=C.sub.1C
where C.sub.1 is a constant and i is the intensity of the current
imposed between the anode and the cathode.
[0146] In general, the value of the constant C.sub.1 can be
determined during a first step and a second step of initial setting
in the factory. The first step consists in carrying out a
calibration of the ISFET transistor probe with solutions having
known pH values, without generating any current between the anode
and the cathode. In another step, different values of current are
applied between the anode and the cathode and the corresponding pH
is measured by means of the ISFET transistor probe. A
characteristic curve linking the value of the measured pH and the
generated current is obtained by linear regression. It establishes
the value C.sub.1 necessary for the in situ calibration, C.sub.1
being its slope. The constant C.sub.1 is thus determined during the
manufacture of the probe.
[0147] The probe comprises means for determining the value of the
pH of the effluent according to the value of the voltage V.sub.GS
measured at the terminals of the gate 24 and the source 21.
[0148] The function linking the pH to the measured voltage V.sub.GS
is:
V.sub.GS=C.sub.2pH+E.sup.0
where C.sub.2 and E.sup.0 are constants to be determined
[0149] This formula corresponds to a generalization of the Nernst
equation:
V.sub.GS=(-2.3 RT/nF)pH+E.sup.0
[0150] With:
[0151] E.sup.0: constant
[0152] R: constant of gases
[0153] F: Faraday constant
[0154] T: temperature in degrees Kelvin
[0155] n: ion charge
[0156] The Nernst equation gives the theoretical values of the
slope C.sub.2 (of the order of 59 mV per pH unit) and of E.sup.0
(intercept point which depends on the threshold voltage of the
transistor and can vary from one sensor to another). However,
E.sub.0 and C.sub.2 are capable of varying for each ISFET
transistor probe. These constants must therefore be determined
precisely during the initial calibration at the manufacture of each
probe with solutions of known pH values.
[0157] In one embodiment, the initial calibration of the probe
comprises a first measurement of the voltage V.sub.GS, called
V.sub.GS1, in a first solution at a first value of pH, pH.sub.1,
and then a second value of the voltage V.sub.GS, called V.sub.GS2,
in a second solution at a second value of pH, pH.sub.2. The values
of the constants E.sub.0 and C.sub.2 can then be computed by
applying the following formulae:
C.sub.2=(pH.sub.1-pH.sub.2)/(V.sub.GS1-V.sub.GS2)
E.sup.0=V.sub.GS1-(pH.sub.1-pH.sub.2)/(V.sub.GS1-V.sub.GS2)pH.sub.1
[0158] The command means and the means for determining the value of
the pH comprise a microcontroller.
[0159] The probe comprises means for calibrating. These means for
calibrating comprise the microcontroller which enables the pH
measurement cycles and probe calibration cycles to be carried out
in alternation.
[0160] The pH curve of the probe is of the
V.sub.GS=C.sub.2pH+E.sup.0 type, C.sub.2 being the slope and
E.sup.0 being the intercept point. The values of E.sup.0 and
C.sub.2 vary over time owing to the ageing of the probe.
[0161] The in situ calibration is aimed at correcting the intercept
point and/or the slope to make sure that the value of pH measured
by means of the probe truly reflects reality.
[0162] During a calibration cycle, the microcontroller is designed
to:
[0163] act on the means for generating current to generate a
current of known intensity I.sub.1, at the anode and the cathode
for an adjustable duration T.sub.1 varying from 1 minute to 30
minutes in order to carry the pH within the membrane to a known
value pH.sub.1;
[0164] act, at the end of the duration T.sub.1, on the means for
generating current and voltage to generate a constant electric
current I.sub.DS and a constant electric voltage V.sub.DS between
the source and the drain in order to bias the transistor; activate
a measurement in the voltage V.sub.GS1 between the gate and the
source; compute the value of intercept point E.sup.0 in applying
the formula E.sup.0=V.sub.GS1-C.sub.2pH.sub.1;
[0165] replace the present value of E.sub.0 by the newly computed
value.
In this embodiment, the original value of the constant C.sub.2 is
kept.
[0166] In another embodiment of calibration, the microcontroller is
designed to:
[0167] act on the means for generating current to generate a first
current of a known intensity I.sub.1, at the anode and the cathode
for an adjustable duration T.sub.1 varying from 1 minute to 30
minutes in order to take the pH within the membrane to a first
known value pH.sub.1;
[0168] act, at the end of the duration T.sub.1, on the means for
generating current and voltage to generate a constant electric
current I.sub.DS and a constant electric voltage V.sub.DS between
the source and the drain in order to bias the transistor;
[0169] activate a measurement of the voltage V.sub.GS1 between the
gate and the source;
[0170] memorize the values of V.sub.GS1, pH.sub.1 and I.sub.1;
[0171] act on the means for generating current to generate a second
current of a known intensity I.sub.2, at the anode and the cathode
for an adjustable duration T.sub.2 varying from 1 minute to 30
minutes in order to take the pH within the membrane to a known
value pH.sub.2;
[0172] act, at the end of the duration T.sub.2, on the means for
generating current and voltage to generate a constant electric
current I.sub.DS and a constant electric voltage V.sub.DS between
the source and the drain in order to bias the transistor;
[0173] activate a measurement of the voltage V.sub.GS2 between the
gate and the source;
[0174] memorize the values of V.sub.GS2, pH.sub.2 and I.sub.2;
[0175] compute the values of the slope C.sub.2 and the intercept
point E.sup.0 and in applying the formulae:
C.sub.2=(pH.sub.1-pH.sub.2)/(V.sub.GS1-V.sub.GS2)
E.sup.0=V.sub.GS1-(pH.sub.1-pH.sub.2)/(V.sub.GS1-V.sub.GS2)ph.sub.1
[0176] replace the present values of C.sub.2 and E.sub.0 by the
newly computed values.
7.2.2. Operation
[0177] A. Measurement of pH
[0178] In order to measure the value of the pH of an effluent E,
this effluent is put into contact with the reference electrode 26
and with the membrane 29 (hence with the gate and the anode) and
with the cathode.
[0179] The microcontroller drives the probe so that no current is
delivered at the anode and the cathode.
[0180] The H.sup.+ ions contained by the effluent E then spread
inside the membrane 29 so that a equilibrium of concentration is
obtained between the interior and the exterior of the membrane 29,
i.e. the effluent, for which the measurement is made. The
concentration in H.sup.+ ions in the membrane 29 is therefore
identical to that of the effluent E.
[0181] The microcontroller acts on the means for generating
electric current and voltage to generate a constant voltage
V.sub.DS as well as a circulation of a constant electric current
I.sub.DS between the source 21 and the drain 22. The values of this
electric current and this electric voltage will be chosen so that
they enable the transistor to be biased.
[0182] The generation of a voltage is then observed between the
source 21 and the reference electrode 26. This voltage is the
voltage V.sub.GS of the IFSET transistor between the gate and the
source. The value of this voltage V.sub.GS is proportional to the
value of the pH of the effluent E. This voltage varies linearly as
a function of the pH for a constant current I.sub.DS and a constant
voltage V.sub.DS. The microcontroller records this voltage
V.sub.GS.
[0183] The microcontroller then determines the value of the pH of
the effluent E according to the value of the electric voltage
V.sub.GS measured at the terminals of the source 21 and the gate 24
in applying for example the formula:
V.sub.GS=C.sub.2pH+E.sup.0.
[0184] B. Calibration
[0185] In order to prevent the appearance of a drift between the
value of pH measured by means of the probe and the real value of
the pH of the effluent, phases of calibration or recalibration of
the probe are implemented regularly, preferably daily.
[0186] According to a first embodiment, during the calibration
cycle, the microcontroller acts on the means for generating current
to generate a current of known intensity I.sub.1 at the anode and
the cathode for an adjustable duration T.sub.1 varying from 1
minute to 30 minutes depending on the time needed to put the
concentration in H.sup.+ ions in equilibrium in the membrane. This
duration is parametrized in the factory. This current intensity
I.sub.1 generates the production of protons for the duration
T.sub.1 and thus carries the pH in the membrane to a known value
pH.sub.1. The H.sup.+ ions generated are far greater in quantity
than the protons present in the effluent (in a log relationship),
which makes it possible to overlook the influence of the pH of the
water outside the membrane for the calibration.
[0187] At the end of the duration T.sub.1, the microcontroller acts
on the means for generating current and voltage to generate a
constant electric current I.sub.DS and a constant voltage V.sub.DS
between the source and the drain: the transistor is then
biased.
[0188] The microcontroller then activates a measurement of the
voltage V.sub.GS1 between the gate and the source. Then it computes
the value of the intercept point E.sup.0 in applying the formula
E.sup.0=V.sub.GS1-C.sub.2pH.sub.1. It then replaces the present
value of E.sub.0 by the newly computed value in the formula
V.sub.GS=C.sub.2pH+E.sup.0.
[0189] In another embodiment of calibration, the microcontroller
acts on the means for generating current to generate a first
current of a known intensity I.sub.1 at the anode and the cathode
for an adjustable duration T.sub.1 varying from 1 minute to 30
minutes depending on the time needed for obtaining equilibrium of
concentration of H.sup.+ ions in the membrane. This duration is
parameterized in the factory. This current intensity I.sub.1
generates the production of protons for the duration T.sub.1 and
thus takes the pH in the membrane to a known value pH.sub.1.
[0190] At the end of the duration T.sub.1, the microcontroller acts
on the means for generating current and voltage to generate a
constant electric current I.sub.DS and a constant electric voltage
V.sub.DS between the source and the drain: the transistor is then
biased.
[0191] The microcontroller then activates a measurement of the
voltage V.sub.GS1 between the gate and the source, and memorizes
the values of V.sub.GS1, pH.sub.1 and I.sub.1.
[0192] The microcontroller then again acts on the current
generating means to generate a second current of a known intensity
I.sub.2 at the anode and the cathode for an adjustable duration
T.sub.2 varying from 1 minute to 30 minutes to take the pH within
the membrane to a second known value pH.sub.2.
[0193] At the end of the duration T.sub.2, the microcontroller
again acts on the means for generating current and voltage to
generate a constant electric current I.sub.DS and a constant
electric voltage V.sub.DS between the source and the drain in order
to bias the transistor.
[0194] The microcontroller then activates a measurement of the
voltage V.sub.GS2 between the gate and the source, and memorizes
the values of V.sub.GS2, pH.sub.2 and I.sub.2.
[0195] The microcontroller then computes the values of the slope
C.sub.2 and of the intercept point E.sup.0 in applying the
formulae:
C.sub.2=(pH.sub.1-pH.sub.2)/(V.sub.GS1-V.sub.GS2)
E.sup.0=V.sub.GS1-(pH.sub.1-pH.sub.2)/(V.sub.GS1-V.sub.GS2)pH.sub.1
[0196] It finally replaces the present values of C.sub.2 and
E.sup.0 by the newly computed values in the formula
V.sub.GS=C.sub.2pH+E.sup.0.
7.3. Trials
[0197] Trials were performed to verify the efficiency of a probe
according to the invention.
[0198] These trials consisted in measuring the value of the pH of
an effluent with a classic glass electrode probe and then with a
probe according to the invention.
[0199] FIGS. 3 and 4 illustrate:
[0200] the real variation of the pH of an effluent: theoretical
value;
[0201] the variation of the pH value measured by means of a probe
according to the prior art: standard measurements;
[0202] the variation of the pH value measured by means of a probe
according to the invention: measurements according to the
invention.
[0203] As can be seen in these figures, in conducting a calibration
step every six hours, the drift between the real value of the pH
and the value of the pH measured by means of a probe according to
the invention is almost zero or at least appreciably smaller than
the drift observed between the real value of the pH and the value
of the pH measured by a prior-art probe.
7.4. Variant
[0204] In one variant, it can be that the membrane will not be
implemented. In this case, the generation of the current at the
anode and the cathode will modify the value of the pH of the
effluent in a controlled manner in proximity to the gate. In other
respects, the structure and the operation of the probe according to
this variant are identical to those of the probe comprising the
membrane.
[0205] In one variant, the reference electrode of the probe could
be replaced by a MOSFET (Metal/Oxide/Semi-conductor Field Effect
Transistor) type transistor. In this case, the gate of the MOSFET
is electrically connected to the gate of the ISFET and a constant
current and voltage are applied between the source and the drain of
the MOSFET to bias it. The measurement of the voltage at the
terminals of the gate of the MOSFET and that of the ISFET which is
proportional to the pH of the solution to be analyzed makes it
possible to deduce the pH from this.
[0206] In other variants, the reference electrode could be
constituted by a reference pseudo-electrode made of silver-silver
chloride wire, gold wire or other types of wire.
[0207] FIG. 5 illustrates an example of an electronic connection
diagram of the MOSFET and ISFET transistors of a probe according to
this variant.
[0208] In this example, a differential cascade is implemented. This
cascade contains a transistor T5 in a feedback connection. The
transistors T3 and T4 are the active load of the MOSFET and of the
ISFET which ensures the equality of the drain in these two
transistors. The transistor T5 (identical to T3 and T4) drives the
gate voltage of the MOSFET. The type of conductivity of the channel
of the transistors T3, T4 and T5 is opposite that of the ISFET and
the MOSFET. To ensure the equality of the drain-source voltages
V.sub.DS of the MOSFET and the ISFET, the condition
I.sub.CS2=0.5I.sub.CS1 is needed.
[0209] Thus, if the potential of the gate V.sub.G1 of the ISFET
increases, the potential on the source V.sub.S1 must increase since
the drain-source current I.sub.DS1 and the potential at the drain
V.sub.D1 are fixed by all the transistors T3, T4, T5. Identically,
the increase in potential at the source (V.sub.S1=V.sub.S2) will
lead to an increase in potential at the gate V.sub.G2 of the MOSFET
and therefore of the output voltage V.sub.out. The potential is
measured relatively to the ground of the circuit. It corresponds to
the voltage between the gates of the MOSFET and the ISFET which is
proportional to the pH of the solution.
[0210] In this embodiment, only the anode and the gate of the ISFET
are put into contact with the effluent to be analyzed.
* * * * *