U.S. patent application number 10/121392 was filed with the patent office on 2002-12-19 for sensor for electrometric measurement.
Invention is credited to Bower, Michael M., Candela, Ellen, Fletcher, Kenneth S., Skinner, David N..
Application Number | 20020189943 10/121392 |
Document ID | / |
Family ID | 24159239 |
Filed Date | 2002-12-19 |
United States Patent
Application |
20020189943 |
Kind Code |
A1 |
Fletcher, Kenneth S. ; et
al. |
December 19, 2002 |
Sensor for electrometric measurement
Abstract
The invention provides a sensor with a reference electrode and a
flowing electrolyte which is particularly useful for measuring the
ion concentration of a process solution. The invention includes a
sensor having a pressurized reservoir which provides flow of an
electrolyte, a non-metallic solution ground and a resistance
temperature device bonded to a non-metallic solution ground. The
invention provides sensors with greater accuracy and stability by
minimizing or eliminating ingress of contaminants from a process
solution through the external junction of the sensor.
Inventors: |
Fletcher, Kenneth S.;
(Hartford, CT) ; Skinner, David N.; (Milton,
MA) ; Candela, Ellen; (Cohasset, MA) ; Bower,
Michael M.; (Wareham, MA) |
Correspondence
Address: |
FOLEY HOAG LLP
PATENT GROUP, WORLD TRADE CENTER WEST
155 SEAPORT BOULEVARD
BOSTON
MA
02110-2600
US
|
Family ID: |
24159239 |
Appl. No.: |
10/121392 |
Filed: |
April 12, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10121392 |
Apr 12, 2002 |
|
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09541358 |
Mar 31, 2000 |
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Current U.S.
Class: |
204/433 |
Current CPC
Class: |
G01N 27/36 20130101;
G01N 27/401 20130101; G01N 27/301 20130101 |
Class at
Publication: |
204/433 |
International
Class: |
G01N 027/414 |
Claims
We claim:
1. A sensor of a fluid parameter, the sensor comprising a reference
electrode, an electrolyte in electrolytic contact with the
reference electrode, a pressurized reservoir for providing flow of
the electrolyte, a reference junction, and an external junction in
electrolytic contact with the reference electrode, wherein the
electrolyte flows between the junctions.
2. The sensor of claim 1 further comprising a piston for subjecting
the electrolyte to a controlled pressure.
3. The sensor of claim 2 wherein the piston is a spring actuated
piston.
4. The sensor of claim 1 further comprising a porous member
positioned between the reservoir and the external junction.
5. The sensor of claim 4 wherein the porous member is made of glass
material.
6. The sensor of claim 1 wherein the reference junction comprises a
cation exchange membrane.
7. The sensor of claim 6 wherein the membrane is a sulphonated
polyfluoroethylene membrane.
8. The sensor of claim 1 wherein the external junction is made of a
ceramic material.
9. The sensor of claim 1 further comprising an external pressure
source for subjecting said electrolyte to a controlled
pressure.
10. The sensor of claim 1 further comprising a non-metallic ground
disposed at a sensing surface.
11. The sensor of claim 10 further comprising a housing wherein the
non-metallic ground extends beyond an end of the housing.
12. The sensor of claim 11 wherein the non-metallic ground is
substantially conical.
13. A sensor comprising a reference electrode, an electrolyte in
electrolytic contact with the reference electrode, a pressurized
reservoir for providing flow of the electrolyte, and a non-metallic
ground disposed at a sensing surface.
14. The sensor of claim 13 wherein the non-metallic ground
comprises a conductive polymer.
15. The sensor of claim 14 wherein the polymer is
polyvinyldifluorine.
16. The sensor of claim 13 further comprising a piston in
communication with the electrolyte for subjecting the electrolyte
to a controlled pressure.
17. The sensor of claim 16 wherein the piston is a spring actuated
piston.
18. The sensor of claim 13 further comprising a porous member
positioned between the reservoir and the external junction.
19. The sensor of claim 18 wherein the porous member is made of
glass material.
20. The sensor of claim 13 wherein the reference junction comprises
a cation exchange membrane.
21. The sensor of claim 20 wherein the membrane is a sulphonated
polytetrafluoroethylene membrane.
22. The sensor of claim 13 further comprising an external pressure
source for subjecting said electrolyte to a controlled
pressure.
23. The sensor of claim 13 further comprising a housing wherein the
non-metallic ground extends beyond an end of the housing.
24. The sensor of claim 23 wherein the non-metallic ground is
substantially conical.
25. A sensor comprising a reference electrode, an electrolyte in
electrolytic contact with the reference electrode, a pressurized
reservoir for providing flow of the electrolyte, a non-metallic
ground disposed at a sensing surface, and a resistance temperature
device bonded to the non-metallic ground.
26. The sensor of claim 25 wherein the non-metallic ground
comprises a conductive polymer.
27. The sensor of claim 26 wherein the polymer is
polyvinyldifluorine.
28. The sensor of claim 25 further comprising a piston in
communication with the electrolyte for subjecting the electrolyte
to a controlled pressure.
29. The sensor of claim 28 wherein the piston is a spring actuated
piston.
30. The sensor of claim 25 further comprising a porous member
positioned between the reservoir and the external junction.
31. The sensor of claim 30 wherein the porous member is made of
glass material.
32. The sensor of claim 25 wherein the reference junction comprises
a cation exchange membrane.
33. The sensor of claim 32 wherein the membrane is a sulphonated
polytetrafluoroethylene membrane.
34. The sensor of claim 25 further comprising an external pressure
source for subjecting said electrolyte to a controlled
pressure.
35. The sensor of claim 25 further comprising a housing wherein the
non-metallic ground extends beyond an end of the housing.
36. The sensor of claim 35 wherein the non-metallic ground is
substantially conical.
37. A pH sensor having a housing and comprising a reference
electrode mounted in the housing, a measuring electrode mounted in
the housing and operatively connected to the reference electrode, a
fluid conduit for containing an electrolyte in electrolytic contact
with the reference electrode, a pressurized reservoir for providing
flow of the electrolyte and in fluid communication with the fluid
conduit, a reference junction encasing the reference electrode, and
an external junction in electrolytic contact with the reference
electrode, wherein the electrolyte flows between the junctions.
38. The pH sensor of claim 37 further comprising a non-metallic
ground disposed at a sensing surface.
39. The pH sensor of claim 38 wherein the non-metallic ground
extends beyond an end of the housing.
40. The pH sensor of claim 39 wherein the non-metallic ground is
sunbstantially conical.
41. The pH sensor of claim 38 wherein the non-metallic ground
comprises a conductive polymer.
42.The pH sensor of claim 41 wherein the polymer is
polyvinyldifluorine.
43. The pH sensor of claim 42 further comprising a piston in
communication with the electrolyte for subjecting the electrolyte
to a controlled pressure.
44. The pH sensor of claim 40 wherein the piston is a spring
actuated piston.
45. The pH sensor of claim 37 further comprising a porous member
positioned between the reservoir and the external junction.
46. The pH sensor of claim 45 wherein the porous member is made of
glass material.
47. The pH sensor of claim 37 wherein the reference junction
comprises a cation exchange membrane.
48. The pH sensor of claim 47 wherein the membrane is a sulphonated
polytetrafluoroethylene membrane.
49. The pH sensor of claim 37 further comprising an external
pressure source for subjecting said electrolyte to a controlled
pressure.
50. The pH sensor of claim 37 wherein the electrolyte is a solution
of AgCl-saturated KCl.
51. The pH sensor of claim 37 wherein the reference electrode is
made of silver-silver chloride.
52. A sensor having a housing and comprising a reference electrode
mounted in the housing, an electrolyte in electrolytic contact with
the reference electrode, fluid motive means for creating and
controlling flow of the electrolyte, and a non-metallic ground
disposed at a sensing surface.
53. The sensor of claim 52 wherein the non-metallic ground extends
beyond an end of the housing.
54. The sensor of claim 53 wherein the non-metallic ground is
substantially conical.
55. The sensor of claim 52 further comprising a reference junction
positioned between the electrolyte and the reference electrode.
56. The sensor of claim 55 wherein the reference junction comprises
a cation exchange membrane.
57. The sensor of claim 56 wherein the membrane is a sulphonated
polytetrafluoroethylene membrane.
58. The sensor of claim 52 further comprising an external junction
in electrolytic contact with the reference electrode.
59. The sensor of claim 58 wherein the external junction is made of
ceramic material.
60. The sensor of claim 52 wherein the fluid motive means comprises
a piston in communication with the electrolyte for subjecting the
electrolyte to a controlled pressure.
61. The sensor of claim 60 wherein the piston is a spring actuated
piston.
62. The sensor of claim 52 wherein the fluid motive means comprises
a porous member.
63. The sensor of claim 62 wherein the porous member is made of
glass material.
64. The sensor of claim 52 wherein the fluid motive means comprises
an external pressure source for subjecting the electrolyte to a
controlled pressure.
65. The sensor of claim 52 wherein the non-metallic ground
comprises a conductive polymer.
66. The sensor of claim 65 wherein the polymer is
polyvinyldifluorine.
67. The sensor of claim 52 further comprising a resistance
temperature device bonded to the non-metallic ground.
68. The sensor of claim 52 further comprising a measuring electrode
operatively connected to the reference electrode.
69. The sensor of claim 52 wherein the electrolyte is a solution of
AgCl-saturated KCl.
70. The sensor of claim 52 wherein the reference electrode is made
of silver-silver chloride.
71. A method of manufacturing a sensor having a resistance
temperature device and a non-metallic ground, the method comprising
the steps of melting the non-metallic ground in contact with the
device, and allowing the non-metallic ground to solidify in contact
with the device.
72. The method of manufacturing a sensor according to claim 71
wherein the non-metallic ground is substantially conical.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a sensor having a reference
electrode, more specifically, a reference electrode with an
internal reference junction. Reference electrodes are commonly used
in connection with ion-selective electrodes to determine the
concentration of ions in solution. For example, a reference
electrode is often used with an electrochemical ion-measuring
electrode, such as a glass pH electrode, to measure the
concentration of hydrogen ions in a process solution. In
particular, the present invention relates to sensors for measuring
the ion concentration of a process solution, e.g., fluids,
slurries, and the like.
[0002] The basis of the electrometric measurement of pH is the
development of a potential gradient across a membrane of specific
composition, when interposed between solutions having different
concentrations of hydrogen ion. The potential developed across the
membrane is quantitatively related to the concentration gradient of
hydrogen ion and can be applied to a known measuring circuit to
measure the pH of the sample. Because the potential developed
across the glass is to be measured, electrolytic contacts must be
made to the solutions on either side of the membrane. The
potentials generated by these contacts are controlled using, for
example, Ag/AgCl reference electrodes with controlled
concentrations of potassium chloride (KCl) solution.
[0003] The conventional, external reference electrode has two
components that contribute to the total potential measured across
the cell: a thermodynamic potential and a liquid junction
potential. The thermodynamic potential is derived from the
electrochemical half-cell, whereas the liquid junction potential is
derived from the difference in the ionic composition of the
internal salt bridge electrolyte and the process solution being
measured. For example, where the reference electrode half-cell
reaction is:
Ag+Cl.sup.-AgCl+e.sup.-
[0004] the potential generated may be fixed by: (1) controlling the
concentration of chloride ion, i.e. Cl.sup.-, at a constant value;
and (2) preventing interfering ions in the process solution from
approaching the reference half-cell. In prior reference electrodes,
these conditions are typically achieved by filling the reference
electrode with potassium chloride (KCl), often within an internal
chamber, which is connected to a salt bridge using an internal
ceramic barrier. In such electrodes, electrolytic contact between
the salt bridge and the process solution is made via an external
ceramic barrier, and the salt bridge is stationary, i e.
non-flowing. In this configuration, both the liquid junction and
the half-cell potential may be compromised during ingress of the
process solution into the internal salt-bridge and reference
half-cell solutions. Thus, accurate measurements require that cell
voltage varies only with the concentration of the ion of interest,
and that the reference electrode potential remain constant, i.e.
unaffected by the composition of the process solution. In fact, it
is known that the reference electrode is often the cause of poor
results obtained from measurements with ion-selective electrodes.
See Brezinski, D. P., Analytica Chimica Acta, 134 (1982) 247-62,
the contents of which are hereby incorporated by reference.
[0005] In addition, the development of process sensors has tended
toward probes with smaller diameters. This trend has made the
construction of highly accurate and stable, sensors more difficult.
For example, in certain sensor designs, positioning the reference
electrode further away from the process solution has resulted in
decreased accuracy, due to decreased thermal accuracy. Thus, it
would be desirable to have a sensor with increased stability and
accuracy of measurement which decreases or eliminates the ingress
of process solution. There is also a need for improved sensors
having smaller diameters while also minimizing the process-wetted
portion of the sensor.
[0006] In view of these considerations, it is an object of this
invention to provide a reference electrode that minimizes or
prevents back-flow of contaminants or materials from the process
solution through the external junction. It is also an object of
this invention to provide a durable, economical and versatile
reference electrode that is easy to fabricate, use, install,
calibrate and maintain. These and other objects are satisfied by
the invention described herein.
SUMMARY OF THE INVENTION
[0007] The present invention provides a sensor with a reference
electrode and a flowing electrolyte. The invention provides sensors
that operate with relatively high accuracy and stability by
minimizing or eliminating ingress of contaminants from a process
solution through the external junction of the sensor. In one
aspect, the invention includes a sensor having a pressurized
reservoir which provides flow of an electrolyte. In another aspect,
the invention provides a sensor having a non-metallic solution
ground. In yet another aspect, the invention includes a resistance
temperature device bonded to a non-metallic solution ground.
[0008] In one embodiment, the invention provides a sensor having a
reference electrode, a flowing electrolyte in electrolytic contact
with the reference electrode, a pressurized reservoir for providing
flow of the electrolyte, a reference junction and an external
unction in electrolytic contact with the reference electrode and
wherein the electrolyte flows between the junctions.
[0009] In another embodiment, the invention provides a sensor
having a reference electrode, a flowing electrolyte in electrolytic
contact with the reference electrode, a pressurized reservoir for
providing flow of the electrolyte, and a non-metallic ground
disposed at a sensing surface.
[0010] In yet another embodiment, the invention provides a sensor
having a reference electrode, a flowing electrolyte in electrolytic
contact with the reference electrode, a pressurized reservoir for
providing flow of the electrolyte, a non-metallic ground disposed
at a sensing surface, and a resistance temperature device bonded to
the non-metallic ground.
[0011] Sensors of the invention may be used to measure various
parameters of a fluid, e.g., ion concentration. In one preferred
embodiment, the sensor is a pH sensor, i.e. a sensor to measure
hydrogen ion concentration, having a reference electrode, a flowing
electrolyte in electrolytic contact with the reference electrode, a
pressurized reservoir for providing flow of the electrolyte, a
reference junction, and an external junction in electrolytic
contact with the reference electrode. The electrolyte flows from
the pressurized reservoir to the external junction. In another
preferred embodiment, the pH electrode includes a non-metallic
ground disposed at a sensing surface. In yet another preferred
embodiment, the pH sensor includes a resistance temperature device
bonded to the non-metallic ground. In a particularly preferred
embodiment, the non-metallic ground extends beyond the end of the
lower housing and, even more preferably, the non-metallic ground is
substantially conical in shape.
[0012] In still another embodiment, the invention provides a method
of manufacturing a sensor having a resistance temperature device
and a non-metallic ground, the method including the steps of
melting the non-metallic ground in contact with the resistance
temperature device and allowing the non-metallic ground to solidify
in contact with the resistance temperature device, thus ensuring
optimal thermal contact. In yet another embodiment, the invention
includes a method of manufacturing a sensor having a resistance
temperature device and a non-metallic ground that is substantially
conical in shape.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1a is a cross-section of one embodiment of a sensor
according to the present invention, taken along section line 1a-1a
of FIG. 1b.
[0014] FIG. 1b is an end view of the embodiment depicted in FIG. 1a
and depicts the sensing surface of the sensor.
[0015] FIG. 2a is a cross-section of another embodiment of a sensor
according to the present invention, taken along section line 2a-2a
of FIG. 2b, and showing aspects of the resistance temperature
device and the solution ground.
[0016] FIG. 2b is an end view of the embodiment depicted in FIG. 2a
and depicts the sensing surface of the sensor.
[0017] FIG. 3a is a cross-section of one embodiment of a sensor
according to the present invention and depicts a solution ground
that is substantially conical in shape.
[0018] FIG. 3b is a view of the embodiment depicted in FIG. 3a
showing aspects of the resistance temperature device and a solution
ground that is substantially conical in shape.
[0019] FIG. 4 is a graph comparing the response time of the
temperature resistance device of a sensor of the invention with the
response time of some commercially available sensors.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
[0020] One aspect of this invention provides a sensor having a
reference electrode for use with electrochemical ion measuring
electrodes, e.g. pH electrodes. The sensor has a flowing
electrolyte that provides electrolytic contact between the internal
reference half-cell and the process wetted junction. This flow of
electrolyte prevents back flow of contaminants or other materials
from the process solution through the external junction and into
the electrolyte, thereby minimizing unwanted liquid junction
potentials in the external junction. Further, this arrangement
reduces the likelihood of reference half-cell contamination. A
sensor in accord with the invention can be manufactured with a
relatively small diameter of, e.g. 0.75 in (1.9 cm). In addition,
sensors of the invention may be designed to reduce the length of
the process wetted portion, for example, to about 0.5 in (1.3
cm).
[0021] A sensor 10 according to one embodiment of this invention
has, as shown in FIGS. 1a and 1b, an upper housing 12 and a lower
housing 14, and includes a pressurized reservoir 20 for electrolyte
22 which is acted upon by a piston 18. The illustrated embodiment
includes a spring 16 acting on the piston 18, to create positive
flow of electrolyte 22. A porous member 24 is provided between the
reservoir 20 and the external junction 26. Preferably, the porous
member 24 is made of glass material. Reference electrode 34 is
encased by internal junction 32, which is a cation exchange
membrane. Preferably, the cation exchange membrane is a sulphonated
polytetrafluoroethylene membrane, such as that commercially
available from DuPont under the trade name NAFION.RTM.. Membrane 40
surrounds measuring electrode 38, which is operatively connected to
reference electrode 34.
[0022] FIGS. 2a and 2b show another sensor 50 according to the
invention, which includes a resistance temperature device 54. As
further shown in the illustrated embodiment, ground wire 56 is
operatively connected to solution ground 58. In a preferred
embodiment, solution ground 58 is made of a non-metallic material.
Most preferably, the solution ground 58 is made of a conductive
polymer, such as conductive polyvinyldifluoride, sold by Elf
Atochem, N.A. under the trade name KYNAe.RTM.. Preferably, the
solution ground 58 is bonded to insulating ground tube 52.
[0023] FIGS. 3a and 3b show another sensor according to the
invention, which includes a substantially conical non-metallic
ground 60. As further shown in the illustrated embodiment, the
substantially conical non-metallic ground 60 extends beyond the end
of lower housing 14. The resistance temperature device 54 extends
into the substantially conical non-metallic ground 60, which is
bonded to ground tube 52. FIG. 3b illustrates ground wire 56 in
operative connection with the resistance temperature device 54.
[0024] In one embodiment, the invention provides a sensor 10 with a
pressurized reservoir 20 for creating and controlling flow of an
electrolyte 22. The reservoir 20 may be pressurized in a variety of
ways. For example, pressure may be imparted by a piston 18 which
subjects the electrolyte 22 to a controlled pressure. In a
preferred embodiment, the piston 18 is a spring actuated piston.
Other fluid motive means known in the art may be used in accordance
with the invention. For example, an external pressure source may be
used to impart flow of the electrolyte, e.g., a pump may be used to
pump electrolyte through a capillary. In a preferred embodiment,
the fluid motive means is a mechanism which creates a pressure drop
across a porous member 24. In another preferred embodiment, the
flow rate of electrolyte 22 is limited to less than about 20
.mu.L/day. Preferably, the pressure exerted on the electrolyte 22
is approximately 200 psig.
[0025] In another embodiment, the invention provides a sensor 50
having a non-metallic ground 58 positioned to contact a process
solution. The ground 58 is disposed at a sensing surface of the
sensor, i.e. any surface which is in contact with the process
solution. In a preferred embodiment, the non-metallic ground 58 is
an electrically conductive polymer. In a most preferred embodiment,
the non-metallic ground 58 is made of polyvinyldifluoride, such as
that commercially available from Elf Atochem, N.A. under the trade
name KYNAR.RTM.. In a preferred embodiment, a non-metallic ground
of electrically conductive polymer is bonded to a non-conductive
polymer tube 52, thus ensuring optimal thermal contact.
[0026] In one particularly preferred embodiment, the invention
provides a sensor having a resistance temperature device 54 that is
bonded to a non-metallic ground 58. The invention also provides a
method of manufacturing a sensor 50 having a resistance temperature
device 54 bonded to a non-metallic ground 58. The method includes
the steps of melting the non-metallic ground 58 in contact with the
temperature device 54 and allowing the non-metallic ground 58 to
solidify in contact with the device 54.The geometrical shape of the
non-metallic ground 58 is not particularly limited, however, in a
preferred embodiment, the non-metallic ground extends beyond the
end of the lower housing 14 and is substantially conical in
shape.
[0027] In yet another preferred embodiment, the invention provides
an internal (reference) junction 32 which includes a cation
exchange membrane. Most preferably, the membrane is a sulphonated
polytetrafluoroethylene membrane, such as that commercially
available from DuPont under the trade name NAFION.RTM.. A cation
exchange membrane, i.e. a membrane that is permeable to many
cations and polar molecules, is preferred as-a material for a
reference junction due to its ability to pass charge as positively
charged cations. The cation exchange membrane is likewise
substantially impermeable to anions and non-polar species.
[0028] In one preferred embodiment, a cation exchange membrane
encases the reference electrode 34. Encasing the reference
electrode 34 in a cation exchange membrane serves to maintain the
Chloride level and minimize effects of contamination from external
sources. It also maintains the Ag+ level due to the fact that Ag+
forms a negatively charged complex of the form
Ag(Cl.sub.n).sup.-(n-1). This also inhibits the AgCl from reaching
the external junction 26, where decreased KCl levels due to
diffusion of the external process may result in the precipitation
of AgCl. Such precipitation may cause clogging of the junction and
a resulting noisy liquid junction potential.
[0029] For a sensor 10 which uses AgCl-saturated, 1 M KCl
electrolyte solution 22, the cation exchange membrane may be
prepared by immersion in a solution of 1 M KCl. This process
creates an electrical junction across the membrane, wherein
potassium ions associate with the membrane. In operation, when a
charge is drawn from an attached measuring device, potassium ions
from the internal solution associate with the membrane, causing
potassium ions to dissociate from the other side of the membrane.
In contrast, conventional porous ceramic junctions require negative
ion movement in the opposite direction in order to maintain charge
balance. Thus, while the flowing electrolyte 22 minimizes back
diffusion of contaminants through the external junction 26, even if
contaminants were to reach this membrane, there would be little
effect on the reference potential until the concentration builds to
an appreciable fraction of the relatively high cation (e.g.,
K.sup.+) concentration.
[0030] According to the present invention, flow of electrolyte 22
may be controlled, in part, by a porous member 24 positioned
between a pressurized reservoir 20 and an external junction 26.
Preferably, electrolyte flow may be controlled to a flow rate in
the range 0.1 to 20 .mu.L/day by creating a pressure differential
across a microporous VYCOR.RTM. glass (Coming Glass code 7930). See
T. H. Elmer, "Porous and Reconstructed Glasses," Engineered
Materials Handbook, Vol. 4:Ceramics and Glasses, which is hereby
incorporated by reference. The particularly useful property of
VYCOR.RTM. in this embodiment is the very narrow pore size
distribution exhibited by this material. This renders flow rates
very predictable and constant. The reference electrode 34 is
located downstream from this porous member 24 and is isolated from
the process by an external liquid junction. As discussed herein,
the external junction 26 is preferably a relatively low porosity
alumina ceramic. Based on a maximum internal fluid capacity of 8 mL
and a useful life of 1-year, the maximum permissible flow rate
should average no greater than about 20 .mu.L/day.
[0031] In a preferred embodiment, the internally pressurized design
of the invention provides an outward flow of electrolyte 22 with a
flow rate sufficient to overcome inward diffusion of process
through the external junction 26. The effectiveness of an
approximately 1 .mu.L/hr flow rate to prevent inward diffusion was
demonstrated experimentally. A multiple syringe pump capable of
accurately delivering controlled flows in the range 0.5 to 2.0
.mu.L/hr was connected into flow cells containing M/871CR
conductivity cells. The cells were connected to 870ITCR
transmitters and a data logger to monitor conductivities in the
range 0 to 100 .mu.S/cm. The diffusion barrier ceramic was placed
at the output of the flow cell at a position up-stream and in close
proximity to the conductivity sensor. At the start of each
experiment, the system, syringe, flow cell and external tube
containing diffusion barrier were filled with deionized, deaerated
water and the assemblies were placed in a thermostated bath to
eliminate thermal expansion effects on the flow-rates. To ensure
against leaks (this minuscule flow-rate is virtually impossible to
detect visually), the output flow was monitored using {fraction
(1/32)}" id capillary tubing (volumetric displacement, 12.5
.mu.L/inch). In each case the system was allowed to operate for
several days to establish a baseline of conductivity with time;
i.e. to ensure no conductivity change due to inwards leaks from the
temperature bath or from corrosion within the flow cells. To start
the salt test, the exit capillary on external tube was carefully
withdrawn using a syringe and replaced with 1 M KCl. The flow
measuring tube was purged of liquid and then reinstalled. No
increase of conductivity at this point signified outflow and, thus,
prevention of diffusion in.
[0032] The electrical resistance of three samples of Ceramtek 244B
type ceramics were tested for electrical resistance and the results
are shown in Table I. The standard procedure measures the iR drop
created by a polarizing current of 0.2 .mu.A across the ceramic
immersed in 1M KCl using two NAFION encased Ag, AgCl/1.0 M KCl
reference assemblies a non-polarizable electrolytic contacts.
1 TABLE I V1.sup.a (mV) V2.sup.b (mV) R, Kohm.sup.c R,
Kohm.sup.corr. NAFION/NAFION 0.20 0.36 0.80 N/A Sample 1 0.89 3.60
13.55 12.75 Sample 2 1.07 4.61 17.7 16.90 Sample 3 0.66 4.33 18.35
17.55 NAFION/NAFION represents the resistance of the two NAFION
assemblies without the junction between them and is essentially the
combined resistance of the two NAFION barriers. .sup.aMeasured
voltage without applied current .sup.bMeasured voltage with 0.2
.mu.A applied .sup.cR, Kohm = {(V2 - V1)/(2E - 6))}.multidot.
10.sup.-3, where the V's are expressed in mV Kohm.sup.corr
represents ceramic after subtracting 0.80K from NAFION/NAFION.
[0033] Although the external junction 26 is not typically used to
control flow rate in normal operation, it presents a significant
restriction to diffusion with minimum electrical impedance.
Experiments were conducted to establish an empirical relationship
between volumetric flow rate and ceramic junction electrical
resistance. Preferably, flow is minimized and electrical resistance
across the ceramic is limited to less than 20 kohms. Reference
conditions for flow rate measurements were determined by mounting
the ceramics in glass tubes to ensure flow through, rather than
around, the ceramic. Ceramics were mounted in Corning Type 0120
glass (potash soda lead) and deionized water, pressurized with 10
psig air provided the flow. Flow was measured as the linear
displacement of the air/water interface along a tube having an id
of {fraction (1/32)}". (12.5 .mu.L/in). Data for two ceramic
materials are shown in Table II.
2TABLE II Flow Rates and Electrical Resistance of External
Diffusion Barriers Ceram-tek 244B, 0.053" diam .times. 0.150" long
Average Flow Rate (.mu.L/hr) Sample 1 22.6 Sample 2 24.4 Sample 3
24.2 Average Resistance, three samples, 15.7K
[0034] A variety of reference electrodes and electrolytes are known
and may be used with sensors of the invention. An ordinarily
skilled artisan can select an electrode/electrolyte combination for
a particular application without undue experimentation. In a
particularly preferred embodiment, the invention provides a pH
sensor. The Ag/AgCl, 1 M KCl, Sat AgCl reference electrode used in
the pH sensor is isolated from the process by an external junction
and an internal reference junction which includes a NAFION.RTM.
membrane barrier. A positive outflow of electrolyte counteracts
inward diffusion of process and additionally inhibits clogging of
the external junction by the process solution. If desired,
diffusional transport of process solution to the reference junction
is further restricted by a relatively long path length between the
external and reference junctions.
[0035] Preferably, the reference electrode 34 of the present
invention produces and maintains a constant (i.e., non-polarizable)
electromotive potential that is unaffected by the small electrical
current requirement of the measuring device to which it is
connected. Further, the reference electrode preferably maintains
its stability over the entire temperature and pressure range
requested and must be protected from exposure to the various
chemical species in the large variety of processes in which these
sensors are applied.
[0036] Silver, silver chloride, in contact with a fixed
concentration of KCl, is preferred for a pH sensor. When properly
constructed, its potential is non-polarizable at the current
densities employed and its temperature dependence closely obeys
theoretical predictions. At equilibrium, the following
electrochemical reaction fixes the electrode potential:
AgCl+e.sup.-Ag.degree.+Cl.sup.-
[0037] Silver chloride, plated on a silver wire, provides the
reference terminal. When current is drawn through the cell, this
reaction can proceed either to the right or left depending on
current direction. The potential will remain constant as long as 1)
sufficient AgCl remains on wire, 2) the chloride concentration
remains constant and 3) extraneous ionic species do not approach
the proximity of the electrode and compete with the chloride
ion.
[0038] Silver chloride solubility is related to concentration of
KCl used in the salt bridge. The solubility of AgCl in 0, 1, 2, 3,
and 4 M KCl is 0.01, 0.1, 0.6 2.2, and 8.0 mM, respectively. The
increase in solubility is due to formation of negatively charged
complex ions having the general formula Ag(Cl.sub.n).sup.-(n-1).
Use of electrolyte 22 having high concentration of KCl is desirable
for limiting electrical resistance over the path that isolates the
internal reference junction 32 physically from the process. Also,
the ability of KCl to form relatively clean junctions with the
process samples with relatively small electrical junction
potentials is desirable. However, when the concentration of KCl is
diluted in the porous junction, AgCl precipitates and clogs it,
causing spurious and erratic liquid junction potentials. Thus, a 1
M KCl solution is preferable because, at this concentration, the
solubility of AgCl is roughly 1% of that in 4 M KCl. This
concentration of electrolyte should be used throughout the probe;
in the glass electrode internal reference electrode (here adjusted
to pH 7), in the working reference electrode and in the electrolyte
22. In this way, the isopotential point for the system is
established at pH 7.
[0039] If desired, the electrolyte used may contain a glycol to
provide freeze protection. For example, the electrolyte used may be
0.33 M KCl with 40 vol. % ethylene glycol, or 1 M KCl with 25%
propylene glycol. NAFION.RTM. membrane resistance may vary
significantly with degree of hydration and it is therefore
necessary to condition the membrane in the electrolyte. This is
done by heating the NAFION membrane in the electrolyte for one hour
at 95-100.degree. C. The membrane is then stored in a closed
container of this electrolyte until used.
[0040] In a pH sensor according to the invention, the pH function
of the glass membrane depends on its bulk composition. It presents
a stable ionic exchange equilibrium with hydrogen ions in contact
with the internal and external surfaces. Electrolytic transport of
cations (usually Na.sup.+ or Li.sup.+) provide sufficient
conductivity across the membrane to allow measurement of this
potential by the connected analyzer with sufficiently high input
impedance. Silicate (SiO.sub.2) forms the stable and durable
anionic framework in glass that provides ion exchange sites
necessary for the pH function. Preferable pH glass formulations
contain at least 50% SiO.sub.2. This property governs the ultimate
temperature limits and chemical compatibility properties of pH
glass membranes. Alkali metal ions such as Li.sup.+, Na.sup.+ and
Cs.sup.+ provide the mobile charge carriers that impart
electrolytic conductivity to these glasses.
[0041] Formulations with Na.sup.+ provide high conductivity, hence
low resistance glasses, but the presence of Na.sub.2O may lead to
high measurement errors in solutions of high pH (the so-called
sodium ion error) and also to increased solubility (corrosion) of
the glass at elevated temperatures. Because of the relatively low
bulk resistivity of this glass it is possible to fabricate this
membrane in a "flat-glass" design for use in applications where
protrusion of a fragile element into the process is objectionable.
This membrane demonstrates ideal Nemstian response over the 2-12 pH
range and 0-85.degree. C. temperature range.
[0042] Lithia glasses (Li.sub.2O) have significantly less
measurement error at high pH than soda glasses and significantly
increased corrosion resistance at elevated temperature. The
tradeoff is that Li+ is significantly less mobile in the glass
yielding higher bulk resistivity. The high resistivity requires
that the membranes be thinner and have larger area than would be
practical with a flat-glass design. Thus, a spherical domed bulb
design is preferable for high temperature wide pH range
measurements.
[0043] The geometric shape of the non-metallic ground in a sensor
of the present invention is not particularly limited. The
nonmetallic ground may be either machined or made by injection
molding according to procedures known in the art. In a preferred
embodiment, the non-metallic ground extends beyond the end of the
sensor housing or body and into the process solution. More
preferably, the geometric shape of such a ground is selected to
provide a relatively large surface area exposed to the process
solution. Additionally, it is preferable to use a non-metallic
ground having relatively thin walls. This combination of relatively
large surface area and relatively thin walls serves to minimize the
response time of the resistance temperature device (RTD), and also
to minimize the possibility of entrapment of any solids present in
the process solution.
[0044] To demonstrate one of the advantages of the present
invention, a sensor according to the invention was compared to
certain commercially available sensors. Specifically, the speed of
thermal response of a probe of the invention was compared with the
speeds of thermal response for various commercially available pH
probes. Briefly, for each probe, the speed of thermal response was
measured by first determining the resistance of the RTD in the
probe at ambient room temperature. Each probe was then placed in
boiling water. The RTD resistance was then measured every 10 to 20
seconds, depending on the rate of response. The response time was
defined as the time a give probe takes to read 90% of the change
from ambient temperature to boiling water.
[0045] FIG. 4 and Table III show a comparison of the response times
of a sensor according to the invention with that of various probes.
The Exemplary Probe used in the experiment was a sensor according
to the invention having a non-metallic solution ground extending
beyond the end of the sensor housing and having a substantially
conical shape. Comparative Probes 1-6 are commercially available pH
probes. In particular, Comparative Probe 1 is a TBI Model 557,
Comparative Probe 2 is a TBI Model 551, Comparative Probe 3 is a
Mettler Inpro 4500, Comparative Probe 4 is a Iontron Ultra 10,
Comparative Probe 5 is a Rosemount TupH, and Comparative Probe 6 is
an ASI Model 68 Versaprobe. Each of Comparative Probes 1 through 5
is a plastic-bodied pH probe with the RTD positioned away from the
process solution interface. Comparative Probe 6 uses a glass/metal
interface with the RTD to achieve its response time. It is clear
from the data presented in FIG. 4 and Table III that the sensor of
the present invention provides dramatically increased response time
as compared to conventional probes and, in fact, is capable of
thermal response times that previously attainable only with a
metallic interface.
3TABLE III Comparison of Thermal Response Times for Various Probes
Probe Manufacturer/Model Response Time (Min.) Comparative Probe 1
TBI Model 557 9.6 Comparative Probe 2 TBI Model 551 8.8 Comparative
Probe 3 Mettler Inpro 4500 4.0 Comparative Probe 4 Iontron Ultra 10
3.2 Comparative Probe 5 Rosemount TupH 3.0 Comparative Probe 6 ASI
Model 68 1.2 Versaprobe Exemplary Probe Foxboro DolpHin 1.2
[0046] The invention also provides a method of manufacturing a
sensor having a resistance temperature device (RTD) 54 and a
non-metallic ground 58. An RTD/ground assembly was prepared as
follows. A wire lead was wrapped around the body of an RTD to form
a subassembly. This subassembly was then inserted into a piece of
electrically conductive polymer (KYNAR.RTM.), using a slip/press
fit. An insulating polymer piece was then placed over the
subassembly. The inner diameter of the insulating polymer
preferably provides a tight fit over the wire lead. The resulting
assembly was placed in a metal heating block to melt the two
polymer pieces to the RTD and wire. The process resulted in: (1) a
hermetic seal between the polymer pieces; (2) an intimate
electrical connection between the lead wire and the assembly; (3) a
mechanical bond between the RTD and the assembly; and (4) an
intimate thermal contact between the RTD and the non-metallic
solution ground.
[0047] Incorporation by Reference
[0048] All patents, published patent applications and other
references disclosed herein are hereby expressly incorporated
herein in their entireties by reference.
[0049] Equivalents
[0050] Those skilled in the art will recognize, or will be able to
ascertain using no more than routine experimentation, that the
apparatus and embodiments described above may be modified without
departing from the broad inventive concept described herein. Thus,
the invention is not to be limited to the particular embodiments
disclosed herein, but is intended to cover modifications within the
spirit and scope of the present invention as defined by the
appended claims.
* * * * *