Note: Descriptions are shown in the official language in which they were submitted.
J ;~6231
Field of the Invention
The present invention relates to n senQor capable
of detecting a gas in a fluid snd more particularly to
a sensor capable of measuring the presence of and the
quantity of oxygen.
Back~round of the Invention
One of the electrochemical methods for oxygen
determination is the amperometric method. This method
is quite rapid, simple in operation and is especially
suited for determining either gaseous or dissolved
oxygen in liquids. Sensors utilized in the
amperometric method of oxygen detection are well known
in the art and generally comprise a hollow body
defining therein a reservoir for electrolyte and the
body is provided with an opening for communication
between the exterior and the reservoir. An anode and
cathode are disposed within the body for contact with
the electrolyte and a thin polymeric membrane, which is
impermeable by the electrolyte but which is permeable
by oxygen, seals the opening of the body. Means are
provided for electrically c;onnecting the electrodes to
an electrical potential and to current measuring means.
A potential is applied between the anode and the
cathode and as sample fluid is brought into contact
with the membrane sealing the sensor body, oxygen
diffuses through the membrane to contact the cathode in
the presence of electrolyte. A current flow results
which is linear with the partial pressure af oxygen
being sampled. The current is measured and correlated
to the amount of oxygen in the sample.
Amperometric oxygen sensors can be said to
generally be one of two types. ~ith the first type of
sensor, oxygen is reduced as it contacts the cathode
thus causing a current flow between anode and cathode.
6231
This type of sensor requires a constant 2 flux through
the membrane to avoid oxygen depletion within the
sensor since 2 is reduced to the hydroxyl group at the
cathode and is not regenerated at the anode. This type
of sensor is very sensitive to the flow rate of the
test fluid and normally requires some means for
maintaining a flow of fluid to the sensor. Current flow
is directly related to the partial pressure of the
oxygen in the test sample. Another type of 2 sensor is
the equilibrium type sensor which employs sufficient
potential to cause continuous reduction of oxygen from
the electrolyte at the cathode and the formation of
oxygen at the anode. Once stabilized in operation, an
equilibrium condition is set up within the sensor which
is disturbed only when the partial pressure Of 2 at
the exterior surface of the membrane changes thus
disturbing the balance of the equilibrium. Disturbance
of equilibrium creates a current flow which is also a
direct measure of the partial pressure of oxygen in the
sample. Although good results are achieved with both
types of sensors, the equilibrium type sensor is
preferred because it is independent of flow of oxygen
past the membrane surface and does not require control
of the sample flow rate or stirring of the sample fluid
in the immediate vicinity of the sensor as is normally
required with the first type of sensor as described
above.
In the equilibrium mode of operation, sensor
operation is maximized when the electrodes are
concentrically disposed in the sensor body with working
surfaces adjacent the inner surface of the membrane.
I~ith concentrically disposed electrodes, it is
necessary that the ratio of anode surface area to
caLhode surface area be at least on the order of 85:1.
Should the ratio of the anode surface to the catl-ode
surface area be less than about 85:1, the sensor ta~es
~.'Z~23~ i
an unduly long period of time, on the order of several
hours, to stabili~e after any stepchange in 2 leYel.
Also, long term stability of the sensor i~ poor even at
constant 2 levels with a resulting continuous loss of
sensitivity. It has also been found that such sensors
exhibit loss of activity over a period of time.
Summary of the Invention
In accordance with the present invention an
improved sensor for dissolved oxygen measurement is
provided which comprises a body having an electrolyte
reservoir and electrolyte contained therein and an
opening for communication between the exterior of the
body and the reservoir. An anode and cathode are
concentrically disposed in the body adjacent the
opening and a thin polymeric membrane permeable to
oxygen and impermeable to the electrolyte seals the
opening. Means are provided to electrically connect the
electrodes to a source of electrical potential and
current measuring means. The working surfaces of the
anode and cathode, that is the surfaces adjacent the
membrane, are spaced apart from the membrane to define
an electrolyte space between the working surfaces and
the membrane. A noble metal screen is disposed on the
working surface of the anode. In the preferred
embodiment of the invention the screen completely
covers the anode working surface and a portion of the
screen which would normally overlie the working surface
of the cathode is cut away so that the cathode working
surface is unobstructed.
Although the reasons are not clearly understood,
the use of the noble metal screen over the anode
surface results in a substantial retention of electrode
stability over a period of time as compared to sensors
similarly constructed but which do not include the
noble metal screen o~er the anode working surface.
1 :~'7623i
In a hi~hly preferred embodiment of the sensor of
the present invention, the cathode is comprised of
rhodium metal and the anode and the noble metal screen
are comprised of platini2ed platinum.
Brief Description of the DrawinRs
The invention will be more fully understood from
the following detailed description taken in conjunction
with the drawings of which:
FIG. 1 is a side elevation, partially in section,
of an oxygen sensor constructed in accordance with the
invention
FIG. 2 is a sectional view in enlarged scale of a
portion of the sensor of FIG. l;
FIG. 3 is an end view of the sensor as illustrated
in Fig.2;
FIG. 4 is a plot of sensor output in microamps
versus temperature plotted over a period of time for an
oxygen sensor constructed in accordance with the prior
art; and
~ IG. S is a plot of sensor output in microamps
versus temperature taken at two time periods for an
oxygen sensor constructed in accordance with the
invention;
Description of the Invention
Referring to FIG. 1, the preferred embodiment of
the present invention is shown as a sensor for oxygen,
shown generally as 10, comprising a body 12 having an
interior defining a reservoir 14 which contains an
electrolyte 17 therein. The body 12 has an opening at
16 for communication between the reservoir 14 and the
exterior of the body. An anode 22 and a cathode 24 are
concentrically disposed in the body 12 adjacent the
opening 16. A thin polymeric membrane 18 seals the
opening 16. The membrane 18 is preferably formed from
i ;~76231
either fluoroethylenepropylene or tetrafluoroethylene
which is permeable to oxygen but which is electrolyte
impermeable. Terminal means 20 are provided to connect
the electrodes electrically to a source of potential 26
and current measuring means 28. The anode 22 and
cathode 24 are supported in the reservoir 14 by a glass
tube 32. The cathode 24 is fused in one end of the
glass tube 32. The anode 22, which is generally disc
shaped, is provided with a central aperture 44 in
which the cathode 24 is concentrically disposed. The
anode 22 is carried by the end of the glass tube 32 and
is affixed thereto by an adhesive material 34 which is
resistant to the electrolyte, such as for example epoxy
adhesives. The anode 22 and the cathode 24 are
connected to the terminal means 20 by means of cathode
1ead 40 and anode lead 42. The membrane 18 is held in
place over the body opening 16 by a threaded annular
cap 36 which cooperates with a ring 38 to clamp the
membrane in position.
When properly positioned in the body 12 the anode
22 and the cathode 24 are located adjacent the opening
16 with the lower working surfaces 30 adjacent to the
membrane 18 but ~slightly spaced therefrom to define an
electrolyte space 46 between the working surfaces and
the membrane. A threaded plug 52 seals a corresponding
threaded opening 54 in the side of the sensor body 12
for adding or removing electrolyte from the reservoir
14.
In accordance with the invention, a noble metal
screen 48 is disposed on the wor~ing surface 3U of the
anode 22. The screen 48 is provided with an opening 50
~hich is aligned with the working surface 30 of the
cathode 24 so that the cathode working surface is fully
exposed to electrolyte in the electrolyte space 46. The
screen, however, covers the entire working surface 30
of the anode 22.
~.;Z'76Z31
The anode 22 is comprised of a noble metal,
preferably platinum over which is deposited 8 layer of
platinum black. The cathode 24 also comprises a noble
metal although in the case of oxygen sensors of the
type to which the present invention relates, it is
highly preferred that the cathode be composed of
rhodium as it is highly resistant to C2 interference.
The ratio of surface area of the anode to the surface
area of the cathode must be at least 85 to 1 and may go
as high as 100 to 1. It is highly preferred to
platinize or deposit platinum black on the working
surface of the anode 22 since the layer of platinum
black apparently increases the surface area of the
anode which improves the aforementioned ratio without
decreasing the size of the cathode unnecessarily or
without increasing the dimensions of the sensor in
order to achieve the desired anode to cathode ratio.
The noble metal screen is preferably comprised of
the same material as the anode 22 and is also
preferably coated with platinum black. The screen 48
is affixed on the working surface 30 of the anode 22 by
suitable means such as spot welding and in the
preferred embodiment the screen is affixed to the
working surface of the anode prior to applying the
platinum black so that the screen and the working
surface are platinized at the same time. The screen
may range between 50 mesh to 1000 mesh. Good results
are achieved with a wire screen of 100 mesh US Sie~e
Series with a wire diameter of about 0.003 inches. The
100 mesh screen size provides an opening on the order
of 0.007 inches which is sufficient to allow ~or a good
buildup of platinum black on the screen and on the
working surface 30 of the anode 22. Although the finer
mesh sizes produce good results if the scrcen 48 is
3S platinized separately from the allode 22, tlle openings
vf the finer mesh screens tend ~o be filled in thus
6Z31
reducing the surface area of the anode working surface
when the screen and anode are assembled and then
platinized. On the other hand, the mesh sizes coarser
than about 50 mesh US Standard Sie~e Series do not
provide the optimum increase in wor~ing surface arca
provided by the finer mesh sizes, i.e., 50 to 1000
mesh.
In a highly preferred embodiment of the invention
the noble metal screen 48 is subjected to a compression
operation prior to assembly on the working surface 30
of the anode 22. It will be understood that the screen
48 in its position on the working surface 30 of the
anode 22 also serves as a spacer between the membrane
18 and the cathode 24 and although it is not fully
understood, it is believed that the compression
operation minimizes the electrolyte space 46 between
the cathode and the membrane thus producing a quicker
response in the sensor. However, it will be understood
that the aforementioned compression step is not
critical and may be eliminated in the assembly of the
sensor in accordance with the present invention.
By way of example, an electrode sensor was
constructed in accordance with the description and
illustration FIG'S. 1 and 2 without a noble metal
screen on the working surface 30 of the anode 22. The
cathode was a 0.040 inch rhodium wire fused in a glass
tube and the anode was a 0.25 inch platinum disc
provided with a 0.05 inch center cutout. The platinum
disc was bonded on the end of the glass tube and the
cathode was concentrically disposed in the opening of
the anode. The anode was provided with a la~er of
platinum ~lack on its wor~ing surface 30. The
electrodes were assembled in a sensor body 12 provided
with an opening 16 which was sealed by a 0.02
fluoroeth)lcl-epropylene membrane. The electrolyte was
a two perccnL ~OH solution.
.
'7~3~
The sensor 90 constructed was installed in nn
environmental chamber which contained ordinary
atmosphere and cycled through a series of temperature
steps, The first temperature step began at 4C for a
minimum period of 4 to 5 hours. At the completion of
this period, the te~perature was increased to 25C for
a minimum period of 4 hours and then stepped up to 44~C
for a minimum of 4 hours. At ~he completion of the
44C temperature period the temperature was reduced to
4~C and the cycle repeated. The sensor was exposed to
the temperature cycles and the signal output recorded
for four complete cycles.
The results are plotted in FIG 4 with line A
representing the signal output at the various
temperatures for the sensor during the first cycle and
line B representing the signal output during the fourth
or final cycle. As can be seen from FIG. 4 the signal
output of the sensor increases with increasing
temperature. More importantly, however, it will be
seen that the signal strength regardless of temperature
is substantially less by the completion of the fourth
cycle. The test represents four weeks of usage of the
sensor. From Fig. 4 it can be seen that over'the test
cycle the sensor exhibits a substantial loss in signal
strength. In fact the loss of signal strength is
greater than the loss allowed by product specificatio`ns
and the sensor is unacceptable for use. Accordingly,
the sensor must be replaced or a new platinum black
surface applied to the working surface of the
electrodes.
At the completion of the four cycles the same
sensor was retrieved from the environmental chamber,
disassembled and a platinized platinum screcn, 100
mesh, was affixed on the working surface of Lhe anode.
The scnsor ~.as reassembled using the same type of
mcmbrane and electrolyte and placed back in the
1 :2 71b;Z31
environmental chamber and subjected to five cycles of
temperature which extended over 5 weeks of continuous
use.
The results are illustrated in FIG. 5. As can be
seen there is only slight decrease in signal strength
over the temperature ranges between the first cycle,
line C, and the fifth cycle, line D. The decrease in
signal strength is within acceptable specified limits
even after 5 weeks of continuous use.
While various embodiments and modifications of the
invention have been described in the foregoing
description and illustrated in the drawings, it will be
understood that minor changes may be made in details of
construction as well as in the combination and
arrangement of parts without departure from the spirit
and scope of the invention as claimed. For example,
the anode 22 may comprise a sintered metal. In
addition, good results are achieved using a tube type
anode rather than the disc shaped anode as illustrated.
The choice of electrolyte will be apparent to those
skilled in the art and the electrolyte may be buffered
or non-buffered as set forth in the examples.
Having described the invention we claim:
