Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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GAS ANALYZER
Back round of the Invention
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The present invention relates to gas
analyzers and, more particularly, to an improved
electrochemical gas analyzer.
Various types of electrolytic oxygen
sensing devices are available for use in measuring
the oxygen content of gaseous mixtures and the
dissolved oxygen content of fluids. Typically, these
devices utilize an electrolytic cell employing a pair
of spaced electrodes immersed in an electrolyte. The
cells utilize an electrical parameter derived from
the reduction of oxygen to determine the
concentration of oxygen.
In US. Patent No. 3,429,796, an
electrochemical gas analyzer is disclosed in which
linearity of response is improved by use of a flat,
planar mesh cathode covered with a plastic membrane.
This provides a uniform electrolyte film between the
cathode and the membrane necessary for accurate
response over a large range.
U. S. Patent No. 3,767,552 discloses an
electrochemical gas analyzer cell which utilizes an
expansion membrane and chamber to eliminate the
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lifting of the cathode membrane in the face of quick
changes of the temperature or pressure due to the
fact that the internal volume changes. on upwardly
protruding, uniformly curved cathode provides intimate
and continuous contact with a stretched,
gas-permeable cathode membrane to provide a highly
stable response.
It has been found that even such a device
gives incorrect results when used in environments in
which the content of the measured substance includes
substantial amounts of nitrous oxide because the cell
acts upon the nitrous oxide to form nitrogen gas
which is less permeable through the plastic cathode
; membrane than is nitrous oxide and may thus cause a
substantial rise in internal volume and internal
pressure, after the expansion membrane reaches its
expanded limit. Also, it has been found that highly
permeable background gases such as hydrogen and
helium can diffuse easily into the cell and produce
excessive internal volume expansions and pressures
which all result in the lifting of the cathode
membrane.
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Summary of the Invention
Therefore, an object of this invention is
to greatly lessen the sensitivity of an
electrochemical gas analyzer cell to internal
pressures caused by highly permeable background gas.
This and other objectives and advantages of
the invention are accomplished by a gas analyzer cell
which includes an insulator body having a central
passage divided into a first cell chamber and a
second expansion chamber by a flexible expansion
membrane. The cell chamber contains an anodic mass
of. a nonpolarizable metal and is enclosed by a
uniformly curved cathodic member of polarizable metal
which is covered by a membrane impervious to liquid
but previous to gas. An electrolyte fills the cell
chamber.
The slightly upwardly protruding curved
cathode and the gas permeable membrane are covered by
a fibrous layer which is previous to gas and is in
turn covered by a metallic mesh held in place by a
ring disc held securely to the insulator body. The
metallic mesh and the fibrous layer hold the cathode
membrane uniformly and tightly against the cathode of
the cell so that the membrane does not lift or change
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its position under conditions of increased internal
pressure and, consequently, incorrect results are
eliminated.
Brief Description of the Drawings
The invention will be better understood by
reference to the detailed description considered in
conjunction with drawings in which:
inure 1 is a sectional view of the gas
analyzer in accordance with the invention; and
Figure 2 is an exploded perspective view of
the assembly forming the analyzer of the invention.
Description of the Preferred Embodiment
Referring now to Figures 1 and 2, an
analyzer cell 10 is shown housed in an insulator body
14. The body 14 of the cell 10 is formed of an
insulating material which in the preferred embodiment
may be a thermoplastic hydrocarbon resin such as
polyethylene. Such a material facilitates the
formation of heat seals with various portions of the
device. The cell 10 is a sealed unit adapted to be
utilized until the available anodic metal is
converted to an oxidized form. The cell 10 is then
discarded and replaced with a new cell.
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The body 14 of the cell 10 is preferably
cylindrical in shape and is adapted for insertion
into a holder, not shown. An axial passage 16
extends through the body 14. The passage 16 is
divided into an upper electrolyte cell chamber 18 and
a lower expansion chamber 20 by means of a flexible,
expansion membrane 22 attached to the body 14 at a
shoulder 24 of the passage 16. The membrane 22 may
be sealed to the body 14 by adhesive or by heat
sealing techniques. Above the shoulder 24, the
passage 16 forms a broad cylindrical aperture 27 and
is constricted at a flange 26 to a smaller
cylindrical aperture 28. An anode 30 has its upper
surface held in place by the flange 26.
The expansion chamber 20 is enclosed by an
end plate 32 attached at a shoulder 34 in the passage
16 of the body 14 by adhesive or heat sealing. The
end plate 32 may be formed of a rigid insulating
material such as glass reinforced epoxy. A bottom
surface 38 of the plate 32 is provided with an anode
contact area and a cathode contact area (neither of
which is shown) by plating or applying contact foil
as is well known to the prior art.
The anode 30 is formed of a porous, high
surface area body of a non-polarizable metal such
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as lead, cadmium, or antimony which does not react
with the electrolyte. In a preferred embodiment, the
anode 30 contains a central aperture 46 for rapid
transmission of pressure waves or surges to the
expansion membrane 22.
The anode 30 is preferably formed of lead
by sistering in situ. More particularly, the
particulate lead is preferably formed into a cohesive
mass and is preliminarily treated to remove any oxide
coating on the surfaces of the particles. By way of
illustration, lead granules having an average size
between 5 and 10 miss are placed within thy boy 14 in
a forming tool and are covered with a 10 percent
solution of potassium hydroxide. While still covered
by the potassium hydroxide, the lead is compressed to
shape so that the particles stinter into a cohesive
mass, and the oxide coating is removed.
A contact wire 52 is connected to the anode
30. The wire 52 is threaded through a small diameter
aperture 54 into a machined bore 56 extending into
the side of the body 14. The wire 52 is welded to a
conductive plug 58, suitably formed of stainless
steel received in the bore 56. A further length of
wire 60 is welded to the exterior of the plug 58.
The wire 60 is reinserted into the body 14 through an
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aperture 64 below the expansion membrane 22. The
wire 52 is then threaded through an aperture 66 in
the end plate 32 and is connected to the anode
contact on the surface 38.
The top surface of the anode 30 may be
enclosed by a disc 70 of a material permeable to
liquid but impermeable to solids to prevent particles
that break away from the anode 30 from moving within
the electrolyte into contact with the cathode and
partially shorting the cell 10. The disc 70 is
suitably formed of filter paper and is retained in
place by a plastic washer 72 which presses the edge
of the disc 70 onto the central top surface of the
anode 30.
The cell chamber 18 is enclosed by a
convex, perforated cathode 74. The outer edge 76 of
the cathode 74 is received in a shoulder 80 on the
aperture 28 of the passage 16 through the body 14. A
gas-permeable, liquid-impermeable membrane 84 is
stretched over the cathode 74 and has its outer edge
heat sealed in a grove 86 provided in the upper
surface of the flange 26. The grove 86 may be filled
with a sealant.
A connecting wire 90 is welded to the
cathode 74 and is threaded through a hole 92 into a
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second bore 94 provided in the side of the cell body
14. The wire 90 is welded to a conductive sealing
plug 96 received in the bore 94. An additional wire
98 is welded to the outside surface of the plug 96
and extends down and into the body 14 through an
aperture 102 in the body below the expansion membrane
22. The wire 98 continues through an aperture 104 in
the base plate 32 into contact with the cathode
contact on the surface 38.
Since smoothness and uniformity of the
outer cathode surface provides more uniform and
intimate contact with the cathode membrane, it is
preferred to utilize a planar, multi-apertured metal
material. The planar, multi-apertured metal material
may be formed from electroformed or electroetched
metals or from perforated metal sheet or metal film.
The cathode surface is formed of a
polarizable metal, suitably a noble metal such as
gold, silver or platinum. The cathode may be formed
of an inner core which is plated or coated with the
noble metal. The core is preferably a resistance
weldable material so as to facilitate connection to
the contact wires. By way of exemplification, the
cathode can be formed from brass first plated with
silver and then with gold.
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The malleable cathode 74 is formed into the
desired convex shape with appropriately shaped
forming tools. The curvature of the cathode is
sufficient to tension the membrane to assure firm
contact with the cathode but does not stretch the
membrane beyond its elastic limit.
The cathode 74 is preferably capable of
assuming and maintaining a convex shape such that the
cathode does not bend, wrinkle or distort under
conditions of usage. Suitably, the gold plated,
brass cathode has a thickness of 10-25 miss to
provide the desired inflexibility.
The cathode membrane 84 is stretched over
the cathode 74 and sealed in place. The cathode
membrane 84 seals the electrolyte within the cell 10
while permitting passage of gas into the cell 10.
The membrane 84 is preferably a synthetic organic
resin inert to the electrolyte and is suitably a
vinyl resin such as polyethylene, polypropylene or
polytetrafluoroethylene.
In order to eliminate the lifting of the
cathode membrane 84 caused by an excessive pressure
within the cell 10, a fibrous element 110 is
positioned over the cathode membrane 84 and a second
mesh element 112 of the same size and shape as the
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cathode 74 and constructed of nickle-plated brass is
positioned there over. The element 112 is held in
place by a stainless steel ring 114 held in a cavity
116 in the body 14 by stainless steel screws 118.
The element 112 is cushioned by the fibrous element
110 so that it conforms to the surface thereof and
lends substantial strength to eliminate lifting of
the cathode membrane 84 which might be caused by
pressure in the cell 10. The fibrous element 110 is
constructed of a material which is porous and
therefore permeable to the gas subject to
measurement. In a preferred embodiment, a porous
Teflon material having a thickness of 0.010 inches
and openings of approximately one to two microns is
utilized.
The expansion membrane 22 is formed of a
flexible synthetic resin and is provided in a
thickness and of a material such that it is more
flexible than the cathode membrane 84. The expansion
membrane 22 is also inert to the electrolyte. By way
of example, the cathode membrane 84 is suitably
formed of polytetrafluoroethylene in thickness from
0.125 to 2.0 miss, and the expansion membrane 22 is
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formed of polyethylene film having a thickness of 1-4
miss, suitably a laminated polyethylene having a
thickness of 2-3 miss.
The electrolyte may be basic, neutral or
acid but is preferably an aqueous solution of any one
or mixture of the following: potassium hydroxide,
potassium carbonate or potassium phosphate, for
example a 10 percent solution of potassium hydroxide.
The cell is fabricated by machining the
various grooves, shoulders and recesses within the
body. The parts of the device are assembled as
illustrated in Figure 2 and as explained above. The
cell is inserted into the holder which connects the
electrodes through an external circuit and during
measurement, the outer surface of the cathode
membrane is immersed in the sample being tested. The
membrane permits the permeation of oxygen into the
cell chamber at a rate which is proportional to the
concentration of oxygen on each side of the membrane.
Since the concentration inside the cell is negligible
when the cell is in dynamic equilibrium, the rate of
influx of oxygen is proportional to the concentration
of oxygen in the sample being tested. The oxygen
reaching the cathode is reduced to form hydroxyl
ions. Simultaneously, the anodically liberated lead
glues
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ions form insoluble lead dioxide. A current
corresponding to the rate of the above reactions
flows in the external circuit and causes a
corresponding indication on an ammeter or recorder.
It is to be understood that only preferred
embodiments of the invention have been described and
that numerous substitutions, alterations and
modifications may be made by those skilled in the art
without departing from the spirit and scope of the
invention. For example, any gas analyzer cell which
utilizes a gas-pervious but liquid impervious
membrane to allow a measured substance to reach an
anode and a cathode surrounded by an electrolyte and
n which the membrane is subject to distorting
internal pressures might well employ an arrangement
for clamping the membrane to preclude its distortion.
Therefore, the invention should be construed in
accordance with the following claims.
WHAT IS CLAIMED IS:
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