U.S. patent application number 13/442669 was filed with the patent office on 2012-11-08 for apparatus and method for measuring hydrogen concentration in molten metals.
This patent application is currently assigned to ENVIRONMENTAL MONITORING AND CONTROL LIMITED. Invention is credited to Mark Anthony Steele Henson, MATTHEW PAUL HILLS.
Application Number | 20120279860 13/442669 |
Document ID | / |
Family ID | 33427927 |
Filed Date | 2012-11-08 |
United States Patent
Application |
20120279860 |
Kind Code |
A1 |
HILLS; MATTHEW PAUL ; et
al. |
November 8, 2012 |
APPARATUS AND METHOD FOR MEASURING HYDROGEN CONCENTRATION IN MOLTEN
METALS
Abstract
The invention concerns a probe for measuring hydrogen comprising
a probe body and a hydrogen sensor. The sensor body has a wall
within which a sealed cavity is defined. The cavity contains a
solid reference material for generating a reference partial
pressure of hydrogen within the cavity. At least a portion of the
wall of the cavity is formed from a solid electrolyte material
carrying a measurement electrode on a surface of the solid
electrolyte outside the cavity and a reference electrode on a
surface of the solid electrolyte within the cavity, exposed to the
reference partial pressure of hydrogen. An electrical conductor
extends from the reference electrode through the wall of the cavity
to an external surface of the sensor body. The probe body comprises
a chamber for receiving the sensor and a reference-signal
connection for connecting to the electrical conductor when the
sensor is received in the chamber.
Inventors: |
HILLS; MATTHEW PAUL;
(Cambridge, GB) ; Henson; Mark Anthony Steele;
(Stafford, GB) |
Assignee: |
ENVIRONMENTAL MONITORING AND
CONTROL LIMITED
Stafford
GB
|
Family ID: |
33427927 |
Appl. No.: |
13/442669 |
Filed: |
April 9, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11664404 |
Feb 4, 2009 |
8152978 |
|
|
PCT/GB2005/003812 |
Oct 3, 2005 |
|
|
|
13442669 |
|
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Current U.S.
Class: |
204/427 |
Current CPC
Class: |
Y10T 29/49826 20150115;
G01N 27/4118 20130101; G01N 27/4114 20130101; G01N 27/4117
20130101; G01N 33/005 20130101 |
Class at
Publication: |
204/427 |
International
Class: |
G01N 27/407 20060101
G01N027/407 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 1, 2004 |
GB |
0421868.1 |
Claims
1. A hydrogen sensor comprising; a sensor body comprising a tube, a
solid electrolyte closing one end of the tube and a sensor cap
closing the other end of the tube, so as to define a sealed cavity
within the sensor body; a solid reference material within the
cavity for generating a reference partial pressure of hydrogen
within the cavity; a measurement electrode on a surface of the
solid electrolyte outside the cavity; a reference electrode on a
surface of the solid electrolyte within the cavity, exposed to the
reference partial pressure of hydrogen; and an electrical conductor
extending from the reference electrode to an external surface of
the sensor body.
2. The sensor according to claim 1, in which the electrical
conductor extends outwardly from the external surface of the sensor
body.
3. The sensor according to claim 1, in which the solid electrolyte
is substantially planar.
4. The sensor according to claim 1, in which the solid electrolyte
is substantially disc-shaped.
5. The sensor according to claim 1, in which a maximum lateral
dimension of the solid electrolyte is less than 10 mm.
6. The sensor according to claim 1, in which a maximum lateral
dimension of the solid electrolyte is less than 6 mm.
7. The sensor according to claim 1, in which a maximum lateral
dimension of the solid electrolyte is less than 4 mm.
8. The sensor according to claim 1, in which a maximum lateral
dimension of the solid electrolyte is about 3 mm.
9. The sensor according to claim 1, in which the thermal expansion
coefficients of the tube and the solid electrolyte are
predetermined either so that they are substantially equal or so
that the solid electrolyte is under compressive stress at an
operating temperature of the sensor.
10. The sensor according to claim 1, in which the cavity contains a
buffer material between the reference material and the sensor
cap.
11. The sensor according to claim 1, in which the sensor cap
comprises the same material as the tube.
12. The sensor according to claim 1, in which the electrical
conductor extends to the external surface of the sensor body
through the sensor cap.
13. The sensor according to claim 1, in which the solid electrolyte
comprises indium-doped calcium zirconate.
14. The sensor according to claim 1, in which the tube comprises
calcium zirconate or magnesia/magnesium aluminate.
15. The sensor according to claim 1, in which the solid electrolyte
is secured to the tube by a glass seal.
16. The sensor according to claim 15, in which the glass is
silica-free glass.
17. The sensor according to claim 1, in which the reference
material comprises a metal/metal hydride reference.
18. The sensor according to claim 1, in which the buffer material
comprises yttrium oxide powder.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S.
application Ser. No. 11/664,404, filed Feb. 4, 2009, now U.S. Pat.
No. 8,152,978, which is the U.S. national stage application of
International Patent Application No. PCT/GB2005/003812, filed Oct.
3, 2005.
[0002] The invention relates to an apparatus and a method for
measuring hydrogen concentration, and in particular for measuring
dissolved hydrogen concentration in molten metals.
[0003] It is important to monitor the concentration of hydrogen
dissolved in molten metals, and in particular in molten aluminium
and its alloys. The solubility of hydrogen in molten aluminium is
much higher than its solubility in solid aluminium, and therefore
when aluminium is cast there is a tendency for dissolved aluminium
in the melt to form bubbles or other flaws in the solid aluminium
product. The hydrogen concentration in molten aluminium can rise
through reaction of the aluminium with moisture in the environment,
and so it is critical to be able to monitor hydrogen concentration
during aluminium casting.
[0004] Many methods have been developed for monitoring hydrogen
concentration in molten aluminium and its alloys, and in other
metals, but all of these suffer disadvantages such as lack of
accuracy, a requirement for cumbersome apparatus, and
disadvantageously long measurement times. A technology which offers
solutions to these problems is the possibility of using a
proton-conducting solid-electrolyte sensor with an internal
solid-state hydrogen reference. This technology has been described
in published prior art, including `The Detection of Hydrogen in
Molten Aluminium` by D P Lapham et al, Ionics 8 (2002), pages 391
to 401, `Determination of Hydrogen in Molten Aluminium and its
Alloys using an Electrochemical Sensor` by C Schwandt et al, EPD
Congress 2003, TMS (The Minerals, Metals and Materials Society),
2003, pages 427 to 438, and in International patent application No.
PCT/GB2003/003967 of Cambridge University Technical Services
Limited. All of these documents are incorporated herein by
reference in their entirety. An advantageous method for taking
measurements from such a probe, termed the `reverse current
technique` has been described in European patent application No. EP
98932375.3 of D J Fray and R V Kumar, which is also incorporated
herein by reference in its entirety.
[0005] However, this technology has not, to date, been developed to
produce a hydrogen probe which meets the practical requirements of
shop-floor use in a foundry. The present invention aims to address
this problem.
BRIEF SUMMARY OF THE INVENTION
[0006] The invention provides in its various aspects a probe, a
hydrogen sensor and a method as defined in the appended independent
claims. Preferred or advantageous features of the invention are
defined in dependent sub-claims.
[0007] In a first aspect the invention may thus advantageously
provide a probe comprising a probe body and a hydrogen sensor. The
sensor is preferably a proton-conducting solid-electrolyte sensor
with an internal solid-state hydrogen reference contained within a
sealed cavity in a sensor body. The solid electrolyte forms at
least a portion of a wall of the sensor body and has a reference
electrode on at least a portion of its surface within the cavity.
The solid reference material generates a reference partial pressure
of hydrogen within the cavity, to which the reference electrode is
exposed. The reference electrode is connected to an electrical
conductor which extends through the wall to an external surface of
the sensor body.
[0008] The probe body preferably comprises a chamber for receiving
the sensor and a reference-signal connection, or connector, for
connecting to the electrical conductor when the sensor is received
in the chamber. The reference-signal connection may then be
electrically connectable to an analyser for generating hydrogen
concentration measurements. In a preferred embodiment, the sensor
may be insertable into the chamber in the probe body and the
reference electrode automatically connected to the reference-signal
connection as the sensor is inserted.
[0009] It is also necessary for the analyser to be electrically
connected to a measurement electrode formed on at least a portion
of a surface of the solid electrolyte outside the cavity. This may
be achieved, for example, either by a second electrical conductor
extending from the measurement electrode, or by means of an
electrical path through the molten metal in which the probe is
immersed during hydrogen sensing.
[0010] Advantageously, the probe body is carried at the end of a
probe support, so that at least an end of the probe body can be
immersed in molten metal for sensing hydrogen concentration. In a
preferred embodiment, an opening is defined at the end of the probe
body which is to be immersed in the molten metal. The sensor is
advantageously insertable through the opening into the chamber in
the probe body and the opening is then sealable by means of a
hydrogen-permeable seal. Thus, when the probe body is immersed in
molten metal, the metal does not pass through the seal but hydrogen
from the melt diffuses through the seal and generates a partial
pressure of hydrogen within the chamber. The measurement electrode
on the solid electrolyte is exposed to the hydrogen and a potential
difference across the solid electrolyte is generated, which is
related to the ratio between the partial pressures of hydrogen at
the measuring electrode and at the reference electrode in known
manner, according to the Nernst equation. The analyser described
above can then measure the potential difference, or use a technique
such as the `reverse current technique` to determine the hydrogen
partial pressure in the chamber, given that the reference hydrogen
partial pressure is known.
[0011] If the measurement electrode is connected to the analyser by
means of an electrical conductor extending from the measurement
electrode, the hydrogen-permeable seal may comprise a conductive or
a non-conductive material. Alternatively, the seal may be
electrically conductive and form part of a conduction path from the
measurement electrode to the molten metal. A separate electrical
connection is then made between the molten metal and the analyser.
In one embodiment, a conductive hydrogen-permeable seal comprises
graphite, for example in the form of graphite wool or a porous
graphite layer. If a graphite seal is used, particularly in a probe
for sensing hydrogen concentration in aluminium or an aluminium
alloy, the outer surface of the graphite is advantageously coated
with titanium diboride to improve wetting with the molten
aluminium.
[0012] In order to reduce the response time of the probe, it may be
advantageous to reduce the volume of hydrogen which needs to
diffuse into the chamber in order to achieve a hydrogen partial
pressure which is in equilibrium with the hydrogen concentration in
the melt. To achieve this, in a preferred embodiment the volume of
the chamber (termed the chamber `dead volume`) is decreased by
placing an insert between the hydrogen-permeable seal and the
sensor.
[0013] Advantageously, the hydrogen-permeable seal may be used
mechanically to retain the sensor in the chamber, for example by
making the seal an interference fit in the opening at the end of
the chamber or by making the seal part of a screw cap covering the
opening.
[0014] Advantageously, the seal is removable to allow removal and
replacement of the sensor, for example in the event of sensor
failure.
[0015] Advantageously, the chamber in the probe body is
hermetically sealed except at the opening. Thus, no seal between
the sensor and the chamber body may be required, either to prevent
diffusion of hydrogen out of the chamber or environmental access
into the chamber. Advantageously, therefore, the arrangement of the
reference-signal connection of the probe body is hermetically
sealed.
[0016] In a preferred embodiment, the sensor body is not fastened
to the probe body and is advantageously a loose fit in the chamber.
In other words, there is preferably sufficient clearance between
the sensor and the chamber to accommodate thermal shocks or
relative thermal expansion of the sensor body and the probe body,
to avoid the application of excessive stresses to the sensor.
Otherwise, such thermal stresses may damage the sensor. This
clearance may also permit hydrogen flow between the sensor body and
the chamber, which may enable the measurement electrode to be
positioned at any point on the surface of the probe body, and not
necessarily at the surface of the probe body nearest to the opening
in the probe body.
[0017] In an alternative embodiment, the probe body may not
comprise a chamber for receiving the probe; in this embodiment, the
probe body is integral with the sensor. In this embodiment the
probe body may advantageously enable some or all of the functions
of providing a hydrogen-permeable seal between the melt and the
solid electrolyte, protecting the sensor from the melt, and
providing a means for coupling the sensor to a probe support. In
one such embodiment, the sensor tube may be incorporated within a
protective ceramic sleeve which is shaped to receive the
hydrogen-permeable seal at one end and to fit onto the probe
support, for example by means of a push fit, at the other end.
Alternatively, the probe body may provide only an external coating
to protect the sensor. In a further alternative the probe-body
coupling means, for coupling the sensor to the probe support, may
comprise a radially-extending flange at an end of the sensor tube,
adapted to engage with a coupling such as a threaded collar to
secure it to the probe support. In a further embodiment, the
function of the hydrogen-permeable seal may be implemented by
inserting the seal into a suitably extended portion of the sensor
tube, which extends away from the sensor chamber beyond a planar
solid electrolyte seated on a recessed seat within the tube.
[0018] In each of these embodiments in which the sensor is
integrated with the probe body, the probe body incorporating the
sensor may advantageously be releasably couplable to the probe
support, as in other embodiments described herein, in order to
achieve the advantage of being able to replace the probe body and
the sensor after degradation or damage during use.
[0019] The dimensions of the components of the probe and the
materials from which the various components are made may
advantageously be selected to ensure that the probe is robust and
reliable when subjected to the thermal shock and cycling involved
in repeated immersion in molten metal.
[0020] Advantageously, the probe is of small size (particularly of
small lateral dimension, or diameter). For example the maximum
lateral dimension of the sensor body is advantageously less than 10
mm, preferably less than 6 mm and particularly preferably less than
4 mm. This may not only reduce the effects of thermal shock but
also advantageously decrease the time taken for the probe to reach
operating temperature when immersed in molten metal and improve the
response time of the probe by reducing the volume of the probe body
chamber and the dead volume therein for the diffusion of
hydrogen.
[0021] In a preferred embodiment, the solid electrolyte comprises a
perovskite, such as indium-doped calcium zirconate. Other portions
of the sensor body are advantageously fabricated from materials of
thermal expansion coefficient compatible with the solid
electrolyte. For example, the remainder of the sensor body may have
the same thermal expansion coefficient as the solid electrolyte or
a slightly smaller thermal expansion coefficient so as to keep the
solid electrolyte in compression at elevated temperature, to
prevent cracking of the electrolyte.
[0022] The solid reference material advantageously comprise a
metal/metal hydride reference, such as titanium/titanium hydride,
zirconium/zirconium hydride or hafnium/hafnium hydride.
[0023] The electrodes on the surfaces of the solid electrolyte are
preferably porous platinum electrodes.
[0024] The probe body is preferably fabricated from a material or
materials which are substantially inert when immersed in the molten
metal, which provide good thermal shock resistance and which have a
suitable thermal expansion coefficient to avoid applying stresses
to the sensor body. In preferred embodiments, the probe body may
comprise aluminium nitride, SiAlON, silicon nitride, dense
graphite, alumina, magnesia, boron carbide or stabilised zirconia.
The probe body may advantageously be coated with a wetting agent or
with titanium diboride. The latter is particularly effective if the
probe body is made of graphite and is for immersion in molten
aluminium.
[0025] The probe may form part of a probe assembly, in which the
probe body is mounted at one end of a probe support. The other end
of the probe support may be provided with a handle for an operator
to hold to immerse the probe in the melt. The probe support may be
tubular, in which case electrical connections may be carried along
its interior between the probe body and the analyser.
[0026] The end of the probe support may be fastened to the probe
body in any convenient manner. In one embodiment, the probe support
is fastened to the probe body by brazing or by means of silica-free
glass. In such an embodiment, the end of the probe support may form
part of the wall of the probe body chamber, in which case the joint
between the probe support and the probe body is advantageously
hermetically sealed. If the probe support is in the form of a tube
carrying, for example, a conductor leading from the reference
electrode, then the tube should advantageously be sealed, for
example using silica-free glass.
[0027] In an alternative embodiment the probe body, including (if
present) the chamber for receiving the sensor and the sensor
itself, may be constructed as a replaceable unit. Similarly, a
probe body incorporating a sensor as an integral unit, as described
above, may be constructed as a replaceable unit or component. In
these embodiments the probe body may advantageously be designed to
be removably couplable to an end of the probe support for easy
replacement. During use, the probe body and, depending on the
design of the probe, a portion of the probe support are repeatedly
immersed in molten metal and may therefore degrade. It may
therefore be economically advantageous to make the probe body
replaceable. In a preferred implementation of this embodiment, a
coupling between the probe body and the probe support both
mechanically supports the probe body during use and makes any
required electrical connections, including (as applicable) any
connections to the reference electrode, the measurement electrode
and a thermocouple.
[0028] In an example of this embodiment a probe-body chamber, in
which a sensor is received, is mounted at one end of a probe-body
shaft. The shaft has an external surface which is substantially
inert to a molten metal in which the probe is to be used and
carries internally along its length any required electrical
conductors. For example, a thermocouple and a connection to the
reference electrode may extend within the shaft, while the melt is
used to provide a connection to the measurement electrode as
described above. The end of the probe-body shaft distant from the
sensor is provided with a suitable coupling for securing it to an
end of the probe support. The coupling may conveniently comprise a
threaded graphite coupling; since the graphite is electrically
conducting this may make contact with the melt during measurements
and so be used to complete a conduction path from the measurement
electrode through the melt to an electrical conductor housed within
the probe support. The probe support may advantageously be tubular
and carry electrical conductors internally, protected from the
melt, to a handle end of the support for taking readings from the
sensor.
[0029] This design may advantageously allow fast and easy
replacement of the probe body and the sensor, which may then be
serviced or discarded. The replaceable unit contains substantially
all of the components of the probe that are subject to
deterioration, such as the hot-end seal (sealing the probe chamber)
and the sensor itself. Advantageously, the probe is designed so
that replacement of the probe body does not require an operator to
connect any electrical connections manually; these are
advantageously automatically completed as the probe body is coupled
to the probe support.
[0030] A probe embodying the invention may be used for measuring
the concentration of hydrogen in molten metals such as aluminium,
magnesium or copper or alloys of these metals. Depending on the
materials used for fabricating the probe, and their thermal
performance, it may be necessary to mount the sensor at a distance
from the probe body opening. For example, the probe as described
above may advantageously be used with molten aluminium, magnesium
or their alloys but copper and its alloys generally melt at higher
temperatures. Thus, for aluminium, magnesium and their alloys the
sensor may be mounted close to the opening from the probe chamber
in order to minimise the chamber volume and reduce the probe's
response time. For use with copper and copper alloys, in order to
expose the sensor to lower temperatures it may be necessary to
mount the sensor further from the opening at the end of the probe
body chamber, and thus further from the molten metal.
[0031] In a further aspect, the invention provides a hydrogen
sensor constructed as follows. The sensor body comprises a tube,
with a solid electrolyte closing one end of the tube and a sensor
cap closing the other end of the tube, so as to define a sealed
cavity within the sensor body. A solid reference material within
the cavity, which is preferably a metal/metal hydride reference,
generates a reference partial pressure of hydrogen within the
cavity. A measurement electrode is provided on a surface of the
solid electrolyte outside the cavity and a reference electrode is
provided on a surface of the solid electrolyte within the cavity,
exposed to the reference partial pressure of hydrogen. An
electrical conductor extends from the reference electrode to an
external surface of the sensor body, preferably through an opening
in the sensor cap, which is sealed by brazing or by a silica-free
glass. In this embodiment, the solid electrolyte is preferably
substantially planar. Advantageously, the tube may be of circular
section and the solid electrolyte substantially disc-shaped.
[0032] Preferably the maximum lateral dimension of the solid
electrolyte is less than 10 mm, preferably less than 6 mm and
particularly preferably less than 4 mm.
[0033] Advantageously, the cavity contains a buffer material
between the reference material and the sensor cap. This may not
only advantageously reduce the volume of the cavity containing the
reference partial pressure of hydrogen but may also protect the
reference material from exposure to the brazing or sealing process
required to secure the sensor cap to the tube.
[0034] This and the other aspects of this invention described above
may, in preferred embodiments, provide a probe for measuring
hydrogen concentration in a molten metal which addresses the
problems of prior art probes. In particular, embodiments of the
invention may provide probes which are robust, of conveniently
small size and which provide accurate measurements with rapid
response times. In addition, economical performance over extended
times and during repeated immersion in molten metal may be achieved
in a preferred embodiment in which the sensor is removable from the
probe body and replaceable with a new sensor, or in which the probe
body incorporating the sensor is removable from the probe support
and replaceable.
DESCRIPTION OF SPECIFIC EMBODIMENTS AND BEST MODE OF THE
INVENTION
[0035] Specific embodiments of the invention will now be described
by way of example, with reference to the drawings, in which;
[0036] FIG. 1 is a longitudinal section of a hydrogen sensor
according to a first embodiment of the invention;
[0037] FIG. 2 is a longitudinal section of a hydrogen sensor
according to a second embodiment of the invention;
[0038] FIG. 3 is an exploded sectional view of a probe
incorporating the sensor of FIG. 1;
[0039] FIG. 4 is an assembled sectional view of the probe of FIG.
3;
[0040] FIG. 5 is a three-quarter view of a sensor according to a
third embodiment of the invention;
[0041] FIG. 6 is a longitudinal section of the sensor of FIG.
5;
[0042] FIG. 7 is an exploded view of a probe incorporating the
sensor of FIG. 5;
[0043] FIG. 8 is a longitudinal section of the probe of FIG. 7, in
its assembled form;
[0044] FIG. 9 is schematic view of a probe assembly embodying the
invention;
[0045] FIG. 10 is a side view of a probe according to a further
embodiment of the invention;
[0046] FIG. 11 is a side view of the probe body of the probe of
FIG. 10;
[0047] FIG. 12 is a longitudinal section of the probe body of FIG.
11;
[0048] FIG. 13 is a longitudinal section of an end portion of the
probe support of the probe of FIG. 10;
[0049] FIG. 14 is a longitudinal section of the probe body and an
end of the probe support of the probe of FIG. 10, in assembled
form;
[0050] FIG. 15 is an enlarged view of the coupling between the
probe body and the probe support of FIG. 14;
[0051] FIG. 16 is a longitudinal section of a probe body
incorporating a sensor, according to a further embodiment of the
invention; and
[0052] FIG. 17 is a longitudinal section of the probe body and
sensor of FIG. 16, coupled to a probe support.
[0053] FIG. 1 is a longitudinal section of a hydrogen sensor 2. The
sensor has a sensor body comprising a tube 4, closed at one end by
a planar solid-electrolyte disc 6. The disc has a porous platinum
electrode 24, 26 formed on each surface and is sealed into a recess
in the end of the tube using a silica-free glass 8. A metal-metal
hydride reference material 10 is inserted into the tube behind the
reference electrode and an electrical conductor 12 extends from the
reference electrode along an internal wall of the tube. A volume
within the tube above the reference material is filled with an
inert buffer material 14 such as Y.sub.2O.sub.3 powder. A sensor
cap 16 is then inserted into an upper end of the tube. An electrode
wire 18 extending through a hole in the sensor cap makes contact
with the electrical conductor 12. The electrode wire is sealed in
the hole and the sensor cap is sealed to the tube using a glass
seal 20, preferably of a silica-free glass. The solid electrolyte
disc, the tube and the sensor cap form the walls of a sensor body
enclosing a sealed cavity. The cavity contains the solid reference
material, which generates a reference hydrogen partial pressure
within the cavity. The electrode wire extends outwardly from the
sensor body, coaxial with the tube.
[0054] The solid electrolyte is preferably of indium-doped calcium
zirconate. The tube and the sensor cap are preferably manufactured
from undoped calcium zirconate, in which case the thermal expansion
of the tube is matched to that of the electrolyte disc and the
sensor cap, allowing the sensor to be thermally cycled without the
build up of excessive thermal stresses. Alternatively, the tube and
sensor cap can be manufactured from magnesia-magnesium aluminate
(MMA), which has a thermal expansion coefficient slightly higher
than the indium-doped calcium zirconate electrolyte. In this case,
the electrolyte is permanently in a state of compressive stress
under measurement conditions (immersed in molten metal), increasing
the thermal shock and thermal cycling resistance of the
electrolyte.
[0055] The diameter of the electrolyte disc in the embodiment is 3
mm and the outer diameter of the tube is 4 mm.
[0056] FIG. 2 illustrates an alternative sensor which differs from
the sensor of FIG. 1 in that the tube and the solid electrolyte
disc are fabricated as a single component, termed a thimble 22.
Thus, in this case, the wall of the sensor body consists of a
closed-ended indium-doped calcium zirconate tube, which is closed
at its open end by a sensor cap and an electrode wire in the same
way as the sensor of FIG. 1. Components common to FIGS. 1 and 2 are
given the same reference numerals in both Figures.
[0057] FIGS. 3 and 4 illustrate the assembly of a probe comprising
a probe body 40 and a sensor 2, as shown in FIG. 1. FIG. 3 is an
exploded view of the probe and FIG. 4 is an assembled view of the
probe.
[0058] The probe body encloses a probe body chamber 42 which
terminates at an opening 44. The probe body is of generally
cylindrical shape and at the end of the chamber opposite the
opening, a central bore in the probe body receives an end of a
probe support 46. An end 48 of the probe support forms a portion of
an end surface of the chamber and is brazed or sealed to the probe
body. A blind bore 50 lined with a metallic tube 52 extends
coaxially from the chamber within the probe support. The blind bore
terminates at an electronic conductor 54 which runs along central
bore within the probe support. The end of the electronic conductor
is sealed at the end of the blind bore using brazing or a glass
seal to ensure that the end of the chamber is hermetically
sealed.
[0059] The chamber 42 is shaped so as to receive the sensor 2 and,
when the sensor is fully inserted in the chamber, the electrode
wire 18 enters and makes electrical contact with the metal tube 52,
which thus forms a reference-electrode connection 56, as shown in
FIG. 4. After the sensor has been inserted into the chamber, a
hydrogen-permeable seal or barrier 58 is inserted, as an
interference fit, into the opening 44, closing the chamber and
mechanically retaining the sensor within the chamber.
[0060] Advantageously, there is sufficient clearance between the
sensor and the probe body to allow free expansion and contraction
of the sensor during the thermal cycling caused by immersion of the
probe into molten metal, without the sensor body being constrained
by the probe body as the probe is heated and cooled.
[0061] With the sensor is in place within the chamber and the
hydrogen-permeable seal in place, the hermetic sealing of the
chamber at its sides and at its end opposite the hydrogen-permeable
seal prevents any leakage of hydrogen out of the measuring chamber
when measurements are made and protects the sensor from
environmental contamination.
[0062] The hydrogen-permeable seal prevents direct contact between
the molten aluminium and the solid electrolyte or other components
of the sensor. It is important that direct contact between molten
aluminium and the electrolyte should be avoided as this causes the
electrolyte to leave the hydrogen-ion-conduction domain and to
enter the oxygen-ion-conduction domain. In that case, the potential
of the measurement electrode would be determined by the oxygen
activity at that electrode rather than the activity of hydrogen,
leading to erroneous readings. The hydrogen-permeable seal is,
however, electrically conductive and serves to make an electrical
connection between the measuring electrode and the molten metal. An
analyser can therefore make electrical contact with the measurement
electrode through the melt, and with the reference electrode
through the electronic conductor within the probe support. Graphite
felt, graphite wool or a grade of graphite with open porosity are
suitable materials for the hydrogen-permeable barrier in this
embodiment.
[0063] The probe body is preferably made of a material of high
density, to avoid any gaseous diffusion through the chamber walls,
of high thermal shock resistance, in order to allow rapid immersion
into the melt without breakage, of low thermal expansion
coefficient, and which is chemically stable in contact with the
molten metal during measurement. Machineable-grade aluminium
nitride is a suitable material as it allows the body to be
manufactured cheaply by machining, preferably with no grinding
being required. Other suitable materials for the probe body are
SiAlON, silicon nitride, dense graphite, alumina, magnesia, or
stabilised zirconia.
[0064] The probe body and the hydrogen-permeable barrier are
preferably painted with a titanium diboride ink. By coating the
probe body in this manner, the response time upon immersion in
molten aluminium or aluminium alloys and the response of the probe
to changes in dissolved hydrogen level may be considerably
improved. The TiB.sub.2 coating enhances wetting in molten
aluminium and is electrically conductive, and so improves
electrical contact between the melt and the hydrogen-permeable
barrier and the remainder of the probe body. This may be
advantageous for example during degassing of the molten metal, when
gas bubbles passing beneath the probe tend to cause loss of
electrical contact with the melt, leading to erratic and unreliable
readings. Coating the probe body with TiB.sub.2 ink helps prevent
loss of electrical contact as the coating provides an electrical
contact around the entire surface of the probe body. Any suitable
electronically conductive coating which is stable in the metal melt
may also be used for this.
[0065] The probe support should be made from an
electrically-insulating material to prevent a short circuit between
the reference and measurement electrodes when the probe is immersed
in the melt. Alumina is a suitable material for the probe support
as long as its diameter is sufficiently small (3 mm or less) to
avoid damage due to thermal cycling. Other suitable materials are
SiAlON or silicon nitride. Importantly, any thermal expansion
mismatch between the probe support and the probe body should be
taken into account to ensure that the two are held tightly together
when the probe is heated to its operating temperature.
[0066] FIG. 5 illustrates a third sensor 60, shown in longitudinal
section in FIG. 6. The structure of this sensor is similar to that
of FIG. 2 in that it is formed from a tube of solid electrolyte
material 62 closed at one end and having a reference electrode 64
and a measurement electrode 66 formed on its inner and outer
surfaces respectively. A metal-metal hydride reference material 68
is inserted into the tube and an electrical conductor 70 extends
from the reference electrode within the tube. The conductor is
helically shaped where it contacts the reference electrode in order
to contact a large area of the reference electrode. A spacer 72 is
inserted into the tube above the reference material, and the
electrical conductor extends through a central bore within the
spacer. An upper end of the tube is packed with an inert buffer
material 74 and closed by a sensor cap 76. The electrical conductor
extends through a central bore in the sensor cap. The sensor cap is
sealed to the tube and the conductor using glass seals or brazing.
The external diameter of the tube surrounding the spacer
progressively increases to form a frusto-conical external surface
78, which provides accurate location of the sensor within a
correspondingly-shaped probe body as described below.
[0067] The materials for fabricating the sensor of FIGS. 5 and 6
are as for the sensor of FIG. 2. The spacer 72 is made from an
inert material such as aluminium oxide and takes up dead volume in
the sensor cavity in order to reduce the response time of the
sensor.
[0068] FIGS. 7 and 8 are exploded and assembled views of a modular
probe 80 for receiving the sensor of FIGS. 5 and 6. A probe body 82
of generally cylindrical shape has an internal wall defining a
probe chamber 84 aligned with its axis of symmetry. The probe
chamber ends at an internally threaded opening 86 at one end of the
probe body. An electrical conductor 88 is wound helically within
the blind end 90 of the probe chamber and extends through and is
sealed within a central bore in the probe body extending from the
blind end of the chamber.
[0069] The probe body is externally threaded at both ends 92,
94.
[0070] The probe chamber is shaped to receive the sensor of FIGS. 5
and 6, with the end of the sensor carrying the measurement
electrode being inserted into the blind end of the chamber so that
the measurement electrode makes contact with the electrical
conductor 88. An externally-threaded insert 96 is then threaded
into the internal thread 86 at the end of the probe chamber to
retain the sensor in position. The electronic conductor extending
from the reference electrode passes through a central bore in the
threaded insert and makes contact with a further electrical
conductor 98 which passes through a sealed bore within the probe
body and emerges parallel to the electronic conductor 88 connected
to the measurement electrode.
[0071] An internally-threaded cap 100 is threaded on to the
external thread 92 of the probe body to provide a
hydrogen-permeable seal at the opening of the probe chamber.
[0072] The thread 94 at the other end of the probe body is threaded
into an end of a tubular probe support 102, the measurement and
reference electronic conductors passing along the inside of the
tube. The end of the probe body within the probe support further
comprises a recess 104 for receiving an end of a thermocouple 106
for measuring the temperature of the probe body adjacent to the
sensor. The measurement and reference electronic conductors and the
leads from the thermocouple pass along the tubular probe support
for connection to an analyser for measuring hydrogen concentration
and temperature.
[0073] In the same way as for the probe of FIGS. 3 and 4, in this
embodiment there is sufficient clearance between the sensor and the
walls of the probe chamber to allow the sensor to expand and
contract freely without being constrained by the probe body when
the probe is heated and cooled. In addition, sufficient clearance
is provided to allow hydrogen flow around the sensor to the region
of the measurement electrode.
[0074] The measurement and reference electronic conductors are both
sealed where they run through the probe body, using silica-free
glass or by brazing, to ensure hermetic sealing of the probe
chamber (other than at the hydrogen-permeable seal).
[0075] The hydrogen-permeable seal is provided by the porous cap
100, which allows the exchange of hydrogen between the melt and the
probe chamber whilst preventing aluminium ingress into the chamber.
If the cap is made from a porous grade of graphite, it is
preferably coated with titanium diboride to ensure good wetting,
and hence good hydrogen exchange, with molten aluminium. However,
the cap may be manufactured from other materials, such as porous
ceramic materials (e.g. porous alumina, porous silicon carbide,
porous silicon nitride) or metallic foam. If these materials are
used, it may not be necessary to use a titanium diboride coating on
the cap in order to obtain an adequate probe response.
Nevertheless, employing a titanium diboride coating should improve
the probe response. It may be noted that because the both
measurement and reference electrodes are connected by electronic
conductors to the analyser, there is no need for electrical
connection to the melt, so the hydrogen-permeable seal may be made
using an electrically-insulating material.
[0076] If the probe cap 100 is of an electronic conductor, such as
graphite or metallic foam, an earth lead may be provided to link
the cap, through the interior of the probe support tube, to earth
in order to reduce electrical noise in the sensor signal.
[0077] Selection of the probe body material is as for the probe of
FIGS. 3 and 4 except that it must be of an electrically-insulating
material in order to prevent short circuiting of the measurement
and reference electronic conductors and, if present, the earth
wire. Suitable materials are SiAlON, silicon nitride, and boron
carbide.
[0078] The probe support should be made from a material providing
good thermal shock resistance, chemical stability in contact with
the molten metal, and chemical stability in air over the
measurement temperature range, which for molten aluminium is
typically 650 to 800 C. Suitable materials include SiAlON, silicon
nitride, aluminium nitride and boron carbide. Graphite may also be
used but may require a protective coating, such as aluminium
orthophosphate, or regular replacement due to its decomposition in
air at between 650.degree. C. and 800.degree. C.
[0079] In each of the described embodiments of the invention, the
stability of the electrical signals from the sensor and, if
present, from the thermocouple, may be improved by screening the
electrical conductors carrying the signals from electrical noise.
In a preferred embodiment in which the conductors are directed
within a tubular probe support, this may be achieved by connecting
the probe support to earth if the probe support material is
manufactured from an electrically-conducting material such as
graphite. If the probe support is made from an insulating material,
its internal wall may be coated with an electrically-conducting
oxidation-resistant material such as silver, gold or platinum,
which is then electrically connected to earth.
Electrically-conducting materials with poor oxidation resistance
may also be used, such as copper, if the layer of conducting
material is protected from exposure to oxygen by, for example, a
glass coating. In an alternative embodiment, screening may be
achieved by running the electrical conductor or conductors
(suitably insulated) within an additional metal tube of, for
example, steel or inconel, placed inside and concentric with the
tubular probe support, electrically connected to earth.
[0080] FIG. 9 is a schematic illustration of a probe assembly
100.
[0081] As shown in FIG. 9, a probe 102 comprising a probe body 104
terminates at a hydrogen-permeable cap 106, similar to the
structure illustrated in FIG. 8. The probe is carried on a tubular
probe support 108, within which are housed electrical conductors
carrying signals from the sensor mounted within the probe body and
from a thermocouple. The end of the probe support distant from the
probe is mounted in a bore within a metal handle 110. It is
important that the probe support is held firmly in place such that
any thermal expansion mismatch between the probe support and the
handle does not result in the probe support cracking, or becoming
loose, upon heating. In the embodiment this is achieved by forming
a circumferential groove on the outer surface of the probe support,
into which a copper ring is fitted. As the probe support is
inserted into the bore in the handle, the copper ring enters the
bore and three grub screws positioned around the circumference of
the handle then screw in, in a radial direction, on to the copper
ring. This not only ensures a secure fastening but achieves an
electrical connection between the copper ring and the handle, which
can be employed to earth the probe support. If the probe support is
of a conducting material, then the earth connection is achieved
automatically. If the probe support is of an insulating material
and is internally screened, a connection between the screen and the
copper ring should be made. For example, if the probe support is
internally coated with a metal coating, the coating can be extended
to the outside of the probe support such that it contacts the
copper ring.
[0082] The handle 110 terminates at a hub 112 from which an
electrical socket 114 extends. The hub houses a ceramic connector
block to which the electrical conductors from the sensor and the
thermocouple (if present) are connected. Corresponding connections
extend from the connector block to the electrical connector 114,
which can be connected to an electronic analyser, preferably by
means of a screened cable. The analyser can then generate hydrogen
concentration and temperature measurements from the probe.
[0083] FIGS. 10 to 15 illustrate various aspects of a probe
according to a further embodiment of the invention.
[0084] FIG. 10 is a side view of the assembled probe 200
incorporating a probe body 202 coupled to a probe support 204. The
end of the support distant from the probe body terminates at a hub
206 and a handle 208. The hub comprises a connector or connection
block 210 for electrical connection of the probe to an electronic
analyser.
[0085] The probe body, disassembled from the probe support, is
illustrated in FIGS. 11 and 12. It comprises a probe chamber 212 in
which a sensor 214 is received. The sensor structure is similar to
that illustrated in FIG. 2, comprising a blind-ended tube, or
thimble, 222 of solid-electrolyte material provided with platinum
electrodes formed on the inner and outer surfaces of the blind end
of the tube. The tube is 13 mm long and of 4.5 mm outside diameter.
The tube contains a metal/metal-hydride reference material (not
shown) and is sealed with a sensor cap 216. An electrode wire 218
extends through a hole in the sensor cap and is connected, within
the sensor body, to the reference electrode on the inner surface of
the sensor cavity (not shown). Inert spacers 220 retain the
metal/metal-hydride reference material in position within the
sensor chamber and reduce the internal volume of the sensor
chamber.
[0086] The probe-body chamber 212 is at one end of a probe-body
shaft 230. The shaft is 44 mm long and 8 mm in diameter and
comprises an electrically-conductive core, or rod, of SiC (2 mm
diameter) extending axially within an electrically-insulating tube
234 of SiAlON. The SiC rod may, for example, be secured within the
SiAlON tube by brazing or by means of a glass seal, or in any other
convenient manner.
[0087] At one end of the shaft 230, the probe-body chamber 212 is
formed by push-fitting a cylindrical tube of AlN (aluminium
nitride, 23 mm long, 9 mm outside diameter) 236 on to a
reduced-diameter portion at the end of the shaft. The sensor is
received within the AlN tube and the electrode wire 218 extends
into an axial blind bore 238 formed at the end of the SiC rod 232.
Electrical contact is thus automatically made between the electrode
wire and the SiC rod as the electrode is received in the probe-body
chamber. After the sensor has been inserted into the chamber,
followed by a graphite wool spacer 241, a hydrogen-permeable seal
or barrier 240 is inserted, as an interference fit to close the
chamber and mechanically retain the sensor within it.
[0088] The end of the shaft distant from the probe chamber is
formed with a radially-extending flange 242 which retains an
internally-threaded graphite collar 244. The collar is slidable
along the shaft but is held captive between the flange 242 and the
AlN sleeve 236, which is of larger external diameter than the
SiAlON sleeve 234; during assembly, the graphite collar must be
placed on the shaft before the AlN cylinder is push-fitted on to
the shaft. FIG. 11 is a side view of the probe body, showing the
graphite collar in a position near the middle of the shaft.
[0089] FIG. 13 is a longitudinal section of the end of the probe
support before it is coupled with the probe body. The support 204
comprises a SiAlON tube 250 approximately 50 cm long and 16 mm
outside diameter, within which two electrical conductors extend
from the hub 206 for connecting the electronic analyser to the
reference electrode and the measurement electrode respectively.
These are the reference-electrode conductor 252 and the measurement
electrode conductor 254.
[0090] At the end of the SiAlON tube to which the probe body is to
be coupled, a SiC boss 256 is joined to the tube 250. The external
surface of the boss comprises a cylindrical portion which is coated
with silver ink, inserted within the end of the tube, and heated to
95.degree. C. (the melting point of silver) to secure the joint
257. Other brazing or glassing jointing techniques may also be
used. Advantageously, no change in cross section of the SiAlON tube
is required, reducing thermal expansion stresses upon immersion of
the end of the tube into molten metal. A portion of the boss
extending from the end of the tube is externally threaded, for
receiving the internal thread of the graphite collar 244.
[0091] The reference-electrode conductor 252 is provided by the
sheath of an Inconel 600 sheathed thermocouple which extends
through the length of the SiAlON tube 250. The sheath of the
thermocouple is insulated from the thermocouple wires within it and
so can be used as the reference-electrode conductor. The
thermocouple sheath 252 is coated with an electrically insulating
layer, 264, which insulates the conductor from the boss and within
the SiAlON tube. The end of the sheath, and reference-electrode
conductor, extends through an axial passage 258 in the boss 256.
The sheath is acted upon by a spring 260 at the hub, which urges
the opposite end 262 of the sheath away from the end of the SiAlON
tube and out of the boss 256.
[0092] The measurement-electrode conductor 254 is an Inconel 600
electrode which extends from the hub 206 to an offset blind bore
266 in the boss. The reference-electrode conductor within the
SiAlON tube is threaded through ceramic beads (not shown) to ensure
electrical insulation from the reference-electrode conductor and
from the probe-support tube 250.
[0093] To couple the probe body to the probe support, the graphite
collar 244 is simply threaded on to the boss 256, as shown in FIG.
14. As the collar is threaded on to the boss, an end 268 of the SiC
rod 232 within the probe-body shaft, which stands slightly proud of
the end of the SiAlON sleeve 234 of the shaft within the graphite
collar, comes into contact with the end 262 of the
reference-electrode conductor. As the collar is threaded further on
to the boss, this contact urges the reference-electrode conductor
into the hub, against the action of the biasing spring 260. This
ensures both that good electrical contact is made between the SiC
rod and the reference-electrode conductor, and that the flange 242
of the probe-body shaft is firmly seated within the graphite
collar. When the collar is fully threaded on to the boss, an end of
the collar butts against an end surface of the SiAlON tube 250 of
the probe support. The assembled structure can be seen in FIGS. 14
and 15.
[0094] In the assembled probe, the reference electrode is
electrically connected to the connector 210 at the hub by means of
the electrode wire 218, the SiC rod 232 and the reference-electrode
conductor 252. The measurement electrode is connected to the
connector 210 through the graphite felt 241, the graphite end cap
240, the metal melt, the graphite collar (which in use is submerged
in the melt), the SiC boss 256 and the measurement-electrode
conductor 254.
[0095] Further features of this embodiment of the invention are as
follows.
[0096] At the hot end of the probe, which is submerged in the melt
during use, SiC is used to make reliable high-temperature
electrical connections, as required of the rod 232 within the
probe-body shaft and at the boss 256. SiC is not subject to
oxidation or deterioration under the measurement conditions, on
immersion in molten aluminium at temperatures of between 600 C and
850 C, either under the reducing atmosphere produced by hydrogen
evolved from the aluminium or the oxidising atmosphere likely to
exist within the probe support tube 250, which is exposed to air at
its handle end.
[0097] The graphite collar, the flanged end of the probe-body
shaft, and the end of the probe support tube 250 can be fabricated
to suitable tolerances such that a gasket is not required in order
to prevent aluminium ingress at the joint between the probe body
and the probe support. This advantageously reduces the complexity
of the structure of the joint.
[0098] It is desirable to avoid changes in cross section of
components that are to be immersed in molten metal. Nevertheless,
in the embodiment there is a reduction in cross section of the
probe-body shaft to allow fitting of the AlN cylinder, to form the
probe-body chamber. However, the push-fit of the AlN cylinder 236
on to the end of the shaft allows a slight relaxation of the joint
on immersion in molten aluminium, reducing or preventing stresses.
In addition, the materials are selected to be closely matched in
thermal expansion, again reducing thermal stresses.
[0099] The probe-body shaft separates the sensor cavity from the
end of the probe support. The graphite collar seals the end of the
probe support against ingress of molten aluminium but it is not gas
tight and the inside of the collar is effectively exposed to the
atmosphere, as noted above. Thus, an artificially low hydrogen
level exists at the end of the probe support. Consequently, the
length of the probe-body shaft and the wetting of the shaft by the
molten aluminium must be predetermined to prevent any local low
hydrogen level affecting the hydrogen activity measured by the
sensor.
[0100] The hydrogen-permeable seal 240 is preferable made of porous
graphite and its porosity can be adjusted to improve the stability
of the probe signal while the melt is being treated by gas
injection. If a gas bubble is situated directly underneath the
graphite membrane, it can cause an undesirable rapid change in the
signal from the probe. The probe can be made less sensitive to such
rapid local fluctuations in the partial pressure of hydrogen at its
end by making the graphite seal less porous. This must be balanced
against the requirement for sufficient porosity so that hydrogen
diffusion through the seal provides a sufficiently rapid
probe-measurement response time.
[0101] The external surface of the SiAlON tube 250 of the probe
support may be coated with SiC. This will earth the melt, making
the probe more resilient to noise, for example in induction-heated
furnaces, and will also provide electrical screening of the
conductors within the tube. The inside surface of the SiAlON tube
may also be coated with SiC and the coating used as the
measurement-electrode conductor. The conductor 254 may then be
omitted.
[0102] As shown in FIG. 10, the probe support is fabricated in two
tubular sections 270, 272, coupled at a joint 274. The
probe-support section 270 to which the probe body is coupled
comprises the SiAlON tube 250 described above. The SiAlON tube 250
and the remaining portion of the probe support 272 are each about
50 cm in length. Since an end portion of the SiAlON tube is
immersed in molten metal when hydrogen concentration readings are
taken, it may degrade over time, although degradation is slower
than for the probe body. Thus, it is advantageous to be able to
replace the SiAlON tube 250 as and when excessive degradation
occurs. This can conveniently be achieved by releasing the coupling
274, withdrawing the SiAlON tube and the boss 256, and replacing
these components. The reference-electrode conductor, the
measurement-electrode conductor and the thermocouple need not be
replaced, as these can be threaded through the replacement SiAlON
tube and boss.
[0103] FIGS. 16 and 17 illustrate a further embodiment of the
invention in which the functions of the probe body and the sensor
are integrated into a single unit. The sensor 300 comprises a
sensor tube 302 that is formed with a recessed internal step 304 on
which a planar disc of solid electrolyte 306 is seated, and bonded
in place. The step is recessed such that an end portion 308 of the
sensor tube extends beyond the solid-electrolyte disc. A disc of
graphite wool 310 is inserted into this recess, followed by a
hydrogen-permeable, push-fitting, graphite disc 312. Within the
sensor tube, behind the solid-electrolyte disc, the sensor
structure is similar to that described in various embodiments
above, including that of FIG. 1.
[0104] The tube contains a solid-state hydrogen-reference material
314, packing material 316 and a sensor cap 318.
[0105] A measurement electrode is formed on an outer surface of the
solid-electrolyte disc and a reference electrode on its inner
surface. The measurement electrode contacts the graphite wool and
thus, through the hydrogen-permeable graphite seal, is in
electrical contact with the melt. The reference electrode is
connected to an electrical conductor within the sensor tube (not
shown) and thus to a reference-electrode conductor 320 which
extends axially through a hole in the sensor cap to terminate
standing proud of the upper end of the sensor 322.
[0106] The sensor tube 302 is formed at its end adjacent the sensor
cap with a flange 324 that extends radially outwards from the
tube.
[0107] The external surface of the sensor tube is coated with a
protective thermal-shock-resistant coating 326.
[0108] FIG. 17 illustrates the coupling of the probe body and
sensor 300 to a probe support, which is the same as illustrated in
FIGS. 13, 14 and 15. A graphite collar 328 is internally threaded
for engagement on the threaded hub 256 and the end of the probe
support, and is formed with an axial hole 330 for receiving the
probe body and sensor, such that the flange 324 engages an inner
surface of the collar. Thus, as the collar is threaded onto the
hub, the reference-electrode conductor 322 makes contact with the
spring-loaded thermocouple sheath 262, which serves as the
reference-electrode conductor within the probe support, as
described in relation to FIGS. 13 to 15.
[0109] The measurement-electrode is electrically connected through
the graphite wool, the graphite seal, the melt and the graphite
collar, in the same way as in previous embodiments.
[0110] It can be seen that in this embodiment the probe body may be
integral with the sensor and comprise a coupling means (in this
embodiment the graphite collar) to provide a probe-body unit which
is releasably couplable to the probe support.
Electronic Analyser
[0111] As described above, the current-reversal measuring technique
may be used to measure hydrogen concentration using the probe, and
to monitor dehydration of the sensor electrolyte. It is also
possible, however, to use a conventional impedance analysis unit.
In this case, the analyser measures sensor EMF, temperature, and
sensor impedance. Only EMF and temperature are required to
calculate dissolved hydrogen level; impedance is used in a
different set of calculations to determine sensor condition, as
described below.
[0112] An EMF is generated between the sensor electrodes according
to the Nernst Equation for a hydrogen ion conductor (1):
EMF = RT 2 F ln p H 2 ref p H 2 meas ( 1 ) ##EQU00001##
[0113] The reference hydrogen partial pressure inside the sensor
(pH2ref) is temperature dependent. The analyser is programmed with
two calibration values (A and B) which allow it to work out what
the reference hydrogen partial pressure is at any given
temperature. These calibration values are obtained by measuring the
sensor EMF in a known partial pressure of hydrogen at two different
temperatures.
[0114] The partial pressure of hydrogen in equilibrium with molten
aluminium is related to the concentration of dissolved hydrogen (H)
by Sievert's law (2):
log H = 1 2 log p H 2 meas - C T + D ( 2 ) ##EQU00002##
The constants C and D depend upon the aluminium alloy and vary
according to how much the chemistry of the different alloys (e.g.
silicon content, magnesium content etc) affects hydrogen
solubility.
[0115] So, effectively the calculation can be broken down into the
following stages:
[0116] From the measured temperature, work out the reference
hydrogen pressure inside the sensor;
[0117] From the measured EMF and (i), use equation (1) to work out
the partial pressure of hydrogen in equilibrium with molten
aluminium;
[0118] Use (ii) and equation (2) to work out the concentration of
dissolved hydrogen in the melt.
[0119] In reality, this can all be combined into one equation
(temperature T in degrees Centigrade here).
H = 10 { 5.03913 T + 273 ( A + ( T - 700 ) 50 ( B - A ) - EMF ) - C
T + 273 + D } ##EQU00003##
[0120] The analyser also monitors sensor impedance, or resistance,
to determine sensor condition as follows. Two calibration
constants, R.sub.700 and R.sub.750, which are the resistance of the
sensor after manufacture at 700 C and 750 C respectively, are
measured and programmed into the analyser. The resistance of the
sensor in its as-manufactured, hydrated state can then be
calculated at any temperature using the Arrhenius dependence of
conductivity on temperature. The analyser monitors the sensor's
actual resistance and measures its deviation from the calculated
value, and flags any deviation greater than a predetermined
threshold, such as SkOhms deviation. This strategy provides an
accurate indication of the condition of the electrolyte, and allows
the analyser to display an appropriate error message if the sensor
becomes dehydrated. The temperatures 700 C and 750 C are arbitrary;
other calibration temperatures could be used.
Other Metals
[0121] The embodiments described above have been presented in the
context of a probe for measuring hydrogen concentration dissolved
in molten aluminium and aluminium alloys. A similar probe may be
used to measure hydrogen concentration dissolved in molten
magnesium and its alloys; any modifications required to ensure
materials compatibility with molten magnesium could be carried out
by the skilled person without inventive effort. With modification
to the embodiments, similar probes could be applied to measure
hydrogen concentration dissolved in molten copper and its alloys.
The maximum operating temperature of the sensors described in the
embodiments is approximately 850.degree. C., beyond which
temperature the performance of the metal-metal hydride reference
degrades. Molten copper is typically at a temperature of about
1100.degree. C. In order to measure hydrogen concentration in
molten copper, the probe body would therefore need to be extended
in order to locate the sensor further away from the melt so as to
keep the sensor temperature below 850.degree. C.
[0122] In conclusion, it can seen that the invention in its various
embodiments overcomes many of the disadvantages of prior art
hydrogen sensors. These advantages include the following.
Portability
[0123] The described embodiments provide a portable probe which can
easily be transported to different measuring locations in a
foundry. Only the light-weight, solid-state probe and the
associated analyser needs to be transported. In addition, the good
portability and fast response time of the probe may advantageously
allow batch measurements to be made, for example so that a quick
check of dissolved hydrogen level can be performed prior to
casting.
Suitability for Repeated Immersion
[0124] The sensor in the embodiments is self-contained and may
advantageously not be joined or sealed to the probe body. Thus, the
sensor may advantageously not be subject to physical constraint by
the probe body, and the associated forces resulting from thermal
expansion mismatch, as in prior art designs. This may
advantageously improve the sensor's resistance to thermal cycling
and make the probe suitable for repeated immersion in molten
metal.
[0125] As a probe is repeatedly dipped in and out of the melt,
particularly for molten aluminium, a blocking oxide layer (e.g. of
aluminium oxide) can build up which impedes the exchange of
hydrogen between the melt and the probe chamber. This can slow down
response time and can cause the chemical potential of hydrogen at
the sensor's measurement electrode to fall below the equilibrium
level in the melt, upon repeated immersions. In prior art probes,
this blocking oxide layer builds up due to the reaction between
oxygen in air contained in the probe chamber and the molten metal.
The self-contained nature of the sensor and probe of the
embodiments described herein allows the probe chamber to be
designed with a minimum of dead volume (free chamber volume), which
may advantageously alleviate both the problems of slow response
time and reducing hydrogen-concentration measurement observed in
prior art probes after repeated immersions.
Low Cost Per Measurement
[0126] Prior art measuring techniques suffer from high cost per
measurement due to the high cost and short lifetime of hydrogen
probes. The embodiments of the present invention described herein
enable the use of replacement components, including replacement
sensors, which may advantageously be manufactured more cheaply and
enable the probe to remain in service for longer. Consequently, the
cost per measurement may advantageously be reduced by comparison
with prior art techniques.
On-Line Monitoring of the Degassing Process
[0127] The embodiments of the present invention are of
advantageously robust construction and may enable rapid response to
changes in dissolved hydrogen level. These probes may therefore be
used in conjunction with a rotary degasser for on-line, real-time
monitoring of the degassing process.
CONCLUSION
[0128] Important advantages of the embodiments of the invention
include the self-contained nature of the sensor and the portability
of the probe. The self-contained nature of the sensor is exploited
in various ways. First, the sensor is preferably not joined or
bonded to the probe body, which may dramatically improve the
thermal shock resistance of the sensor and lead to longer sensor
lifetime in terms of cycles to failure. Second, the probe body and
the sensor may preferably be miniaturised, giving the following
benefits; reducing the dead volume in the probe chamber may
advantageously improve response time to changes in hydrogen
concentration and prevent accumulation of metal oxide on the probe
surface upon repeated immersion; the pre-heat time of the probe on
immersion may be advantageously reduced and the probe's thermal
shock resistance improved. Finally, the cost of manufacture of the
probe and replacement sensors may be advantageously low.
* * * * *