Probe for the determination of gas concentration in molten metal

The invention provides a new immersion head probe for use in apparatus for the determination of the concentration of gas dissolved in a molten metal. Such determinations are needed to facilitate removal of the gas, which can cause bubbles in the solidified metal and subsequent processing difficulties. The apparatus circulates an inert carrier gas through the probe in gas exchange contact with the molten metal to entrain dissolved gas until an equilibrium mixture is obtained; the concentration of the dissolved gas in the mixture then is representative of its concentration in the molten metal. The head consists of a monolithic or integral body of a porous gas-permeable material of sufficient mechanical strength at the temperature of the molten metal and of sufficient porosity, pore size and permeability to permit the necessary gas diffusion in a reasonable period of time. The body of the probe preferably is thin in one direction to minimize the path length for the gas to diffuse therein, and is provided with a carrier gas inlet and outlet suitably spaced apart to ensure that the gas diffuses throughout the body. If the test is to be carried out in a stationary body of molten metal, the probe may be vibrated or the metal may be stirred to increase the probe/metal contact, both decreasing the time required for the gas mixture to reach sufficient equilibrium.

FIELD OF THE INVENTION 
The present invention relates to a probe for use in apparatus for measuring 
the concentration of a gas such as hydrogen dissolved in a molten metal, 
so as to permit the total content of the gas in the metal to be 
determined, and to apparatus employing such a probe. More particularly, 
the invention is concerned with a probe and apparatus for direct 
measurement of the content of hydrogen dissolved in liquid metal, more 
specifically molten aluminum and alloys thereof. 
REVIEW OF THE PRIOR ART 
Many metals including aluminum and its alloys when in the liquid state 
react chemically quite readily with the moisture in the atmosphere to form 
gaseous hydrogen which, owing to its high solubility will dissolve readily 
in the liquid metal. This is particularly true of aluminum and its alloys 
and for convenience the following discussion will make reference 
principally to this metal. Thus, the solubility of hydrogen in aluminum 
and its alloys is particularly high, about 1 mL STP/100 grams at the 
melting temperature (about 700.degree. C.), but the solubility in the 
solid metal is only about one-tenth of this value, and this dissolved 
hydrogen can generate serious problems during further processing of the 
solid metal. For example, during solidification there is a strong tendency 
for the excess gas to be expelled from the metal, leading to the formation 
of blow holes and gas bubbles which are trapped therein. Such bubbles lead 
to the formation of cracks in the cast ingots, which can have disastrous 
consequences during subsequent rolling operations, and can ruin the 
surface finish of thin foil products. There is therefore an increasing 
requirement to degas the molten metal prior to the metal casting process. 
Degassing processes usually comprise the introduction of chlorine gas 
and/or an inert gas such as nitrogen or argon into the molten body or 
stream of metal in the form of a dispersion of fine bubbles. Typically 
dilute mixtures of chlorine in argon are used with one or more lances or 
rotating impellers to introduce the degassing media into the melt. The 
efficient operation of the degassing process requires an accurate 
knowledge of the concentration of the hydrogen gas in the metal, so that 
its total content can be determined, and numerous techniques exist for 
such measurement. Most of these techniques require the preparation of a 
solid sample and access to sophisticated analytical equipment suitable 
only for use in a laboratory setting and not the relatively arduous 
conditions of a metal casting shop. Moreover, although these methods are 
precise they are relatively slow and do not allow the necessary 
information to be obtained "on-line" during the progress of a casting 
operation. 
There is at present only one method known to the applicants which enables 
direct measurement within the molten metal and allows on-line analysis in 
the plant, namely the "Telegas" process, as described in U.S. Pat. No. 
2,861,450 of Ransley et al. The "Telegas" apparatus comprises a probe 
immersion head which is immersed in the molten metal, the head comprising 
an inverted collector cup or bell of heat resistant impervious ceramic 
material whose mouth is closed by a ceramic filter to form a chamber 
within its interior. A first capillary tube extends downward through the 
head and the filter, while a second such tube extends upward from the 
interior of the chamber. A fixed quantity of an inert gas, usually 
nitrogen, is circulated in the apparatus by feeding it down through the 
first tube and withdrawing it through the second tube, so that it bubbles 
into the molten metal adjacent the head, the bell collecting the 
upwardly-moving bubbles, while the ceramic filter prevents the molten 
metal from entering the enclosure. The nitrogen entrains some of the 
hydrogen in the adjacent metal and is constantly recirculated for a 
sufficient length of time, usually about 5 to 10 minutes, until the 
partial pressure of the hydrogen gas in the nitrogen/hydrogen mixture 
reaches an equilibrium value. Owing to the high mobility of the dissolved 
hydrogen in the molten metal, this will accurately represent the hydrogen 
concentration throughout the body of the melt. 
As equilibrium is approached the concentration of the hydrogen in the 
carrier gas is monitored by measuring the difference in electrical 
resistance of two like hot-wire detecting elements disposed in respective 
equal measuring cells, one of which receives the nitrogen/hydrogen mixture 
and the other of which has an atmosphere whose thermal conductivity is 
substantially equal to that of the nitrogen, usually air. The difference 
in resistance is measured by a bridge circuit, the value being calibrated 
to correspond to the hydrogen gas concentration value, as determined by 
any of the laboratory-type analytical apparatus mentioned above. This 
measured value will need to be compensated for melt temperature, and also 
for the different solubility of hydrogen in the specific metal or alloy 
with which the apparatus is employed, by any of the methods well known to 
those skilled in this particular art. 
There are several technical problems connected with this type of immersion 
head. Firstly, the probes are made of high density ceramic materials in 
order to be resistant to the molten metal and also to be impervious to 
diffusion of the hydrogen therethrough, so that faulty readings will not 
be obtained. Such materials have very low resistance to thermal and 
mechanical shock, and any mishandling leads to damage or even destruction. 
For example, it is essential in practice to preheat the probe before 
immersion by positioning it close to the body of molten metal, and to 
insert it and withdraw it slowly from the metal in order to prevent such 
thermal shocks. Again, such a probe theoretically should be effective for 
20 to 30 analyses before requiring replacement, but it is not unknown for 
them to become useless after only three immersions in the melt. The usual 
cause of this is splashing of the liquid metal during the part of the 
analysis cycle in which the gas mixture is purged from the probe, this 
metal blocking the porous ceramic element so that it cannot perform its 
function. Further, because of the design the probes are relatively 
expensive to produce. Difficulties also arise in obtaining rapid and 
accurate analyses, owing to the particular shape of the probe. Thus, if 
the probe is not kept vertical in the molten metal, some of the carrier 
gas may escape from beneath the cup to the surface, leading to an 
erroneous reading. Moreover, the gas that bubbles from the first conduit 
ideally should disperse uniformly in the adjacent body of metal, but 
instead tends to stay close to the outside wall of the conduit, so that 
the recirculation time is considerably increased. 
Another form of immersion probe has been disclosed in a paper by R. N. 
Dokken and J. F. Pelton, of Union Carbide Corporation, entitled "In-Line 
Hydrogen Analysis in Molten Aluminum" and presented in an international 
seminar on refining and alloying of liquid aluminum and ferro alloys held 
in Trondheim, Norway on Aug. 26-28, 1985. This probe was intended to 
replace the "Telegas" probe with the intention of correcting deficiencies 
perceived therein, such as the possibility that the recirculating gas 
forms an envelope around the tip of the probe to cause a loss of carrier 
gas and consequent inaccuracy. This probe is described in the paper as 
comprising two long concentric metallic tubes attached to two heavier 
metallic tubes. The outer tubes are protected from dissolution into the 
aluminum by having a woven ceramic blanket covering their outer surfaces. 
The two heavier tubes are the measuring head of the probe, with the spaces 
within the ceramic fiber weave providing a zone for the transfer of 
hydrogen from the molten aluminum to argon carrier gas in these spaces. 
This carrier gas is recirculated through the two long concentric tubes up 
to the measuring portion of the instrument. 
This probe is essentially a steel structure in which the area of the 
gas/aluminum exchange surface is of the same order as that of the 
steel/aluminum contact surface. Hot steel at the operative temperature is 
quite permeable to hydrogen and is subject to oxidation; the resulting 
oxidized steel can develop an exothermic reaction with the molten 
aluminum, and the oxide can react with the hydrogen to form water, leading 
to false readings. Owing to its design, the regions enclosed by the 
ceramic weave are effectively "dead" zones having little or no direct 
contact with the circulating carrier gas, and there is moreover the clear 
possibility of the inflowing gas "short-circuiting" directly from the 
inlet to the outlet, leading to longer equilibrium times. 
DEFINITION OF THE INVENTION 
It is therefore a principal object of the present invention to provide a 
new apparatus for determining the concentration of gas dissolved in a body 
of molten metal, particularly to a method that provides an "on-line" 
direct measurement of such gas concentration, and more particularly to an 
apparatus that permits such measurement of the concentration of hydrogen 
in aluminum. 
In accordance with the present invention there is provided an immersion 
probe for immersion in a molten metal for determination of the 
concentration of a gas dissolved therein, the probe comprising: 
a probe body consisting of a gas-permeable, liquid-metal-impervious 
material of sufficient heat resistance to withstand immersion in the 
molten metal; 
the body having a gas inlet to its interior, and a gas outlet therefrom; 
the gas inlet and outlet being spaced from one another so that gas passing 
from the inlet to the outlet traverses a substantial portion of the probe 
body interior for entrainment of gas diffusing to the interior of the body 
from the ambient molten metal. 
Also in accordance with the invention there is provided in an apparatus for 
the determination of gas concentration in a molten metal the combination 
of: 
an immersion probe for immersion in the molten metal, the probe comprising: 
a probe body consisting of gas-permeable, liquid-metal-impervious material 
of sufficient heat resistance to withstand immersion in the molten metal; 
the body having a gas inlet to its interior and a gas outlet therefrom; 
the gas inlet and outlet being spaced from one another so that gas passing 
from the inlet to the outlet traverses a substantial portion of the probe 
body interior for entrainment of gas diffusing to the interior of the body 
from the ambient molten metal; 
carrier gas supply means; 
a recirculation gas pump for the carrier gas and gas entrained therein; 
a gas concentration determining means adapted to determine the proportion 
of the gas present in a mixture thereof with a carrier gas; and 
conduit means connecting the carrier gas supply means, the gas inlet, the 
gas outlet, the gas recirculating pump and the gas concentration 
determining means in a closed circuit for circulating a carrier gas 
through the probe to entrain therein gas that has diffused into the probe 
body from the molten metal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1 there is shown therein a probe element 10 of the 
invention, consisting of a monolithic body 12 of gas-permeable, 
liquid-metal-impervious material, immersed in a body 14 of molten metal, 
specifically of molten aluminum or an alloy thereof. The body 14 may be 
stationary, as would be obtained in a ladle or a laboratory sample, or it 
may be a stream of metal, as would be obtained in a transfer trough 
leading from a casting furnace. The specific structure of the probe 
element will be described in detail below. A fine bore tube 16 extends 
from a gas inlet 18 in the body of the probe element to a recirculation 
pump 20 via a non-return valve 22, and thence via another non-return valve 
24 to the gas outlet of the sensing cell 26 of a katharometer 28. Another 
fine bore tube 30 extends from a gas outlet 32 from the body 12 to the gas 
inlet to the katharometer sensing cell 26, so as to complete a closed 
circuit including the probe element, the pump and the cell. The tube 30 
includes a T-junction by which the gas circuit is connected to a 
controllable flushing valve 34 which when opened admits a flushing gas, 
usually nitrogen, into the circuit from a suitable source, usually a 
cylinder of the compressed gas (not shown). 
In the embodiment of FIG. 1, the comparison cell 36 of the katharometer is 
open to atmosphere, since ambient air is a suitable comparison medium when 
the carrier gas is nitrogen. However, if some other carrier gas is used, 
such as argon, it would then be necessary either to seal the comparison 
cell containing said gas, or to flow the gas continuously through the 
cell. Each cell contains a respective fine resistance wire 38 and 40 
connected as the respective adjacent arms of a bridge circuit 42. The 
other bridge arms are constituted in well known manner by resistors 44 and 
46, the bridge is supplied with operating current from battery 48 via 
adjusting resistor 50, and a bridge meter 52 or other measuring device 
being connected in known manner between the two opposite junctions. A 
thermocouple 54 is mechanically connected to the probe element 10 so that 
it is immersed therewith into the molten metal 14 and provides the 
necessary measurement of the metal temperature. 
The thermocouple 54, the pump 20, the flushing valve 34, and the bridge 
measuring device 52 are all connected to a computer controller 56 which is 
arranged to automatically control the apparatus through each concentration 
determining cycle of operations, and to feed the results of the cycle to 
one or more display and/or recording devices which will be apparent to 
those skilled in the art. 
A typical measurement cycle will begin with the flushing valve 34 being 
opened by the controller 56, so that dry nitrogen under pressure 
circulates through the entire circuit, entering at both the probe gas 
inlet 18 and the outlet 32 and exiting through the porous body of the 
probe element; this circulation is maintained long enough to ensure that 
only nitrogen remains in the circuit. On start-up it is also desirable to 
maintain the flushing for a sufficiently long period to ensure that all 
moisture has been eliminated. The flushing operation is maintained until 
the probe has been lowered into the melt when the valve 34 is closed and 
the pressure of the nitrogen in the circuit will quickly reach a steady 
value. In practice the flushing is carried out at a gas pressure of about 
20 to 50 KPa (3 to 7 p.s.i.), which reduces to a range of about 2 to 8 KPa 
(0.25 to 1 p.s.i.) during the test procedure. The operation of the pump 
motor causes the volume of carrier gas in the circuit to be constantly 
recirculated therein, passing in the body 12 from the inlet 18 to the 
outlet 32. 
Owing to the very high mobility of hydrogen in liquid aluminum at the usual 
temperatures involved (700.degree. C.), it will rapidly and easily enter 
the porous probe body in attempting to establish concentration equilibrium 
and become entrained in the carrier gas, the circulation of this gas being 
maintained for a period of time known to be sufficient to establish 
equilibrium, usually of the order of 1 to 10 minutes. At the end of this 
period the controller is operative to take a measurement of the difference 
in resistivity of the resistance wires 38 and 40 in the katharometer. The 
nitrogen/hydrogen mixture causes increased cooling of the wire 40 because 
of the presence of the hydrogen, this increase being a measure of the 
partial pressure or concentration of the hydrogen in the nitrogen/hydrogen 
mixture, and thus of the concentration of the dissolved hydrogen in the 
metal body. The controller will usually be arranged to compute the 
concentration value directly, as will be apparent to those skilled in the 
art, including the application of a correction factor from an 
operator-adjusted circuit 58 to account for the different solubility of 
hydrogen in different metals and alloys. Upon conclusion of the 
measurement portion of the cycle the circuit is flushed as described 
above, so that it is ready for a new cycle. The probe may be removed from 
the metal or left in place at the choice of the operator. 
The improved operation of the probes of the present invention is best 
described by comparison with the "Telegas" probe which consists of dense 
gas-impervious ceramic body from which the nitrogen carrier gas is bubbled 
into the metal body in direct contact with the metal and the hydrogen 
dissolved therein. It has been considered necessary for such direct 
contact to take place to obtain effective entrainment of the hydrogen in 
the carrier gas. The difficulties in practice obtained with this apparatus 
have been described above and do not require to be repeated. 
By contrast a probe element 10 of the invention, by elimination of this 
bubbling and its replacement with direct diffusion and mixing of the gases 
within the interstices of the probe body, can consist of a single 
monolithic or unitary block of material of suitably chosen porosity, pore 
size and permeability provided with a gas inlet and a gas outlet spaced 
sufficiently apart that the circulating carrier gas must traverse a 
substantial portion of the interior of the probe body. The small probe 
body almost immediately reaches the temperature of the ambient metal, and 
the hydrogen therefore readily diffuses in the pores of the block, so that 
it will quickly mix with the carrier gas and attain the necessary 
equilibrium of concentration. 
The porosity of a body is usually expressed as a percentage and is simply 
the proportion of the total volume of the body that is occupied by the 
voids within the body, a highly porous body having a high percentage of 
voids. A high porosity has the advantage that the material is usually more 
resistant to thermal shock, so that the probe can be plunged directly into 
the metal without prewarming, and removed without having to cool it 
slowly, and there is greater opportunity for diffusion of the hydrogen 
into the body, circulation of the nitrogen in the body, and mixing of the 
two gases together. However, a high porosity body inevitably has many 
large pores and is usually structurally weaker, to the extent that it may 
be difficult to anchor the tubes 16 and 30 in the body, and the probe may 
become too fragile for satisfactory handling under industrial testing 
conditions. Again, because of the large pores of a highly porous body 
difficulty may be encountered in the liquid metal seeping into the body. 
The range of porosity for the probe bodies of the invention is from a 
minimum of about 5% to a maximum of about 80%, but preferably is in the 
range of about 20% to about 60%, and more preferably is in the range from 
about 35% to about 40%. 
A second important consideration in the choice of suitable materials for 
the probe body is the pore size, and this can vary over a wide range, 
namely from about 0.5 micrometers to 2,000 micrometers, since the size of 
the hydrogen molecules in the metal is of the order of 2.times.10.sup.-4 
micrometers (2 Angstroms), and both gases can diffuse easily even in the 
smallest size pores. The lower limit is determined more by the impaired 
resistance of fine-pored materials to thermal shock, while the upper limit 
is dictated by mechanical assembly problems, as described above and the 
increased possibility of the molten metal entering the larger pores. For 
example, with aluminum under normal operating conditions penetration of 
the metal into the pores will start to become excessive above 1,000 
micrometers. The preferred pore size is therefore in the range 10 
micrometers to 1,000 micrometers, and more preferably is in the range 50 
micrometers to 200 micrometers. 
The third important consideration in the material choice is its 
permeability. A body of porosity and pore size within the preferred ranges 
may still be unsatisfactory if the cells or voids are completely "closed" 
off from one another, or are so poorly interconnected that the gases 
cannot diffuse and mix together within a reasonable period of time. 
As previously described, the porosity of the probe body must be due 
predominantly to interconnected pores or voids so that it is sufficiently 
permeable to the gases. Permeability may be generally defined as the rate 
at which a gas or liquid will pass through a material under a specified 
difference of pressure. Permeability of any given material can be measured 
by determining the quantity of a fluid (in this case air) that will flow 
through a thin piece of the material of specified dimensions under a 
specified low pressure differential. 
For flows occurring under low pressure differentials, D'Arcy's Law states: 
##EQU1## 
where Q=Air flow (m.sup.3 /s) 
P.sub.e =Specific permeability (m.sup.2) 
L=Sample thickness (m) 
A=Sample cross-sectional area (m.sup.2) 
u=Air viscosity at the temperature of measurement (1.84.times.10.sup.-5 
Kg/m-s at 20.degree. C.) 
P=Pressure (Pa) 
The permeability is usually expressed in Darcy units, 
where: 
EQU 1 Darcy=1.times.10.sup.-12 m.sup.2 
Therefore equation (1) can be written: 
##EQU2## 
where P.sub.D is the specific permeability expressed in Darcies. 
For air at 20.degree. C. and using a pressure differential of 2 in. H.sub.2 
O (500 Pa): 
##EQU3## 
With the probes of the invention it is preferred that the permeability be 
in the range about 2 to about 2,000 Darcies, more specifically in the 
range about 10 to about 100 Darcies. 
The pore size of the material must be such that both of the carrier gas and 
the hydrogen will diffuse readily therethrough and become mixed with one 
another, while it must be impossible for the metal to enter more than the 
surface layer of the probe body. Thus, it is acceptable to find after the 
conclusion of a measurement cycle that a thin skin of solidified metal has 
mechanically adhered to the exterior surface of the probe, since this can 
readily be stripped away before the next cycle without damage to the 
probe. Theoretically, it would seem to be advantageous for the exterior 
surface of the probe body to be metal-wettable, so as to obtain a 
high-diffusion interface between the metal and the probe, but in practice 
it is found that reproducible results can be obtained with a monolithic 
body of non-wettable material, particularly if the probe and/or the metal 
are stirred as described below. The presence of the above-described thin 
skin of aluminum on the probe surface indicates that the surface has 
become wetted and once this has taken place the surface will remain 
wetted. Wetting can be facilitated by precoating the body with a thin 
layer of a suitable metal such as aluminum, silver, nickel or platinum, as 
indicated diagrammatically in FIGS. 2 and 3 at 59 by the broken outline. 
The metal layer can be applied by any of the well-known processes for such 
deposition, such as dipping, spraying, electrolylic, electroless, etc., 
the layer being preferably of about 10 micrometers (0.0004 in) to 1000 
micrometers (0.04 in) in thickness. 
It is found particularly advantageous to employ for the coating 59 a 
material that has a catalytic action toward the hydrogen, promoting 
association from its monatomic state in the molten aluminum to the 
molecular diatomic state in the probe body for its entrainment in the 
carrier gas. A particularly suitable metal for this purpose is platinum, 
which can readily be deposited in the desired very thin layers from 
commercially available electroless platinising solutions. Because of its 
metallic nature platinum will in addition facilitate wetting as described 
above. As an example of a suitable process the body 12 is immersed in the 
platinising solution for a brief period which may be from about 5 seconds 
to about 5 minutes (the specific time depending upon the solution 
concentration and the coating thickness desired), the solution normally 
consisting of about 3% concentration of platinum chloride (PtCl.sub.4) or 
hydroplatinochloride (H.sub.2 PtCl.sub.4) in hydrochloric acid, optionally 
including lead acetate as a buffer. The body is then baked at a 
temperature above 500.degree. C., usually about 800.degree. C., to ensure 
that no residual hydrochloric acid remains. The coating obtained is 
estimated to be of thickness of about 1 micrometer (0.00004 in.) to 100 
micrometers (0.004 in.) and thicknesses of about 0.1 micrometer (0.000004 
in.) to 1000 micrometers (0.04 in.) are considered to be suitable. It is 
found that in use the catalytic coating does eventually dissolve away and 
if the probe body still has sufficient useful life it can easily be 
re-coated. Other materials that will function in this manner are, for 
example, palladium, rhodium and nickel. 
The shape of the probe is not at all critical, but it is advantageous that 
in at least one dimension it be as small as is practical, so as to provide 
a corresponding minimum path length for the hydrogen to diffuse into the 
block interior. Preference is also given to shapes that maximize the 
active metal/probe surface area for a given probe volume. These 
considerations give preference to the shape of a thin wafer, as 
illustrated by FIGS. 2 and 3, that is rectangular in all elevations. It 
will be noted that wherever possible edges of the body are rounded so as 
to avoid as much as possible sharp corners that are particularly 
susceptible to mechanical shock. The thickness of the probe to provide the 
desired minimum path length should be between about 0.5 cm and 1.5 cm, the 
minimum value being determined also by the mechanical strength of the 
material and thus of the resultant wafer. Advantageously the volume of the 
probe is between 1 cc and 10 cc, preferably from 2 cc to about 5 cc. 
Referring again to FIGS. 1 to 3, it will be seen that in this particular 
embodiment the probe body 12 is provided with two parallel bores 60 and 62 
which respectively receive the ends of the two tubes 16 and 30; the bores 
extend into a groove 64 in which the tubes are bent to lie and into which 
they are fastened by a layer of a suitable heat resistant cement 66 (FIG. 
1). This structure brings the two tubes closer together, as seen in FIG. 
1, to facilitate their enclosure in a sheath 68 of a heat resistant 
material, such as a material woven from an alumina fibre, and at the same 
time provides added resistance to torques that are applied to the body 
during its handling and its immersion, etc. in the body of liquid metal. 
In constructing an apparatus of this type it is desirable to keep the 
volume of carrier gas that is required as small as possible, so as to 
decrease the time required for equilibrium to be reached, and this 
consideration dictates the use of narrow bore tubes 16 and 30, a miniature 
recirculating pump 20 and a probe 10 of small volume. It will be 
understood that the volume of gas to fill the probe will be at most the 
volume of the voids therein. A practical volume for a complete system is 
between 1 cc and 5 cc, while a practical gas flow rate to obtain a 
reasonably short response time is from about 50 cc to about 200 cc per 
minute. However, as the volume of the probe is reduced there is a 
correspondingly reduced access of the metal and the hydrogen in the melt 
to the carrier gas and a compromise is therefore necessary. A very 
successful probe of the invention consists of a porous circular-segment 
alumina disc as shown in FIG. 4 of porosity about 35% to 40%, average pore 
size about 120 micrometers and permeability about 25 Darcies. The body has 
a thickness 0.64 cm (0.25 in.) and diameter 2.5 cm (1.00 in.) to have a 
volume of about 3 cc (0.3 inch cubed). 
It will be seen that a simple monolithic block of such shape is easy to 
manufacture by well known procedures. Because of its compact 
configuration, such a body inherently has high resistance to mechanical 
shock. Moreover, since it is operative totally immersed in the liquid 
metal with the exchange of hydrogen between probe and metal taking place 
through the probe body surface, and the hydrogen entrainment into the 
carrier gas taking place entirely within the interior of the probe body, 
then its atitude and positioning in the metal body is completely 
non-critical avoiding this possibility of error. It will also be noted, 
that because of this internalization of the mixing or entrainment 
mechanism the probe is able to operate successfully in a fast-moving 
stream of metal, such as in a transfer trough, which is not the case with 
a probe relying on external bubbling for entrainment, when the bubbles may 
be swept away before they can return into the probe. The material must be 
refractory in nature, namely able to withstand the temperature of 
immersion without softening to an unacceptable degree, and as non-reactive 
as possible with the metal, since such reactivity will eventually require 
the probe body to be replaced. A very satisfactory probe material for use 
in aluminum is fused. granular alumina, the grains being held together by 
a porcelanic bond; such materials of a wide range of porosities are 
commercially available. 
It will be seen that the probes of the invention can easily be made 
entirely of non-metals, avoiding problems of corrosion and diffusion of 
the hydrogen, which at the temperatures involved will diffuse through most 
commercially useful metals. By suitable choice of the porous material used 
for the body, it is possible to obtain a large gas exchange surface in a 
compact monolithic or unitary integral body, with a maximum of the body 
volume occupied by the pores and minimum of "dead,volume" occupied by the 
solid material. 
The probes of the invention can take a number of different forms, and some 
examples are shown in FIGS. 4 through 12. As previously described, the 
embodiment of FIG. 4 is formed as the major segment of a flat circular 
disc, while that of FIG. 5 is a complete circular disc, the tubes 16 and 
30 extending different distances into the body 12 to increase the length 
of the flow path between the inlet 18 and outlet 32. FIG. 6 shows a 
rectangular body that is somewhat longer than it is wide, with the tubes 
16 and 30 extending different distances into the body, as with the 
structure of FIG. 5, while FIG. 7 shows a probe with a cylindrical body, 
the tubes 16 and 30 entering at opposite ends. FIG. 8 illustrates a 
triangular-shaped probe body and FIG. 9 an elliptical-shaped body, while 
FIG. 10 shows that a quite irregular-shaped body of a suitable material 
can be provided with a gas inlet and outlet and function successfully. 
FIG. 11 illustrates the fact that the body is not necessarily monolithic, 
i.e. formed from a single block of material, but instead can be an 
integral body that is assembled from more than one piece joined together 
by a suitable cement (not shown), care being taken to ensure that the 
cement layer does not constitute a barrier to free diffusion of the gases 
through the body from the inlet to the outlet. The bores 60 and 62 are in 
this embodiment constituted by mating semi-circular cross-section grooves. 
FIG. 12 illustrates another integral structure containing a large open 
void 70 into which the tubes 16 and 30 discharge, the hydrogen diffusing 
into this volume through the wall of the probe body; such a structure does 
permit a somewhat less porous material to be used for the body, since 
hydrogen diffuses more easily than nitrogen and only the hydrogen needs to 
diffuse through the body. The size of the void 70 should not be such that 
it increases substantially the response time of the probe. 
The probes of the invention have been described in connection with the 
determination of hydrogen concentration in aluminum and its alloys, but 
can of course be used for the determination of this and other gases in 
other metals, such as magnesium, copper, zinc, steel and their alloys. 
There is a wide range of manufactured and naturally occurring materials 
that can be used to form an immersion probe of the invention, provided of 
course that upon test they are able to meet the requirement of the 
combination of mechanical strength, porosity, pore size and permeability. 
Examples of synthetic materials are: 
(a) Porous ceramics that are sufficiently refractory in nature to be used 
with the metal under test, including the carbides, nitrides and oxides of 
aluminum, magnesium, silicon, zirconium, tungsten and titanium; 
(b) Ceramic foams and fibres; 
(c) Grinding materials and synthetic minerals, particularly the silicates 
and spinels; 
(d) Composites of fibres in metal matrices; 
sintered metal powders of sufficiently high melting point, e.g. steel, 
titanium and tungsten; since such materials are metal-wettable they should 
be provided with a gas-permeable coating of a metal non-wettable material; 
(e) Porous graphite and other carbon based materials, including fibres of 
such materials in mat form or embedded in a suitable matrix; and 
(f) Filtered porous glasses of sufficiently high melting point, such as 
pyrex and aluminosilicates; porcelains. 
Examples of naturally-occurring materials are mullites, sandstones, and 
pumices. The materials can be prepared to have the necessary properties 
and shape by any of the well known techniques, such as sintering, 
pressing, binding, gas forming, moulding, drilling, grinding, etc. 
When use of the probes of the invention involves their immersion in a 
moving stream of metal, the movement of the metal past the probe 
(typically of the order of 5 cm/sec) ensures adequate contact between the 
probe surface and the metal to obtain a reasonably short response time to 
nitrogen/hydrogen equilibrium. However, as with any probe this period is 
increased if the bath is static. Owing to the inherent structure of the 
probes it is possible to shorten the test time in a static bath by 
creating an artificial relative movement between the probe and the metal. 
This is not possible with prior art probes using external bubbling because 
of the danger of loss of the circulating carrier gas if it does not remain 
sufficiently close to the probe to be recaptured thereby. Thus, it is 
found that the response time with the probes of the invention can be 
reduced to values of about 2 to 5 minutes by use of the embodiments 
illustrated by FIGS. 13 to 15. 
With the apparatus of FIG. 13 the probe element 10 is mounted on a vibrator 
72, the movements of the probe produced by the vibrator 72 facilitating 
the diffusion of the hydrogen across the probe/metal interface. The 
vibrator can be of mechanical or magnetostrictive type and vibrates the 
probe in any mode that it produces. 
With the apparatus of FIG. 14 the probe is mounted to rock about a pivot 74 
under the action of a motor-driven eccentric 76 connected to the probe 
support by a shaft 78. With both systems the range of movement of the 
probe is preferably in the range 0.5 to 5 Hertz, more preferably in the 
range 1 to 2 Hertz, and with a mechanical excursion in the range 10 to 100 
mm. 
With the apparatus of FIG. 15 the probe is stationary during the test, and 
instead the molten metal is circulated around the probe by means of a 
small impeller 80 driven by a motor 82, this circulation again 
facilitating diffusion at the probe/metal interface. An impeller of about 
8 cm diameter rotating at speeds in the range of 100 to 400 r.p.m. is 
found to be completely effective. 
To determine the effectiveness of the probes of the invention 28 different 
probes were employed in comparison tests that were confirmed using 
existing laboratory instruments. Each probe was tested under static 
conditions for three repeat measurements being taken out of the metal 
bath, comprising a small laboratory furnace at temperatures from 
700.degree. C. to 750.degree. C., between each test. The values obtained 
ranged from 0.05 to 0.45 ml/100 g, with most values in the range 0.15 to 
0.25 ml/100 g for four different alloy types, namely: 
(a) commercially pure aluminum (99.5%); 
(b) aluminum/magnesium alloys including up to 5% by weight Mg; 
(c) aluminum/zinc/magnesium alloys including up to 5% by weight Zn and up 
to 2% Mg 
(d) aluminum/lithium alloys including up to 3% by weight Li 
The overall probe to probe reproducibility (84 values) was 0.017 ml/100 g, 
while the average repeatability of the same probe was 0.012 ml/100 g. The 
usual response time under these static conditions was 8 to 10 minutes. The 
precision of these values may be compared with the reproducibility values 
of 0.03 to 0.05 ml/100 g obtained with a nitrogen carrier fusion 
laboratory-type analyser. 
FIGS. 16 through 18 are test results obtained with the following metals: 
FIG. 16: Unalloyed aluminum at 705.degree. C. 
FIG. 17: Al/Zn/Mg alloy with 5% Zn and 2% Mg at 709.degree. C. 
FIG. 18: Al/Li alloy with 2.5% Li at 720.degree. C. 
The reproducibility of all of the results will be noted. Adequate 
equilibrium for testing was reached with the unalloyed aluminum in 5 
minutes with an acceptable value at 4 minutes. The results obtained with 
the Al/Zn/Mg alloy were even faster with acceptable equilibrium at a 
little over 2 minutes and complete equilibrium at 3 minutes. Complete 
equilibrium was reached with the Al/Li alloy in 2 minutes, with the 
reproducibility differing the most, namely over the range 0.26 to 0.29 ml 
per 100 g. Lithium alloys are difficult to test with conventional 
laboratory methods. In most laboratory test procedures as a solid sample 
of the alloy is heated to a temperature sufficient to release the hydrogen 
the lithium also is released and good reproducibility is correspondingly 
difficult to obtain. Its alloys therefore require special handling.