Note: Descriptions are shown in the official language in which they were submitted.
W092/02794 PCT/US9t/0~32
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Title: Thermocouple Equipped with Ceramic Insulator
and Sheath and Method of Making Same i
Backaround of the Invention:
The present invention relates to temperature
sensing apparatus and more particularly to those devices
adapted for use with internal combustion enqines, nuclear
reactors, and other apparatus which require high temperature .
monitoring.
In the past, various ap~ratus which operate at
high temperatures have been equipped with thermocouples,
particularly the "~ and "K~ types, which have been inserted
to monitor reaction temperatures. The materials which
jacketed these temperature sensing elements were typically
formed from stainless steel or other conductive metals.
Much of the reaa-out instrumentation to which the
thermocouple was attached required electrically ungrounded
thermoelements. Accordingly, the metal jackets were
electrically isolated from the iron-constantin or
chromel-alumel thermoelement wires, typically by filling the
space between the eIements and jacket.with powdered
magnesium oside. The major deficiency of.these previous
. .
thermocoupie designs was the use of a.hygroscopic mineral ~ . .
o~ide ~o insulate the metallic thermoelement wires from the
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metal, protective jacket. The insulative capacity of the
mineral oxide was compromised by water vapor invading the .
insulative material from the surrounding atmosphere. Even a
slight amount of moisture in the powdered mineral 02ide
permitted formation of at least a temporary ground between
the jacket and the thermocouple wires, thereby rendering the
device non-functional. Over the years, this problem was
tolerated, with many operators simply discontinuing use of
the thermocouple and associated pyrometer for a period of
time. However, with the growth in use of unmanned
machinery, the constant monitoring required for computer
control and computer monitoring, and the dangers posed by
escessive temperatures in combustion reactions, the need for
a durable and reliable thermocouple has become paramount. :
For example, use of thermocouples in internal
combustion engines, particularly in monitoring eshaust gas
temperatures, has become more widespread and critical. In
addition to the above-mentioned grounding problem,
conventional thermocouples were frequently sized to project `
deeply into the exhaust gas stream. Naturally, the metallic
jackets for such elongated thermocouples were subject to
considerable wear from esposure to hot and corrosive gases.
In addition, many earlier thermocouples employed
tapered pipe threads for attachment to the eshaust
mani~olds. Removal and replacement typically involved a
cutting torch, thus requiring engine shutdown, so that
possibly combustible gases in a classified location (i.g.,
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W092/02794 PCT/US91/05432
Class 1, Group D, Division 1 or 2) or other engine room were
not ignited by sparks from the torch. Likewise, in the
absence of a relatively permanent thermowell from which the
spent thermocouple could be e~tracted, engine shutdown was
reguired so that poisonous exhaust gases would not escape
through the opening in the exhaust manifold formed when the
old thermocouple was removed.
Accordingly, the present inventor was faced with
the problems of devising a thermocouple which was
substantially impervious to moisture, capable of
withstanding substantial heat and vibration, less exposed to
the corrosive forces within the high temperature
environment, and easily removed and replaced.
Summarv and Ob;ects of the Invention:
The thermocouple which is the subject of the
present invention basically comprises at least one
thermoelement, a unitary ceramic insulator formed with at
least one thermoelement-receiving passageway, and a unitary
ceramic sheath formed with an insulator-receiving chamber.
In addition, the present thermocouple may be equipped with a
metal shell formed with a sheath-receiving bore and provided
with cooperative fastening-means for securing the
thermocouple in an operative position. Preferably, the
above-mentioned cooperative fastening means includes~
standard spark plug threads, but may also use NPT threads.
It is also preferable for the above-mentioned metal shell or
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jacket to be constructed in such a manner that a sensor end
of the thermoelement e~tends into the sampling area less
deeply than what has been the norm in the past.
A primary object of the present invention is to
provide a thermocouple which is practically impervious to
moisture in the atmosphere surrounding the apparatus into
~hich it is inserted. Another object of the present
thermocouple is to be more durable and less intrusive in the ;
sample area which it monitors than was obtained heretofore.
Yet another object of the present invention is to provide a
thermocouple which is readily adapted for insertion in an
existing thermowell, as well as being adapted to receive an
outer threaded sleeve or relatively complete cover. A ~ `;
further object of the presént thermocouple is to be readily
removable from the sample area it is intended to monitor.
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Brief Description of the Drawinqs:
Fig. 1 is a front elevational view of a
thermocouple according to the present invention equipped
with a threaded metallic outer shell;
Fig. 2 is a vertical sectional view taken along
line 2-2 of Fig. 1 and particularly illustrating the manner
in which the present thermocouple is constructed;
Fig. 3 is a front elevational view of a
ther,mocouple according to the present invention~equipped
with an elongated metal tube or thermowell; and
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Fiq. 4 is a vertical sectional view taken along
line 4-4 of Fig. 3 and particularly illustrating the manner
in which the present thermocouple is mounted in the tube or
thermowell.
Detailed Descri~tion of the Preferred Embodiment:
As best indicated in Fiqs. 2 and 4, the present
thermocouple, generally designated 10, basically comprises
at least one and preferably two thermoelements 11, a unitary
ceramic insulator 12 formed with at least one and preferably
two relatively spaced apart thermoelement-receiving
passageways 13, and a unitary ceramic sheath 14 formed with
an insulator-receiving chamber 15. Prefera~ly, the
thermoelsments are either iron-constantin or chromel-alumel
wires, although other thermocouple alloys may be employed.
The thermoelement insulator 12 is preferably an
elongated, cylindrical articlé fabricated from high purity
calcined alumina oxide powder which is milled and blended
with some plasticizers and flexing agents to provide an
homogeneous material of ninety-six percent alumina oxide- ;-
content. Other material compositions fall within the scope
of the present invention which is not to be restricted to
this one preferred embodiment. Typically, this alumina
oxide material is e~truded to form the insulator 12, and is
then sintered at elevated temperatures to form it to its
final dimensions. One of the bare thermoelement wires 11 is
inserted into and through each of the passageways, and a
con~entional thermocouple junction 16 is provided at a
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sensor end of the ceramic insulator 12 adjacent to the
closed end of the insulator-receiving chamber 15 of the -
ceramic sheath 14.
Preferably, the oeramic sheath 14 is manufactured
by those methods commonly used to fabricate spark plug
insulators from high purity alumina oxide formulas. In this
instance, the calcined alumina oxide powder is milled and
blended with plasticizers and fluxing agents to form an
homogeneous material of ninety-six percent alumina oxide ;
content. This material is then extruded or isostatically
pressed into blanks. The resulting blank is then shaped to
its unfired profile and dimensioned by lathe cutting or form
grinding. Alternatively, the unformed alumina oxide
material may be injection molded into the desired shape.
Once the ceramic sheath 14 is shaped, it is sintered under `
controlled firing conditions at elevated temperatures where
it is reduced in size to the final dimensions, within
standard commercial tolerances.
The thermoelement-bearing ceramic insulator 12 is
then coated with a high temperature alumina ceramic cement
and inserted into the ceramic sheath 14. This assembly is
then cured under controlled temperature conditions to set
the adhesive~ Note that fillers, such as powdery fillers -
(e.g., sand, magnesium oxide) are not necessary in the
assembly of the present invention. If the foregoing -
as~embly is intended for use in an e~isting thermowell, then-
no more needs to be done to the assembly 10-other than to
apply ANSI standard colored insulation to the thermoelement
'.
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wires ll so that the connection to the instrumentation (not
shown) will result in accurate temperature levels being
monitored. Once the sensor assembly 10 is inserted into the
thermowell, an externally threaded gland 18 (Fig. 4) may be
screwed onto the cooperative internal threads of the
thermowell in surrounding and overlying relation to an upper
shoulder portion l9 of the ceramic sheath 14.
As indicated in Figs. 3 and 4, the present ceramic
thermocouple 10 may be fitted cartridge-style into a
stainless steel tube 20. This tube 20 may be somewhat
permanent to the manifold or other housing into which it is
installed and thus may constitute the above-referenced
thermowell. The subject tube or thermowell 20 is formed
with a sheath-receiving chamber ~1, the sensor end o which
is closed. Preferably, the tube 20 is designed to project
into the manifold or other chamber no more than one-quarter
of the distance between opposing walls thereof.
Advantageously, the outer tube or thermowell 20 is equipped
with esternal threads for cooperative engagement with a
threaded coupling (not shown) on the mani~old wall. In
addition, a hesagonal or octagonal head or shoulder 23 is
formed on the stainless steel tube for extracting and
fastening the tube to the manifold or chamber wall.
Alternatively, as indicated in Figs. l and 2, the
present ceramic thermocouple 10 may be fitted înto a steel
shell 23 which is hermetically sealed to the ceramic sheath
14 in a manner well known in the art of sealing a spark plug
insulator to its surrounding metal shell. Preferably, the
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W092/02794 PCT/US91/05432
q ~n~
shell is formed with standard SAE spark plug threads 24, a
gasket 25 and a hesagonal or octagonal head 26. Preferably,
the shell 23 is mounted on the ceramic thermocouple lO so
that the sensor end 16 of the thermocouple projects at least
one inch into the chamber which is to be monitored. In the
case of exhaust gas temperature monitoring, the sensor end I ;-
16 preferably extends less than one-half of the distance
between opposing walls of the manifold in which it is
mounted.
As an alternative to SAE threads 24, the shell may
be provided with IS0 or DIN standard spark plug threads, as
well as with NPT threads. In this manner, the subject ;
thermocouple may be replaced as easily as a spark plug
and/or retrofitted into an existing threaded socket. Since
the majority of prior art eshaust gas thermocouples have
used one-ourth, three-eights, one-half or three-fourths
inch NPT threads, it would be advantageous to employ 18
millimeter by 1.5 millimeter SAE or IS0 spark plug threads.
Preferably, a range of thread lengths would be provided,
typically 12.7 millimeters, 18.5 millimeters and 2S.4
millimeters. The 18.5 millimeter length would be virtually
identical to the thread length of a one-half inch NPT pipe ' ,~
thread. - 1
In addition, using TIG-welding procedures--typically !.
used in the manufacture of specialized industrial spark -
plugs, the manuiacturer or o~erator may attach additional
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` W092/02794 PCT/VS9l/05432
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threads to the shell 23, thereby allowing for attachment of
various hazardous location protective heads (not shown) for
suppressing sparks and/or corrosion.
Thus, the present invention provides a thermocouple
which is readily adapted for use in a variety of
applications, either in combination with a metal tube or
thermowell projecting into the area whose temperature is to
be monitored or in association with a threaded metal shell :
similar in construction to those employed with spark plugs. -
The present ceramic thermocouple 10 is preferably of
cartridge design for ready replacement and for relatively
permanent attachment to a spark plug-type threaded metal
shell 23. ~y constructing the present thermocouple 10 from
a unitary alumina oside insulator 12 which houses the
thermoelements and a surrounding unitary alumina oxide
sheath 14, the present invention is able to withstand the
forces at work in the chamber being monitored and in the
surrounding environment which tends to cause conventional
thermocouples to ground out.
