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Патент USA US2405075

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Filed Nov. 27, 1945
Patented July 30, 1946
Brown Instrument Company, Philadelphia,
Pa., a corporation of Pennsylvania
Application November 27, 1943, Serial No. 512,058
1 Claim. (Cl. 136-4)
The present invention relates to temperature
measurement, and more particularly to a protect
ing tube which is used to protect a temperature
measuring thermocouple.
When a thermocouple such, for example, as
chromel-alumel is used in a metal tube contain
ing iron, at temperatures above 1500° F., the
wires thereof change their composition for some
It is customary in the measurement of tem
reason not at present known. It is thought how
peratures by means of thermocouples to enclose
ever that this change in composition may be due
to the fact that the atmosphere within the tube
the latter in a protecting tube. This is done for
several reasons amongst which is to prevent con
is oxygen free. Above 1500° F. the iron in the
tamination of the thermocouple and to increase its
metal tube will combine with the oxygen of the
life. The protecting tube must have the char 10 air in the tube to form an iron oxide. The re
acteristic of being unaffected by the medium
maining constituents of the air, or the iron oxide,
with the heat cause a change in the composition
which surrounds it and of rapidly transmitting
of the thermocouple wires, with a consequent
heat from the process to the temperature re
sponsive element. It has been particularly dif
change in the thermocouple calibration.
In order to avoid this change in thermocouple
?cult to ?nd a protecting tube that will satisfy 15
calibration, a gas impervious ceramic tube 4 is
these requirements when used in a molten salt
bath of the carburizing type.
inserted within the metal tube 2. The thermo
It is, accordingly, an object of
to provide a protecting tube for
which will withstand the action
baths, and particularly baths of
this invention
couple to be used is inserted in this ceramic tube.
The tube 4 is preferably made of a ?ne grain
of carburizing 20 vitri?ed ceramic material which is known as sil
limanite and which has the chemical composition
cyanide salts,
of AlzSiOs. A tube of this type will not permit
for a longer period than has heretofore been
the passage of gases through it at the tempera
tures that are encountered in a carburizing bath
It is a further object of the invention to pro
so that the atmosphere therein is normal. Such
vide a protecting tube for thermocouples which
will permit the rapid transfer of heat from its
a tube, however, if immersed directly in the bath
would be disintegrated by the action of the salts
exterior to a thermocouple located in the same,
forming the same. Thus the ceramic tube 4 pro
and with a minimum thermal lag.
tects the thermocouple from the action of the
It is a further object of the invention to pro
vide the combination of a carburizing bath with 30 atmosphere that is found in the interior of the
metal tube 2, and the ceramic tube is protected
a protecting tube of the type disclosed herein.
from the action of the carburizing salt bath by
The various features of novelty which charac
metal tube 2.
terize this invention are pointed out with particu
larity in the claim annexed to and forming a part
Since the dead air space between the two tubes
of this speci?cation. For a better understand 35 2 and 4 will slow down the heat transfer from the
exterior of the protecting tube to the thermo
ing of the invention, however, its advantages and
couple, the space between the two tubes is ?lled
speci?c objects obtained with its use, reference
with a refractory material 5 which will rapidly
should be had to the accompanying drawing and
transfer heat so that temperature changes to
descriptive matter in which is illustrated and de
which the protecting tube is subjected will be
scribed a preferred embodiment of the inven
more quickly re?ected by the thermocouple than
would otherwise be possible. A suitable refrac
In the drawing the single ?gure shows a pro
tory material has been found to be silicon car
tecting tube made in accordance with the pres
bide. This refractory material is powdered and
ent invention.
The protecting tube is made of a plurality of 45 is poured into the space between the metal and
the ceramic tubes as shown in the drawing.
parts each of which has its own characteristics
and serves a de?nite purpose. As shown in the
The upper end of the metal tube 2 is shown as
drawing the outer portion of the protecting tube
being threaded at 6 so that any convenient type
of support may be attached thereto. Such a sup
is formed of a closed end metal tube 2 that is
port would also serve to hold the thermocouple
made of mild steel. This tube is covered with a
thin layer of chromium 3 which may be deposited
wires so that they could be connected with a
thereon in any suitable manner such as electro
suitable pyrometer. It is noted that the protect
plating. A tube of this type when properly heat
ing tube of the present invention can be used
in other places than in carburizing salt baths
treated will withstand the action of molten cya
nide salts which are used for carburizing baths. 55 although it is primarily intended for that pur
2,405,075 '
pose. Any desired type of thermocouple may be
used in the protecting tube to measure the tem
vantage without a corresponding use of other
perature thereof provided this thermocouple is
Having now described this invention, what I
claimas new anddesire to secure by Letters Pat
suitable for the ranges of temperatures that are
Ul ent is:
While in accordance with the provisions of the
A thermocouple protecting tube comprising an
statutes, I have illustrated and describedithe best
form of thisinvention now knowntome, it would:
outer closed-end tube made ‘of mild steel and
covered with a thin layer" ofochromium', an inner
closed-end tube received in telescoping relation
be apparent to those skilled in the art that
changes may be made in the form of the'appa
ratus disclosed without departing from the spirit’
10. :by
said outer tube and made of a ?ne grained,
vitri?ed, gas impervious ceramic material, the
of this invention as set forth in the appended I space between said tubes being ?lled with a granue
and that may
in some
cases .certainrfeatures
vvbe;iilsreld ‘toiled-‘ft
lar refractory material. "
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