Protectors For Thermo-couples





Thermo-couples must be protected from the danger of mechanical

injury. For this purpose tubes of various refractory materials

are made to act as protectors. These in turn are usually protected

by outside metal tubes. Pure wrought iron is largely used for this

purpose as it scales and oxidizes very slowly. These tubes are

usually made from 2 to 4 in. shorter than the inner tubes. In lead

baths the iron tubes often have one end welded closed and are used

in connection with an angle form of mounting.






Where it is necessary for protecting tubes to project a considerable

distance into the furnace a tube made of nichrome is frequently used.

This is a comparatively new alloy which stands high temperatures

without bending. It is more costly than iron but also much more

durable.



When used in portable work and for high temperatures, pure nickel

tubes are sometimes used. There is also a special metal tube made

for use in cyanide. This metal withstands the intense penetrating

characteristics of cyanide. It lasts from six to ten months as

against a few days for the iron tube.



The inner tubes of refractory materials, also vary according to

the purposes for which they are to be used. They are as follows:



MARQUARDT MASS TUBES for temperatures up to 3,000 deg.F., but they will

not stand sudden changes in temperature, such as in contact with

intermittent flames, without an extra outer covering of chamotte,

fireclay or carborundum.






FUSED SILICA TUBES for continuous temperatures up to 1,800 deg.F. and

intermittently up to 2,400 deg.F. The expansion at various temperatures

is very small, which makes them of value for portable work. They

also resist most acids.



CHAMOTTE TUBES are useful up to 2,800 deg.F. and are mechanically strong.

They have a small expansion and resist temperature changes well,

which makes them good as outside protectors for more fragile tubes.

They cannot be used in molten metals, or baths of any kind nor

in gases of an alkaline nature. They are used mainly to protect

a Marquardt mass or silica tube.



CARBORUNDUM TUBES are also used as outside protection to other

tubes. They stand sudden changes of temperature well and resist

all gases except chlorine, above 1,750 deg.F. Especially useful in

protecting other tubes against molten aluminum, brass, copper and

similar metals.



CLAY TUBES are sometimes used in large annealing furnaces where they

are cemented into place, forming a sort of well for the insertion of

the thermo-couple. They are also used with portable thermo-couples

for obtaining the temperatures of molten iron and steel in ladles.

Used in this way they are naturally short-lived, but seem the best

for this purpose.






CORUNDITE TUBES are used as an outer protection for both the Marquardt

mass and the silica tubes for kilns and for glass furnaces. Graphite

tubes are also used in some cases for outer protections.



CALORIZED TUBES are wrought-iron pipe treated with aluminum vapor

which often doubles or even triples the life of the tube at high

temperature.



These tubes come in different sizes and lengths depending on the

uses for which they are intended. Heavy protecting outer tubes

may be only 1 in. in inside diameter and as much as 3 in. outside

diameter, while the inner tubes, such as the Marquardt mass and

silica tubes are usually about 3/4 in. outside and 3/8 in. inside

diameter. The length varies from 12 to 48 in. in most cases.



Special terminal heads are provided, with brass binding posts for

electrical connections, and with provisions for water cooling when

necessary.





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