The Thermo-couple





With the application of the thermo-couple, the measurement of

temperatures, between, say, 700 and 2,500 deg.F., was made more simple

and precise. The theory of the thermo-couple is simple; it is that

if two bars, rods, or wires of different metals are joined together

at their ends, when heated so that one junction is hotter than the

other, an electromotive force is set up through the metals, which

will increase with the increase of the difference of temperature

between the two junctions. This electromotive force, or voltage, may

be measured, and, from a chart previously prepared, the temperature

determined. In most pyrometers, of course, the temperatures are

inscribed directly on the voltmeter, but the fact remains that

it is the voltage of a small electric current, and not heat, that

is actually measured.



There are two common types of thermo-couples, the first making use

of common, inexpensive metals, such as iron wire and nichrome wire.

This is the so-called base metal couple. The other is composed of

expensive metals such as platinum wire, and a wire of an alloy of

platinum with 10 per cent of rhodium or iridium. This is called

the rare metal couple, and because its component metals are less

affected by heat, it lasts longer, and varies less than the base

metal couple.



The cold junction of a thermo-couple may be connected by means

of copper wires to the voltmeter, although in some installations

of base metal couples, the wires forming the couple are themselves

extended to the voltmeter, making copper connections unnecessary.

From the foregoing, it may be seen that accurately to measure the

temperature of the hot end of a thermo-couple, we must know the

temperature of the cold end, as it is the difference in the

temperatures that determines the voltmeter readings. This is absolutely

essential for precision, and its importance cannot be over-emphasized.



When pyrometers are used in daily operation, they should be checked

or calibrated two or three times a month, or even every week. Where

there are many in use, it is good practice to have a master pyrometer

of a rare metal couple, which is used only for checking up the

others. The master pyrometer, after calibrating against the melting

points of various substances, will have a calibration chart which

should be used in the checking operation.



It is customary now to send a rare metal couple to the Bureau of

Standards at Washington, where it is very carefully calibrated

for a nominal charge, and returned with the voltmeter readings

of a series of temperatures covering practically the whole range

of the couple. This couple is then used only for checking those

in daily use.



Pyrometer couples are more or less expensive, and should be cared

far when in use. The wires of the couple should be insulated from

each other by fireclay leads or tubes, and it is well to encase them

in a fireclay, porcelain, or quartz tube to keep out the furnace

gases, which in time destroy the hot junction. This tube of fireclay,

or porcelain, etc., should be protected against breakage by an

iron or nichrome tube, plugged or welded at the hot end. These

simple precautions will prolong the life of a couple and maintain

its precision longer.



Sometimes erroneous temperatures are recorded because the cold

end of the couple is too near the furnace and gets hot. This always

causes a temperature reading lower than the actual, and should be

guarded against. It is well to keep the cold end cool with water,

a wet cloth, or by placing it where coal air will circulate around

it. Best of all, is to have the cold junction in a box, together

with a thermometer, so that its temperature may definitely be known.

If this temperature should rise 20 deg.F. on a hot day, a correction of

20 deg.F. should be added to the pyrometer reading, and so on. In the

most up-to-date installations, this cold junction compensation is

taken care of automatically, a fact which indicates its importance.



Optical pyrometers are often used where it is impracticable to

use the thermo-couple, either because the temperature is so high

that it would destroy the couple, or the heat to be measured is

inaccessible to the couple of ordinary length. The temperatures of

slag or metal in furnaces or running through tap-holes or troughs

are often measured with optical pyrometers.



In one type of optical pyrometer, the observer focuses it on the

metal or slag and moves an adjustable dial or gage so as to get

an exact comparison between the color of the heat measured with

the calor of a lamp or screen in the pyrometer itself. This, of

course, requires practice, and judgment, and brings in the personal

equation. With care, however, very reliable temperature measurements

may be made. The temperatures of rails, as they leave the finishing

pass of a rolling mill, are measured in this way.



Another type of optical pyrometer is focused on the body, the

temperature of which is to be measured. The rays converge in the

telescope on metal cells, heating them, and thereby generating a

small electric current, the voltage of which is read an a calibrated

voltmeter similar to that used with the thermo-couple. The best

precision is obtained when an optical pyrometer is used each time

under similar conditions of light and the same observer.



Where it is impracticable to use either thermo-couples or optical

pyrometers, sentinels may be used. There are small cones or cylinders

made of salts or other substances of known melting points and covering

a wide range of temperatures.



If six of these sentinels, melting respectively at 1,300 deg., 1,350 deg.,

1,400 deg., 1,450 deg., 1,500 deg., and 1,550 deg.F., were placed in a row in a

furnace, together with a piece of steel to be treated, and the

whole heated up uniformly, the sentinels would melt one by one and

the observer, by watching them through an opening in the furnace,

could tell when his furnace is at say 1,500 deg. or between 1,500 deg. and

1,550 deg., and regulate the heat accordingly.



A very accurate type of pyrometer, but one not so commonly used as

those previously described, is the resistance pyrometer. In this

type, the temperature is determined by measuring the resistance to an

electric current of a wire which is at the heat to be measured. This

wire is usually of platinum, wound around a quartz tube, the whole

being placed in the furnace. When the wire is at the temperature of

the furnace, it is connected by wires with a Wheatstone Bridge, a

delicate device for measuring electrical resistance, and an electric

current is passed through the wire. This current is balanced by

switching in resistances in the Wheatstone Bridge, until a delicate

electrical device shows that no current is flowing. The resistance

of the platinum wire at the heat to be measured is thus determined

on the Bridge, and the temperature read off on a calibration

chart, which shows the resistance at various temperatures.



These are the common methods used to-day for measuring temperatures,

but whatever method is used, the observer should bear in mind that

the greatest precision is obtained, and hence the highest efficiency,

by keeping the apparatus in good working order, making sure that

conditions are the same each time, and calibrating or checking

against a standard at regular intervals.





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