PLACING OF PYROMETERS.--When installing a pyrometer, care should be taken that it reaches directly to the point desired to be measured, that the cold junction is kept cold, and that the wires leading to the recording instrument are kept in good shape. The length of these lead wires have an effect; the longer they are, the lower the apparent temperature.

When pyrometers placed in a number of furnaces are connected up in series, and a multiple switch is used for control, it becomes apparent that pyrometers could not be interchanged between furnaces near and far from the instrument without affecting the uniformity of product from each furnace.

Calibration can best be done without disturbing the working pyrometer, by inserting the master instrument into each furnace separately, place it alongside the hot junction of the working pyrometer, and compare the reading given on the indicator connected with the multiple switch.

Protection tubes should be replaced when cracked, as it is important that no foreign substance is allowed to freeze in the tube, so that the enclosed junction becomes a part of a solid ma.s.s joined in electrical contact with the outside protecting tube. Wires over the furnaces must be carefully inspected from time to time, as no true reading can be had on an instrument, if insulation is burned off and short circuits result.

If the standard calibrating instrument used contains a dry battery, it should be examined from time to time to be sure it is in good condition.

THE LEEDS AND NORTHRUP POTENTIOMETER SYSTEM

The potentiometer pyrometer system is both flexible and substantial in that it is not affected by the jar and vibration of the factory or the forge shop. Large or small couples, long or short leads can be used without adjustment. The recording instrument may be placed where it is most convenient, without regard to the distance from the furnace.

ITS FUNDAMENTAL PRINCIPLE.--The potentiometer is the electrical equivalent of the chemical balance, or balance arm scales. Measurements are made with balance scales by varying known weights until they equal the unknown weight. When the two are equal the scales stand at zero, that is, in the position which they occupy when there is no weight on either pan; the scales are then said to be balanced.

Measurements are made with the potentiometer by varying a known electromotive force until it equals the unknown; when the two are equal the index of the potentiometer, the galvanometer needle, stands motionless as it is alternately connected and disconnected.

The variable known weights are units separate from the scales, but the potentiometer provides its own variable known electromotive force.

The potentiometer provides, first, a means of securing a known variable electromotive force and, second, suitable electrical connections for bringing that electromotive force to a point where it may be balanced against the unknown electromotive force of the couple. The two are connected with opposite polarity, or so that the two e.m.f.s oppose one another. So long as one is stronger than the other a current will flow through the couple; when the two are equal no current will flow.

Figure 107 shows the wiring of the potentiometer in its simplest form. The thermo-couple is at _H_, with its polarity as shown by the symbols + and -. It is connected with the main circuit of the potentiometer at the fixed point _D_ and the point _G_.

[Ill.u.s.tration: FIG. 110.--Simple potentiometer.]

A current from the dry cell _Ba_ is constantly flowing through the main, or so-called potentiometer circuit, _ABCDGEF_. The section _DGE_ of this circuit is a slide wire, uniform in resistance throughout its length. The scale is fixed on this slide wire. The current from the cell _Ba_ as it flows through _DGE_, undergoes a fall in potential, setting up a difference in voltage, that is, an electromotive force, between _D_ and _E_. There will also be electromotive force between _D_ and all other points on the slide wire. The polarity of this is in opposition to the polarity of the thermo-couple which connects into the potentiometer at _D_ and at _G_. By moving _G_ along the slide wire a point is found where the voltage between _D_ and _G_ in the slide wire is just equal to the voltage between _D_ and _G_ generated by the thermo-couple. A galvanometer in the thermo-couple circuit indicates when the balance point is reached, since at this point the galvanometer needle will stand motionless when its circuit is opened and closed.

[Ill.u.s.tration: FIG. 111.--Standard cell potentiometer.]

The voltage in the slide wire will vary with the current flowing through it from the cell _Ba_ and a means of standardizing this is provided. _SC_, Fig. 111, is a cadmium cell whose voltage is constant.

It is connected at two points _C_ and _D_ to the potentiometer circuit whenever the potentiometer current is to be standardized.

At this time the galvanometer is thrown in series with _SC_. The variable rheostat _R_ is then adjusted until the current flowing is such that as it flows through the standard resistance _CD_, the fall in potential between _C_ and _D_ is just equal to the voltage of the standard cell _SC_. At this time the galvanometer will indicate a balance in the same way as when it was used with a thermo-couple. By this operation the current in the slide wire _DGE_ has been standardized.

[Ill.u.s.tration: FIG. 112.--Hand adjusted cold-end compensator.]

DEVELOPMENT OF THE WIRING SCHEME OF THE COLD-END COMPENSATOR.--The net voltage generated by a thermo-couple depends upon the temperature of the hot end and the temperature of the cold end. Therefore, any method adopted for reading temperature by means of thermo-couples must in some way provide a means of correcting for the temperature of the cold end. The potentiometer may have either of two very simple devices for this purpose. In one form the operator is required to set a small index to a point on a scale corresponding to the known cold junction temperature. In the other form an even more simple automatic compensator is employed. The principle of each is described in the succeeding paragraphs, in which the a.s.sumption is made that the reader already understands the potentiometer principle as described above.

As previously explained the voltage of the thermo-couple is measured by balancing it against the voltage drop _DG_ in the potentiometer.

As shown in Fig. 111, the magnitude of the balancing voltage is controlled by the position of _G_. Make _D_ movable as shown in Fig.

112 and the magnitude of the voltage _DG_ may be varied either from the point _D_ or the point _G_. This gives a means of compensating for cold end changes by setting the slider _D_. As the cold end temperature rises the net voltage generated by the couple decreases, a.s.suming the hot end temperature to be constant. To balance this decreased voltage the slider _D_ is moved along its scale to a new point nearer _G_. In other words, the slider _D_ is moved along its scale until it corresponds to the known temperature of the cold end and then the potentiometer is balanced by moving the slider _G_. The readings of _G_ will then be direct.

[Ill.u.s.tration: FIG. 113.--Another type of compensator.]

The same results will be obtained if a slide wire upon which _D_ bears is in parallel with the slide wire of _G_, as shown in Fig.

113.

AUTOMATIC COMPENSATOR.--It should be noted that the effect of moving the contact _D_, Fig. 113, is to vary the ratio of the resistances on the two sides of the point _D_ in the secondary slide wire. In the recording pyrometers, an automatic compensator is employed.

This automatic compensator varies the ratio on the two sides of the point _D_ in the following manner:

The point _D_, Fig. 114, is mechanically fixed; on one side of _D_ is the constant resistance coil _M_, on the other the nickel coil _N_. _N_ is placed at or near the cold end of the thermo-couple (or couples). Nickel has a high temperature coefficient and the electrical proportions of _M_ and _N_ are such that the resistance change of _N_, as it varies with the temperature of the cold end, has the same effect upon the balancing voltage between _D_ and _G_ that the movement of the point _D_, Fig. 114, has in the hand-operated compensator.

Instruments embodying these principles are shown in Figs. 115 to 117. The captions making their uses clear.

[Ill.u.s.tration: FIG. 114.--Automatic cold-end compensator.]

PLACING THE THERMO-COUPLES

The following ill.u.s.trations from the Taylor Instrument Company show different applications of the thermo-couples to furnaces of various kinds. Figure 118 shows an oil-fired furnace with a simple vertical installation. Figure 119 shows a method of imbedding the thermo-couple in the floor of a furnace so as to require no s.p.a.ce in the heating chamber.

[Ill.u.s.tration: FIG. 115.--Potentiometer ready for use.]

Various methods of applying a pyrometer to common heat-treatment furnaces are shown in Figs. 120 to 122.

[Ill.u.s.tration: FIG. 116.--Eight-point recording pyrometer-Carpenter Steel Co.]

LEEDS AND NORTHRUP OPTICAL PYROMETER

The principles of this very popular method of measuring temperature are sketched in Fig. 123.

[Ill.u.s.tration: FIG. 117.--Multiple-point thermocouple recorder--Bethlehem Steel Co.]

[Ill.u.s.tration: FIG. 118.--Tycos pyrometer in oil-fired furnace.]

The instrument is light and portable, and can be sighted as easily as an opera gla.s.s. The telescope, which is held in the hand, weighs only 25 oz.; and the case containing the battery, rheostat and milliammeter, which is slung from the shoulder, only 10 lb.

[Ill.u.s.tration: FIG. 119.--Thermocouple in floor of furnace.]

[Ill.u.s.tration: FIG. 120.--Pyrometer in gas furnace.]

A large surface to sight at is not required. So long as the image formed by the objective is broader than the lamp filament, the temperature can be measured accurately.

[Ill.u.s.tration: FIG. 121.--Tycos multiple indicating pyrometer and recorder.]

[Ill.u.s.tration: FIG. 122.--Pyrometer in galvanizing tank.]

Distance does not matter, as the brightness of the image formed by the lens is practically constant, regardless of the distance of the instrument from the hot object.

[Ill.u.s.tration: FIG. 123.--Leeds & Northrup optical pyrometer.]

The manipulation is simple and rapid, consisting merely in the turning of a knurled k.n.o.b. The setting is made with great precision, due to the rapid change in light intensity with change in temperature and to the sensitiveness of the eye to differences of light intensity.

In the region of temperatures used for hardening steel, for example, different observers using the instrument will agree within 3C.

[Ill.u.s.tration: FIG. 124.--Too low.

FIG. 125.--Too high.

FIG. 126.--Correct.]

Only brightness, not color, of light is matched, as light of only one color reaches the eye. Color blindness, therefore, is no hindrance to the use of this method. The use of the instrument is shown in Fig. 127.