The Working of Steel / Annealing, Heat Treating and Hardening of Carbon and Alloy Steel

by Fred H. Colvin; K. A. Juthe

THE THERMO-COUPLE

With the application of the thermo-couple, the measurement of temperatures, between, say, 700 and 2,500°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°F. on a hot day, a correction of 20°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°, 1,350°, 1,400°, 1,450°, 1,500°, and 1,550°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° or between 1,500° and 1,550°, 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.