OPTICAL SYSTEM AND ELECTRICAL CIRCUIT OF THE LEEDS & NORTHRUP OPTICAL PYROMETER.--For extremely high temperature, the optical pyrometer is largely used. This is a comparative method. By means of the rheostat the current through the lamp is adjusted until the brightness of the filament is just equal to the brightness of the image produced by the lens _L_, Fig. 123, whereupon the filament blends with or becomes indistinguishable in the background formed by the image of the hot object. This adjustment can be made with great accuracy and certainty, as the effect of radiation upon the eye varies some twenty times faster than does the temperature at 1,600F., and some fourteen times faster at 3,400F. When a balance has been obtained, the observer notes the reading of the milliammeter. The temperature corresponding to the current is then read from a calibration curve supplied with the instrument.
[Illustration: FIG. 127.--Using the optical pyrometer.]
As the intensity of the light emitted at the higher temperatures becomes dazzling, it is found desirable to introduce a piece of red glass in the eye piece at _R_. This also eliminates any question of matching colors, or of the observer's ability to distinguish colors. It is further of value in dealing with bodies which do not radiate light of the same composition as that emitted by a black body, since nevertheless the intensity of radiation of any one color from such bodies increases progressively in a definite manner as the temperature rises. The intensity of this one color can therefore be used as a measure of temperature for the body in question. Figures 124 to 126 show the way it is read.
CORRECTION FOR COLD-JUNCTION ERRORS
The voltage generated by a thermo-couple of an electric pyrometer is dependent on the difference in temperature between its hot junction, inside the furnace, and the cold junction, or opposite end of the thermo-couple to which the copper wires are connected. If the temperature or this cold junction rises and falls, the indications of the instrument will vary, although the hot junction in the furnace may be at a constant temperature.
A cold-junction temperature of 75F., or 25C., is usually adopted in commercial pyrometers, and the pointer on the pyrometer should stand at this point on the scale when the hot junction is not heated.
If the cold-junction temperature rises about 75F., where base metal thermo-couples are used, the pyrometer will read approximately 1 low for every 1 rise in temperature above 75F. For example, if the instrument is adjusted for a cold-junction temperature of 75, and the actual cold-junction temperature is 90F., the pyrometer will read 15 low. If, however, the cold-junction temperature falls below 75F., the pyrometer will read high instead of low, approximately 1 for every 1 drop in temperature below 75F.
With platinum thermo-couples, the error is approximately 1/2 for 1 change in temperature.
CORRECTION BY ZERO ADJUSTMENT.--Many pyrometers are supplied with a zero adjuster, by means of which the pointer can be set to any actual cold-junction temperature. If the cold junction of the thermo-couple is in a temperature of 100F., the pointer can be set to this point on the scale, and the readings of the instrument will be correct.
COMPENSATING LEADS.--By the use of compensating leads, formed of the same material as the thermo-couple, the cold junction can be removed from the head of the thermo-couple to a point 10, 20 or 50 ft. distant from the furnace, where the temperature is reasonably constant. Where greater accuracy is desired, a common method is to drive a 2-in. pipe, with a pointed closed end, some 10 to 20 ft. into the ground, as shown in Fig. 128. The compensating leads are joined to the copper leads, and the junction forced down to the bottom of the pipe. The cold junction is now in the ground, beneath the building, at a depth at which the temperature is very constant, about 70F., throughout the year. This method will usually control the cold-junction temperature within 5F.
Where the greatest accuracy is desired a compensating box will overcome cold-junction errors entirely. It consists of a case enclosing a lamp and thermostat, which can be adjusted to maintain any desired temperature, from 50 to 150F. The compensating leads enter the box and copper leads run from the compensating box to the instrument, so that the cold junction is within the box. Figure 129 shows a Brown compensating box.
[Illustration: FIG. 128.--Correcting cold-junction error.]
If it is desired to maintain the cold junction at 100: the thermostat is set at this point, and the lamp, being wired to the 110- or 220-volt lighting circuit, will light and heat the box until 100 is reached, when the thermostat will open the circuit and the light is extinguished. The box will now cool down to 98, when the circuit is again closed, the lamp lights, the box heats up, and the operation is repeated.
[Illustration: FIG. 129.--Compensating box.]
BROWN AUTOMATIC SIGNALING PYROMETER
In large heat-treating plants it has been customary to maintain an operator at a central pyrometer, and by colored electric lights at the furnaces, signal whether the temperatures are correct or not. It is common practice to locate three lights above each furnace-red, white and green. The red light burns when the temperature is too low, the white light when the temperature is within certain limits--for example, 20F. of the correct temperature--and the green light when the temperature is too high.
[Illustration: FIG. 130.--Brown automatic signaling pyrometer.]
Instruments to operate the lights automatically have been devised and one made by Brown is shown in Fig. 130. The same form of instrument is used for this purpose to automatically control furnace temperatures, and the pointer is depressed at intervals of every 10 sec. on contacts corresponding to the red, white and green lights.
[Illustration: FIG. 131.--Automatic temperature control.]
AN AUTOMATIC TEMPERATURE CONTROL PYROMETER
Automatic temperature control instruments are similar to the Brown indicating high resistance pyrometer with the exception that the pointer is depressed at intervals of every 10 sec. upon contact-making devices. No current passes through the pointer which simply depresses the upper contact device tipped with platinum, which in turn comes in contact with the lower contact device, platinum-tipped, and the circuit is completed through these two contacts. The current is very small, about 1/10 amp., as it is only necessary to operate the relay which in turn operates the switch or valve. A small motor is used to depress the pointer at regular intervals. The contact-making device is adjustable throughout the scale range of the instrument, and an index pointer indicates the point on the instrument at which the temperature is being controlled. The space between the two contacts on the high and low side, separated by insulating material, is equivalent to 1 per cent of the scale range. A control of temperature is therefore possible within 1 per cent of the total scale range.
Figure 131 shows this attached to a small furnace.
[Illustration: FIG. 132.--Portable thermocouple testing molten brass.]
PYROMETERS FOR MOLTEN METAL
Pyrometers for molten metal are connected to portable thermocouples as in Fig. 132. Usually the pyrometer is portable, as shown in this case, which is a Brown. Other methods of mounting for this kind of work arc shown in Figs. 133 and 134. The bent mountings are designed for molten metal, such as brass or copper and are supplied with either clay, graphite or carborundum tubes. Fifteen feet of connecting wire is usually supplied.
The angle mountings, Fig. 134, are recommended for baths such as lead or cyanide. The horizontal arm is usually about 14 in. long, and the whole mounting is easily taken apart making replacements very easy. Details of the thermo-couple shown in Fig. 132 are given in Fig. 135. This is a straight rod with a protector for the hand of the operator. The lag in such couples is less than one minute.
These are Englehard mountings.
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.
[Illustration: FIG. 133.--Bent handle thermocouple with protector.]
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 3,000F., 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.
[Illustration: FIG. 134.--Other styles of bent mounting.]
FUSED SILICA TUBES for continuous temperatures up to 1,800F. and intermittently up to 2,400F. 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 2,800F. 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 1,750F. 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.
[Illustration: FIG. 135.--Straight thermocouple and guard.]
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.