Document 50rLqQwR8qRjOaa4JR3dpdQVJ

1222 , CHAPTER 52 1957 Guide cated or recorded on conveniently located instruments; average tempera ture may be readily obtained by connecting several couples in parallel or in series; and temperatures may be obtained within thin materials, narrow spaces, or otherwise inaccessible locations. Thermocouples in series with every alternate junction maintained at a common temperature will give an emf which, divided by the number of couples to give the average emf10 per couple; may be used to find the aver age temperature. This series arrangement of thermocouples, often called a thermopile, can have extreme sensitivity and is useful in detecting very small changes in temperature. Thermocouples in parallel, having the similar metals of a number of couples connected together and run to a common cold junction, will cause an indication on a potentiometer which is the true average emf only if the electrical resistances of the parallel junctions are the same.10'11 Temperatures of surfaces below red heat are difficult to determine by any other means than thermocouples. For this purpose, a thermocouple of fine wires is preferable to minimize the possibility of error due to the conduction of heat along the wires. It may be attached to a metal surface in any of several ways. For permanent installations, soldering, brazing or peening may be desirable. A small hole is drilled for the peening opera tion; the thermocouple is inserted and the metal is peened to retain it. The fact that the thermocouple is in electric contact with the surface is unimportant in usual circuits. For temporary arrangements, couples may be attached by means of surgical or cellophane tape. For many boiler or furnace surfaces, furnace cement serves very well. The thermocouple may be attached by means of the cement when the surface is cold, and must be treated gently and usually supported until the cement dries, due to heat, and hardens--after which it has ample strength. It is good practice to use as little cement as possible, and also to plaster the wires to the surface for an inch or so from their junction to avoid errors due to heat conduction along the wire. Electric insulation between the wires should be perfect except at the junction since, otherwise, the indicated emf will be between those existing at the junction and at the short circuit. Resistance Thermometers Resistance thermometers depend for their operation upon the. change of electric resistance of metal with change in temperature. The resistance generally increases with rising temperature. Their use largely parallels that of thermocouples, although readings tend to be unstable above 950 F. Two-lead temperature elements are not recommended, since they do not permit correction for lead resistance. Three leads to. each resistor are necessary to obtain consistent readings. A typical circuit used by several manufacturers is shown in Fig. 2. In this design a differential galvanometer is used, in which coils L and H exert opposing forces on the indicating needle. Coil L is in series with the ther mometer resistance AB, and coil H is in series with the constant resistance R. As the temperature falls, the resistance of AB decreases allowing more current to flow through coil L than through coil H. This causes an in crease in the force exerted by coil L, pulling the needle down to a lower reading. Likewise, as the temperature rises the resistance of AB increases, causing less current to flow through coil L than through coil H. This forces the indicating needle to a higher reading. Rheostat S must be ad justed occasionally to maintain a constant flow of current. Instruments and Measurements 1223 As compared to the thermocouple, the resistance thermometer does not require a cold junction, and it can be simply scaled for more accurate meas urements; but, generally because of its construction it is more costly and is apt to have considerable lag. It gives best results when used to meas ure steady or slowly changing temperature. For accurate results the entire thermometer coil must be exposed to the temperature to be measured. Pyrometers The pyrometer is the usual instrument for measuring high temperatures such as those of incandescent bodies or furnace interiors. There are two types. In the radiation pyrometer the radiant energy from an observed surface falls on a thermopile, and the emf generated by the pile, measured Fig. 2. Typical Resistance Ther mometer Circuit and Connections by a galvanometer or potentiometer, is an index of the surface temperature. With the optical pyrometer a narrow spectral band, usually red, emitted by the surface, is matched visually with the filament of a special electric lamp. The emf necessary to cause the filament to match the surface in brightness is the index of the temperature of the surface. Pyrometers are calibrated by means of various metals with known melting or freezing points. Portable as well as laboratory models are manufactured. Color Indicating Crayons Crayons are available, the marks of which change color or melt at speci fied temperatures. Such crayons have been sold in boxes covering the range from about 100 F to about 800 F in 100 deg steps-, with a precision of some 10 deg. They are a rough but convenient means of determining temperatures, and of locating isothermal lines on surfaces below red heat. PRESSURE MEASUREMENT Pressure Gages The Bom-don is the most common type of pressure gage, and its appear ance probably is familiar to any one having an acquaintance with power plants or laboratories. The essential element of such a gage is the Bourdon