Document MMNmX3vZYV7XZ8YQJVevwq2Xk

194 CHAPTER, 11 1949 Guide D = number of degrees on the thermometer scale which are not immersed. = temperature indicated on the thermometer, Fahrenheit degrees. ti = temperature of the non-immersed mercury column, Fahrenheit degrees. 0.00009 = difference in the coefficient of expansion of the mercury and glass. When a thermometer is used in a liquid or in air or gas near room tem perature the effects of radiation can often be ignored, but when the tem perature of hot air or gas is desired, means are usually provided for mini mizing the effect of radiation.7 These include bright metallic shields around the thermometer bulb and the use of aspirated thermometers in which a stream of the air or gas is drawn at considerable speed across the bulb. This increases the influence of the gas temperature on the thermom eter indication. In any case, to prevent errors in temperature measure ments, there should be ample circulation so that the thermometer will indicate a true temperature of the medium under observation, and ample time should be allowed for the thermometer to reach the same temperature as the medium. In reading a thermometer the eye should be at. the same level as the top of the liquid to avoid parallax. Industrial-type thermometers are available for permanent installation in pipes or ducts. These instruments are fitted with metal guards to prevent breakage and are useful for many purposes. However, the con siderable heat capacity and conductance of the guards or shields do not permit such thermometers to follow the fluctuations in a medium of vary ing temperature as well as those of the etched stem type. Thermocouples When two wires made of dissimilar metals are joined by soldering, weld ing or merely by twisting, a thermocouple or thermo-junction is formed and an electromotive force, which depends upon the temperature of the junc tion, is found to exist between the wires. When the wires are joined at two points a thermocouple circuit is formed, and if one junction is kept at a temperature different from the other, an electric current flows through the circuit due to the difference in emf developed by the two junctions. This phenomenon is employed for temperature measurements in thermocouple systems, one junction being ordinarily kept at a constant temperature, as in an ice bath, while the other junction is placed at a point at which it is desired to observe the temperature. In practice it is desirable to utilize emf to indicate temperature because, at small or zero current flow, the resistance of the circuit is unimportant. A high resistance millivolt meter is useful in some cases but the potentiometer yields better results. -In the potentiometer the electromotive force' generated by the thermocouples is balanced against an electromotive force from the battery so that observa tions are made with no flow of current through the thermocouple circuit. A conventional arrangement is illustrated in Fig. 1. The thermocouple leads A-B are so connected that their polarity opposes that of battery C. If the position of E oh the graduated slide wire rheostat DF is adjusted until galvanometer G shows no current flowing, resistance DE will indicate directly the voltage generated by the thermocouple. In order to calibrate the instrument, switch S is thrown over to the standard cell circuit while rheostat B is adjusted so that the galvanometer shows zero current. Battery C then exerts the known voltage of the standard cell at DH. The choice of materials for thermocouple wires is determined by the range of temperature to be measured. Up to about 600 F base metals such as iron-constantan or, preferably from the corrosion standpoint, copperconstantan are satisfactory and develop a relatively large emf of 40 to 60 Instruments and Measurements 195 rovolts per degree. Chromel-alumel couples are useful in the .flue gas rature range while platinum- (platinum-rhodium) couples are used for temperatures.- Impurities make large differences in the perform- nce of thermocouple wires and for this reason calibration of samples from each spool of wire is essential for precise work. Data on wire can usually be obtained from the manufacturer. The act of adjusting rheostat D-F (Fig. 1) for zero current flow is known as balancing the potentiometer. Automatic self balancing instruments are on the market of both the indicating and recording types. They usu ally contain an automatically compensating cold junction to avoid the use of an ice bath; and special thermocouple wire is furnished with them from' the factory. With a suitable potentiometer, small wires serve as well for thermo--i-- torero ones and the fineness of the wires is limited only by consid eration of mechanical strength and convenience in handling. Small couples respond more promptly to changes in temperature and are less affected by radiation than large ones. For use in' heated air or gases, thermocouples are often shielded as are thermometers and aspirated ther mocouples are sometimes used. An arrangement has been described, also for avoiding error due to radiation, which involves several thermocouples of different sizes the. true temperature being estimated by extrapolation to zero diameter8. By the use of thermocouples, temperatures at remote points may be indi cate! 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, harrow 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 emf9 per couple, may be used.to find the average 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 emf only if the electrical- resistances of. the parallel junctions are the same9,10- .X