Modern Gasworks Practice

tO os Oi MODERN GASWORKS PRACTICE couple, comprising two wires—one of which is composed of a pure metal and the other of an alloy—is generally used. For high temperature work it is further neces-■sary that both the metal and the alloy should be inf usible. In. some of the best-known modem pyrometers—particularly among those used on gasworks—the sensitive element employed is merely a thermo-couple constructed. from special metals. As regards the direction of the current, this undergoes reversal when. the temperature of the hot junction rises above 550°. This is known as thermo-electric inversion, and it has been found that certain metals do not exhibit the phenomenon of reversal. It is those metals which are invariably employed in the construction of thermo-■couples made use of in the practical pyrometer. The electro-motive force set up is proportional to the difference in temperature between the hot and cold junctions, and as the cold junction is maintained at a constant temperature the E.M.F. is proportional to the increase in temperature of the hot junction. This E.M.F. is measured by introducing into the circuit a sensitive gal-vanometer or millivoltmetei calibrated to read directly in degrees of temperature. At 1,100° C., a couple formed of iron and copper-nickel wires will generate 50 millivolts. For measur-ing temperatures up to 95° C.-, a thermo-couple of bismuth and antimony is best; for temperatures up to 500° C., a satisfactory thermo-couple consists of one iron wire and one 60 per cent, nickel and 40 per cent, copper; for temperatures as high, as 1,000° C., a very satisfactory base-metal thermo-couple is one wire of 90 per cent, nickel and 10 per cent, chromium, and the other wire of 98 per cent, nickel and 2 per cent, aluminium. For constant service above this, a thermo-couple, one wire of which is chemically pure platinum and the other 90 per cent, platinum and 10 per cent, rhodium, is recommended. The thermo-couple instrument is found, broadly speaking, in two distinet forms. The “ direct-insertion ” instrument is very similar to that shown in Fig. 134, with the exception that a thermo-couple, fitted witli the necessary leads, is substituted for the resistance coil. Probably, however, the most familiar instrument on gasworks is that in which the Optical and thermo-couple principles are combined. These pyrometers possess the advantage that they are operated at a distance from the hot body through the action of radiant heat, and as no part actually comes in contact with the heated object, there is no harmful influence upon the life of the instrument. With careful operation they will read within 1 or 2 per cent, of accuracy.