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244 CHAPTER 16 1965 Guide And Data Boole Table 1 .... Measurement of Temperature - No. Ateotsrweaf Means fcmffe, F Pmuwii, F 1 Glass-stem thermometers Mercury-glass thermometer Temp of gaaes and liquids by contact -38/575 Alcobo1-glass thermometer Pentane-glass thermometers Jena or quarts mercury nitrogen thermometers 2 Gas thermometer 8 Resistance therfnometeri ^ v * Platinum-resistance b\ S3TSAKD Primary standard r;T;v n\A A v? Precision; remote readings; temp of fluids or solids by contact -100/100 -200/70 -38/1,000 -459/1,000 -320/1,800 Nickel-resistance, thermometer- e- .Remote, readings;<temp,,by^ contact ;'Vl56/3W'' T<riM th^n Less than Trifta tJrnn 0.02 to 5 0.3 In gases, accuracy *f. feeted by radiation Requires enmarferwble skill to use High cost; accuracy af fected by radiation in gases * .Accuracy, affected by radiation m' gases ' 4 Thermocouples Pt-Pt-Rh.thermocouple _ . ___ Standardfor,thermocouples..... 500/3,000 0.1 to 5.. High cost; also, requires expensive measuring d&r vice . Chromel-atumel.thermocouple A . General testing of high temp; remote Up to 2,200. 0.1 to 15. Leasaccuratethanabove rapid readings by^dSect contact" ' IroD-constantantherinocouple-1' Up'to'1,500 o.i ti> is Subject to oxidation -. Copper constantan thermocouple Same,as above, especially suited for ' Up Ur600 .'. 0.1 to 15 low temp *i , 5 Beckman thermometers . .(metastatic} For differential teinp in same applica Tc'd'iff tions as in glass stem thermometer o.oic Must be set for temp to be measured ''! ' 6 Bimetallic thermometers ' For approx temp .. 0/1,000- 1, usually - Time lag; unsuitable for much more remote.use; unreliable , 7 . Pressure-bulb thermometers _ Gas-filled bulb ' * ' " '.Remote^testiag -200/1-.000 J-2 ' Vapor-filled bulb J""' > Liquid-filled bulb 8 Optical pyrometers ^ ` >> : '::i - - . '1 - . - ; 2O/50PrM -50/2,100 For intensity of narrow spectral band 1,500 upward of high temp radiation (remote) `v. 2. '2 - 15 Caution' must be'exer cised so that installation is correct .. 9;] Radiation pyrometers For intensity of total high temp radi Any range ation (remote) " '' 1 10 "Seger-cones (fusion pyrometers) *.- Approx temp (within temp, source); ? -1,000/3;600. 50 .>.11 - Tempi] Sticks .. Approx temp' (oh 'surface) .' , ; , 125/900;.-; l!percent 12 Melting'&nd boiling points of ini' Standards" ' 'tennis'1' - All except ex Extremely' For laboratory use only tremely high - precise . .temp- .. 'To' prevent errors in. temperature, imeasurements,; ample .time should2 be allowed for the thermometer to attain .tem perature equilibrium:with the surrounding fluid. In-reading a thermometer, the eye should be at the sunw level as.the top of'the-liquidrcohmro to avoid parallax.' -rs ,;// Themocouples '.'...V'! When'two :wires made of dissimilar metals are-joined'.by Bolderingj welding,'or merely by -twisting, a thermocouple.or thermo-junction is formed.-An electromotive force, which de pends upon the temperature ofithe junction;'is found to'exist between the wires; When-the'wires are^joined at'twoipoints a thermocouple circuit is fdrmedv 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 devel oped' by the'two junctions:! This' phenomenon,is;employed Tor-temperature measurements'in.thermocouple systems,-one -junction'being ordinarily-kept at aicohstant temperaturei'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 utilise emf to indicate temperature because, at small or zero current flow, the resistance of the'circuit is un- important..A'highresistence millivolt meter is'useful in sortie oases ibut the potentiometer yields better'results.' In'the po tentiometer-the electromotive force generated by the thermo- -couples is balanced against an electromotive .force from the .battery;.so that observations are made with no flow of cur rent through, the thermocouple circuit., A conventional ar* >rangement is illustrated in Fig. 1. The thermocouple leads A-B are so connected that their polarity opposes.that of bat- .tery C.-If the position qf E on the graduated slide.wire rheo- stat pf'. b adjusied until galvanometer G shbws.no current flowing, resistance DE will indicate directly the voltage gen erated by the thermocouple. In order 'to calibrate the instal lment/. switch S is throwm over to, the standard, cell circuit while rheostat R is adjusted so that the galvanometer shows zero current. Battery C then exerts the known voltage of thb standard cell at1 DH. ` .... -V/ Mediuremeht and Instnmienb 245 TM nfudiustiii* rheostat D-F (Fig. 1) for icro current ` the potentiometer. Automatic eelf- instruments of both the indicating and recording on the market. They usually contain an automatinT^mnensating cold junction to avoid the use.of an.ice and'special thermocouple wire is furnished with them fmm the factory.The choice of materials for thermocouple .wire is deter- mined by the temperature to be measured, the protection from corrosion afforded the couple, and the . precision and He required. In general, copper vs. constantan is mUjLhle for temperatures up to 700.F, iron vs. constanton.up alumel'up to 2200 F/ Higher tem- m.ratures'require the use of noble metal thermocouples (plati- ..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 circuits are the same.1* * The thermocouple is.particularly useful.in determining a surface temperature. It may be attached to.a metal surface in any of several-ways. For permanent installations, solder ing, brazing, or peeping may be desirable! A small hole is drilled.for the peening operation; the thermocouple.is. in serted and the metal is peeped to retain it. The fact that the thermocouple is in electric contact with the surface is unim portant in usual circuits. For temporary arrangements, cou ples may be attached by mpans of surgical or cellophane tape.- For boiler or'"furnace surfaces, ..furnace cement serves very well. To''minimize the possibility of error due to heat conduc tion along the wires, a surface thermocouple should be made of fine wires,' and the wires should be held in close contact with`the surface for an inch or so from the junction. Electri cal insulation between the wires must be perfect except at the junction. ; Resistance Thermometers fig. 1.... Baric Circuit and Connections for Thermocouple and Potentiometer cost of a noble metal couple, it does not develop as high an emf as the base metal couples do. Impurities make large dif ferences' in the performance 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 ob tained from the manufacturer. With a suitable potentiometer, small wires serve as well for thermocouples as large ones, and the fineness of the wires is limited only by consideration 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. Heavy gage couples, however, are necessary for High temperature.work where corrosion of the wire is a problem. For-use in heated air or gases, thermo couples are often shielded,1 as are thermometers, and aspi rated thermocouples are sometimes used.- An arrangement has been described for avoiding error due to radiation. It in volves the use of several thermocouples of different wire sizes, the true temperature being estimated by extrapolation-of readings to zero diameter.* - ') By the use of thermocouples, temperatures at remote points may be indicated or recorded on conveniently located instru ments, and temperatures may be obtained'within thin mate rials, narrow spaces, or otherwise inaccessible locations.. Thermocouples in series, with every alternate junction maintained at a common temperature, will- produce ah emf which, when divided by the'number of couples," gives the average emf corresponding to the true-average temperature. This series arrangement of thermocouples;'often called'a thermopile, have extreme sensitivity and is useful in1 de tecting very small changes in temperature. ,:i Resistance'thermometers depend for their operation upon the change of electric resistance of metal with change in tem perature. The, resistance generally increases with rising tem perature. 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 a differential galvanometer is used, in which coils L and H exert opposing forces onthe indicating' needle.-Coil'L is in series with the thermometer-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 increase 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 flowthrough coil L than through-coil H. This forces the in dicating needle to a higher reading. Rheostat'S must be ad justed occasionally to maintain a'constant flow of current. - As compared to the thermocouple, the resistance ther mometer does not require a cold' junction, and it. can be simply scaled for more accurate measurements; but, gen erally because of its construction-it is more cosily-and is apt to have considerable lag. It: gives best results when.