Page 284 - Instrumentation Reference Book 3E

268 Temperature measurement
represented by a triangle). Further increase in the junctions at temperatures tl and t? the e.m.f. is
temperature of the hot junction will result in a not altered if one or both junctions are opened
decrease in the thermoelectric e.m.f. (the e.1n.f. and one or more other metals are interposed
represented by the second triangle will be in the between metals A and B, provided that all
opposite sense). When the average temperature of the junctions by which the single junction at tem-
the two junctions is 275 "C. or what comes to the perature tl may be replaced are kept at ti; and all
same thing, the sum of the two temperatures is those by which the junction at temperature t?
550 "C, the areas of the two triangles will be equal may be replaced are kept at tl.
and there will be no thermoelectric e.m.f.: 275 "C This law has a very important bearing on the
is the "neutral temperature" for the copper-iron application of thermocouples to temperature mea-
couple. With circuits of other materials, the neu- surement, for it means that, provided all the
tral point will occur at different temperatures. apparatus for measuring the thermoelectric
Further increase in the temperature of the hot e.m.f., connected in the circuit at the cold junc-
junction will produce a thermoelectric e.m.f. in tion, is kept at the same temperature, the presence
the opposite direction: from iron to copper at the of any number of junctions of different metals
hot junction; which will again increase with will not affect the total e.m.f. in the circuit. It also
increasing temperature of the hot junction as means that if another metal is introduced into the
was seen with zinc and iron in Figure 14.28. hot junction for calibration purposes it does not
In choosing two materials to form a thermo- affect the thermoelectric e.m.f., provided it is all
couple to measure a certain range of temperature, at the temperature of the hot junction.
it is very important to choose two which have
thermoelectric lines which do not cross within Law of interinedinte teniperatzrr.es The e.m.f.
the temperature range, that is, the neutral tem- E1-3 of a thermocouple with junctions at tem-
perature must not fall within the range of tem- peratures tl and t3 is the sum of the e.m.f.s of
perature to be measured. If the neutral two couples of the same metals, one with junc-
temperature is within the temperature range. tions at temperatures fl and t? (e.m.f. = EI-~),
there is some ambiguity about the temperature and the other with junctions at tl and
indicated by a certain value of the thermoelectric t3 (e.m.f. = EzL~), Figure 14.30:
see
e.ni.f., for there will be two values of the tempera-
ture of the hot junction for which the thermo- El-> + E?-3 = El-3 (14.22)
electric e.m.f. will be the same. For this reason
tungsten-molybdenum thermocouples must not This law is the basis upon which thermocouple
be used at temperatures below 1250 "C. measuring instruments can be manufactured.

14.5.1.6 Addition of therrnoelectvic e.nz.fs 14.5.1.7 Cold jziiicfion compensation
In measuring the e.m.f. in any circuit due to It is not normally practical in industrial applica-
thermoelectric effects, it is usually necessary to tions to have thermocouple cold junctions main-
insert some piece of apparatus, such as a milli- tained at O"C, but with the cold junctions at
voltmeter, somewhere in the circuit, and since this ambient temperature cold junction compensation
generally involves the presence of junctions other is required. To achieve cold junction compensa-
than the two original junctions, it is important to tion consider a thermocouple with its hot junction
formulate the laws according to which the e.m.f.s at t "C and its cold junction at ambient, its e.m.f.
produced by additional junctions may be dealt being The instrument must indicate an
with. These laws, discovered originally by experi- e.m.f. equivalent to having the cold junction at
ment, have now been established theoretically. OT, i.e., an e.m.f. of Eo-,. This requires that an
e.m.f. must be added at E+r to provide the
required signal:
Law of interntediate nzetals In a thermoelectric
circuit composed of two metals A and B with EO-r = E,, + EO-a (14.33)

'1 <L> f2 f t2<L> '3 = tl

B B
t2
Figure 14.30 Lawof intermediate metals.