Page 281 - Instrumentation Reference Book 3E

Measurement techniques: thermocouples 265
being proportional to I’R, depends only upon
the size of the current and the resistance of the
conductor and does not change to a cooling effect
when the current is reversed. The amount of heat
liberated, or absorbed, is proportional to the
quantity of electricity which crosses the junction,
and the amount liberated, or absorbed, when unit
All parts of circuit at same temperature-no EMF current passes for a unit time is called the Peltier
coefficient.
(a) As heat is liberated when a current does work
in overcoming the e.m.f. at a junction. and is
absorbed when the e.m.f. itself does work, the
existence of the Peltier effect would lead one to
believe that the junction of the metals is the seat
of the e.m.f. produced in the Seebeck effect. It
would appear that an e.m.f. exists across the
junction of dissimilar metals. its direction being
Junction “J”hotter than remainder of circuit- from copper to iron in the couple considered. The
EMF generated and current flows
e.m.f. is a function of the conduction electron
(b) energies of the materials making up the junction.
Figure 14,24 Basicthermocouple circuit In the case of metals the energy difference is small
and therefore the e.m.f. is small. In the case of
semiconductors the electron energy difference
may be much greater, resulting in a higher e.m.f.
at the junction. The size of the e.m.f. depends not
only on the materials making up the junction but
also upon the temperature of the junction. When
both junctions are at the same temperature, the
emf. at one junction is equal and opposite to
that at the second junction: so that the resultant
e.m.f. in the circuit is zero, If, however. one junc-
tion is heated, the e.m.f. across the hot junction is
u greater than that across the cold junction. and
Figure 14.25 Simple thermocouple. there will be a resultant e.m.f. in the circuit which
is responsible for the current:
e.m.f. in the circuit = F2 - PI
absorbed at the junction when the current flows
in one direction and liberated if the current is where PI is the Peltier e.m.f. at temperature TI.
reversed. Heat is absorbed when a current flows and P’ is the Peltier e.m.f. at temperature Tz
across an iron-copper junction from copper to where T, > TI. Peltier cooling is used in instru-
iron. and liberated when the current flows from mentation where a small component is required
iron to capper. This heating effect should not be to be cooled under precise control. Figure 14.26
confused with the Joule heating effect; which shows diagrammatically the construction of such

Blocks of
thermoelectric
Ceramic

+
Electrical connection
connection
Figure 14.26 Peltiercaoler.