Page 290 - Instrumentation Reference Book 3E

274 Temperature measurement

Temperature 'C
Figure 14.35 Summary of thermoelectric properties of precious metal thermocouples. Broken lines indicate areas for
intermittent service.

some applications the junction being connected to
the plant earth via the sheath tip can be unaccept-
able, so in such cases insulated thermocouples
must be used. The principal advantages are their
quick response and mechanical flexibility, being
able to be bent into almost any shape. Care must
be taken if re-using MI thermocouples, for though
they can be straightened or rebent to a new shape
this cannot be done too often. Either the wires
break, or the insulation gets displaced and the
thermocouple becomes short-circuited. As shown in
Figures 14.36 and 14.38, MI thermocouples can be
supplied fitted with a variety of terminations. A
further useful advantage of MI thermocouples is
that the cable can be bought in rolls together with
suitable terminations. and the thermocouples can
be made up to the required specifications on site.
Also, in situations where robust cabling is
required, MI thermocouple cable can be used in
lieu of compensating cable (see Section 14.5.3.9).

Conductors (a)
Figure 14.36 Examplesof industrial thermocouple
probes. Courtesy ABB. Metal sheath Insulation

any length requircd. Thc junction can be either Conduccorl lbl
insulated (a) or weldcd (b) to the tip of the sheath
as shown in Figure 14.37. The latter arrangement ~i~~~~ 14.37 Mineral insulated thermocouples:
has the advantage of very quick response. For (a) insulated junction, (b) junction welded to sheath.