Page 289 - Instrumentation Reference Book 3E

Measurement techniques: thermocouples 273

I I I I
-8 -4 0 4 8 12 16
Thermocouple EMF mV
Figure 14,34 Type K thermocouple characteristic.

Table 14.74 Thermocouples commercially available but not covered by British Standards (composition and accuracy to be
agreed with manufactuer).

Tipe Coridiictors Output for indicated Service teriiperatwe
(positive conductor first) temperature (cold jiazction ut 0 'C) (~u.Y. irztermirterzt senice)
-~
W Tungstenhngsten: 26% 34.1 mV at 2000°C 20-23OO'C (1600°C)
rhenium
w5 Tungsten: 3% Rhenium/ 32.404 mV at 2000 "C
tungsten: 26% rhenium
w3 Tungsten: 3% rhenium/ 35.707mV at 2000 "C (W, suitable for hydiogen
tungsten: 25% rhenium atmosphere)
Tungstenlmolpbdenum
Rhodium: iridium/rhodium Typically 6.4mV at 1200°C
Iron/eold: nickel/chromium 1 to 2 300K
Iron:-gold/silver 1 to 2 300K

need to be interchangeable with other types of 14.5.3.3 i2~izerLrl-iiiszilnted tlzerrnocoi~ples
temperature measurement equipment, are avail-
able sheathed in steel or stainless steel designed Probably the most versatile format for thermo-
for direct insertion into process vessels or for use couples is the mineral-insulated (MI) construc-
in a thermometer pocket. Figures 14.36(a) and (b) tion. In this form the thermocouples are made
show typical insertion probes. Where thermo- from mineral-insulated cable similar in concept
couples are to be immersed in very corrosive to the MI cable used for electrical wiring applica-
process fluids or into very high temperature tions. It differs, however, in that the conductors
locations they are available constructed in are of thermocouple wire and the sheath is
ceramic sheaths as in Figure 14.36(c). Sheathed usually stainless steel. The insulation. however,
thermocoaples. especially the ceramic ones, suffer is similar, being in the form of finely powdered
from a slow response time, typically a minute and densely compacted ceramic, usually alumi-
or more. However. the locations where they num oxide or magnesium oxide. Figure 14.36
are essential for their mechanical properties shows MI thermocouples at (d). (e). and (f).
are usually in heavy plants where temperatures They are available in diameters from 1 milli-
do not normally move fast in any case meter up to 6 millimeters and can be sapplied in