Page 287 - Mechanical Engineer's Data Handbook

ENGINEERING MEASUREMENTS 275

Thermocouple circuit with extension leads 7.4.5 Thermoelectric sensitivity of
materials
Two cold junctions at the same temperature are used
and copper extension leads to the measuring instru- Therrnodectne sensitivity of tbermocwple materials
*
ment. relative to platinum (reference jonctioa at OT)
~
Sensitivity Sensitivity
Metal (pV"C- l) Metal (pV "C - l)

Bismuth - 72 Silver 6.5
Constantan - 35 Copper 6.5
Nickel - 15 Gold 6.5
Practical thermocouple Potassium -9 Tungsten 7.5
Sodium -2 Cadmium 7.5
The wires pass through ceramic beads inside a protec- Platinum 0 Iron 18.5
tive metal sheath. Mercury 0.6 Nichrome 25
Carbon 3 Antimony 47
Aluminium 3.5 Germanium 300
Lead 4 Silicon 440
Tantalum 4.5 Tellurium 500
Thermopile Rhodium 6 Selenium 900

This consists of a number of thermocouples connected
in series to give a higher e.m.f.

.<=e

Thermopile
7.4.4 Thermocouple pairs and
temperature limit

Temperature ("C)

Materials Minimum Maximum Applications

Copper/constantan (57%cu, 43%Ni) -250 400 Flue gases, food processes, sub-zero
temperatures
Iron/constantan - 200 850 Paper pulp mills, chemical reactors,
low-temperature furnaces
Chrome1 (90%Ni, lO%Cr)/Alumel 0 1100 Blast-furnace gas, brick kilns,
(94%Ni, 3%Mn, 2%A1, l%Si) glass manufacture
Platinum/platinum rhodium 0 1400 Special applications
Tungsten/mol ybdenum 1250 2600 Special applications