1. THeRmOCOUpleS THeORy
11
Mineral insulation
The table below shows only a portion of the materials that can
be used; however, the four shown are the most common.
The most important parameters to be considered in selecting
the mineral insulations are the upper temperature limit and the
performance characteristics at the temperature. Of course there
are other parameters which should also be considered such as:
resistively, purity and crushability. However, they are secondary
to temperature. For example: MgO, the most commonly used
(as exhibited by this table) has an upper temperature limit of
2,395 °C with high resistivity, excellent purity and very good
crushability.
Mineral insulated thermocouple sheath material
Sheath
material
Melting
point °C
Max. temperature
in air °C
Operating #
atmosphere
Continuous
max. temp °C
304 SS
1400°C
1048°C
O,R,N,V
895°C
310 SS
1400°C
1071°C
O,R,N,V
1145°C
316 SS
1250°C
960°C
O,R,N,V
930°C
321 SS
1415°C
815°C
O,R,N,V
871°C
347 SS
1425°C
915°C
O,R,N,V
871°C
Inconel
1398°C
1095°C
O,N,V (c.)
1145°C
Copper
1082°C
315°C
O,R,N,V (b)
315°C
Aluminium
660°C
425°C
O,R,N,V
371°C
Platinum
1770°C
1648°C
O,N (c.)
1648°C
Molybdenum
2620°C
535°C
V,N,R
2626°C
Tantalum
3004°C
400°C
V
2760°C
Titanium
1815°C
315°C
V,N
1090°C
STABILITY
Insulation
material
Formula
Melting
point °C
Max. limit
in
oxi. Atm °C
Thermal
shock
res.
Reducing
Atm
Carbon
Acid
slag
Basic
slag Metal
Alumina
Al 2 O 3 2037°C 1954°C
Good Good
Fair Good Good Good
Magnesium MgO
2760°C 2395°C
Fair
Poor
Good Poor Good Fair
Thoria
ThO 2 3315°C 2700°C
Poor
Good
Fair Poor Good Exc.
Zirconia
ZrO 2 2590°C 2510°C
Fair
Good
Fair Good Poor Good
# Key
O = Oxidizing
R = Reducing
N = Neutral
V = Vacuum
(b) = Scales readiliy in oxidizing atmospheres
(c) = Sensitive to sulphur corrosion