Thermocouple
Types Type K --- Chromel ( Nickel-Chromium
Alloy ) / Alumel ( Nickel-Aluminium Alloy )
This
is the most commomly used "general purpose" thermocouples. They are
available in the ~ -200 °C to +1200 °C range. The characteristic of the
thermocouple undergoes a step change when a magnetic material reaches its Curie
point. This occurs for this thermocouple at ~ 352°C. Sensitivity is ~ 41 µV/°C.
Type K and N are the most oxidation resistant base metal thermocouples. Not recommended
for temperature of 1200C for long period.
Type
J --- Iron / Constantan
Limited range (~ -40
to +750 °C) makes type J less popular than type K. The main application is
with old equipment that cannot accept modern thermocouples. J types cannot be
used above 760 °C as an abrupt magnetic transformation causes permanent decalibration.
The magnetic properties also prevent use in some applications. Type J's have a
sensitivity of ~ 53µV/°C. Not recommended for temperature higher than
480C because of the higher oxidation rate of the iron element.
Type
T --- Copper / Constantan
Good for temperature
in the ~ -200 to 350 °C range. Non-magnetic, type T thermocouples are a popular
choice for strong magnetic fields and subzero environments. Type T thermocouples
have a sensitivity of ~ 43 µV/°C. Most accurate base-metal thermocouple.
Can be used in vacuum and in oxidizing, reducing, or inert atmospheres.
Type E --- Chromel / Constantan ( Copper-Nickel Alloy )
Type E has a high output ~ 68µV/°C which makes it well suited
to cryogenic use. It is also non-magnetic. Recommended for use up to 870C in oxidizing
or inert atmospheres.
Type
N --- Nicrosil (Nickel-Chromium - Silicon Alloy) / Nisil (Nicke l- Silicon Alloy)
High stability and resistance to high temperature
oxidation makes type N suitable for high temperature measurements without the
cost of platinum (B, R, S) types. They can withstand temperatures above ~1200
°C. Sensitivity ~ 38 µV/°C at 900°C.
Thermocouple
types R, S and B are all noble metal thermocouples
and exhibit similar characteristics. They are the most stable of all thermocouples,
but due to their low sensitivity ~ 10 µV/°C they are usually used for
high temperature measurement (>300 °C). Type
B --- Platinum 30% Rhodium / Platinum 6% Rhodium
Good
for high temperature measurements up to ~ 1800 °C. Type B thermocouples (due
to the shape of their temperature to voltage curve) give the same output at 0
°C and 42 °C. This makes them useless below 50°C. Type
C --- Tungsten 5% Rhenium / Tungsten 26% Rhenium
Good
for temperature ~ 35 to 4200°F. This thermocouple is well suited for vacuum
furnaces at extremely high temperatures and must never be used in the presence
of oxygen at temperatures above 500°F. Type
R --- Platinum 13% Rhodium / Platinum
Good for
high temperature up to ~1600 °C. Low sensitivity ~10 µV/°C and high
cost makes them unsuitable for general purpose use.
Type
S --- Platinum 10% Rhodium / Platinum
Good for
high temperature measurements up to 1600 °C. Low sensitivity ~10 µV/°C
and high cost makes them unsuitable for general purpose use. Due to its high stability,
type S is used as the standard of calibration for the melting point of gold (1064.43
°C).
Type M --- Nickel Alloy 19 /
Nickel-Molybdenum Alloy 20
This type is used
in the vacuum furnaces as well for the same reasons as with type C above. Upper
temperature is limited to 2490°F. | 
