Spectrum Controls 1769sc-IF8u - Manual (Page 90)

80 Compact IO ™ Universal Input Module
Both thermoelements of type K thermocouples are reasonably stable,
thermoelectrically, under neutron irradiation since the resulting changes in
their chemical compositions due to transmutation are small. The KN
thermoelements are somewhat less stable than the KP thermoelements in
that they experience a small increase in the iron content accompanied by a
slight decrease in the manganese and cobalt contents.
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards
[7] specifies that the initial calibration tolerances for type K commercial
thermocouples be ±2.2°C or ±0.75 percent (whichever is greater)
between 0°C and 1250°C, and ±2.2°C or ±2 percent (whichever is
greater) between -200°C and 0°C. In the 0°C to 1250°C range, type K
thermocouples can be supplied to meet special tolerances that are equal to
approximately one-half the standard tolerances given above. Type K
thermocouple materials are normally supplied to meet the tolerances
specified for temperatures above 0°C. However, the same materials may
not satisfy the tolerances specified for the -200°C to 0°C range. If
materials are required to meet the tolerances below 0°C, this should be
specified when they are purchased.
The suggested upper temperature limit of 1260°C given in the ASTM
standard [7] for protected type K thermocouples applies to AWG 8 (3.25
mm) wire. It decreases to 1090°C for AWG 14 (1.63 mm), 980°C for
AWG 20 (0.81 mm), 870 for AWG 24 or 28 (0.51 mm or 0.33 mm), and
760°C for AWG 30 (0.25 mm). These temperature limits apply to
thermocouples used in conventional closed-end protecting tubes and they
are intended only as a rough guide to the user. They do not apply to
thermocouples having compacted mineral oxide insulation.
Type N Type N Type N Type N Type N
T TT T T h e r h e r h e r h e r h e r m o c o u p l e s m o c o u p l e s m o c o u p l e s m o c o u p l e s m o c o u p l e s
This section describes Nickel-Chromium-Silicon Alloy Versus Nickel-
Silicon-Magnesium Alloy thermocouples, commonly referred to as type N
thermocouples. This type is the newest of the letter-designated
thermocouples. It offers higher thermoelectric stability in air above 1000°C
and better air-oxidation resistance than types E, J, and K thermocouples.
The positive thermoelement, NP, is an alloy that typically contains about 84
percent nickel, 14 to 14.4 percent chromium, 1.3 to 1.6 percent silicon, plus
small amounts (usually not exceeding about 0.1 percent) of other elements
such as magnesium, iron, carbon, and cobalt. The negative thermoelement,
NN, is an alloy that typically contains about 95 percent nickel, 4.2 to 4.6
percent silicon, 0.5 to 1.5 percent magnesium, plus minor impurities of iron,
cobalt, manganese and carbon totaling about 0.1 to 0.3 percent. The type
NP and NN alloys were known originally [16] as nicrosil and nisil,
respectively.
The research reported in NBS Monograph 161 showed that the type N
thermocouple may be used down to liquid helium temperatures (about 4K)
but that its Seebeck coefficient becomes very small below 20K. Its
Seebeck coefficient at 20K is about 2.5mV/K, roughly one-third that of