Spectrum Controls 1769sc-IF8u - Manual (Page 88)

78 Compact IO ™ Universal Input Module
rusts in moist atmospheres and may become brittle, type J thermocouples
are not recommended for use below 0°C. In addition, they should not be
used unprotected in sulfurous atmospheres above 500°C.
The positive thermoelement, iron, is relatively insensitive to composition
changes under thermal neutron irradiation, but does exhibit a slight
increase in manganese content. The negative thermoelement, a copper-
nickel alloy, is subject to substantial composition changes under thermal
neutron irradiation since copper is converted to nickel and zinc.
Iron undergoes a magnetic transformation near 769°C and an alpha-
gamma crystal transformation near 910°C [6]. Both of these
transformations, especially the latter, seriously affect the thermoelectric
properties of iron, and therefore of type J thermocouples. This behavior
and the rapid oxidation rate of iron are the main reasons why iron versus
constantan thermocouples are not recommended as a standardized type
above 760°C. If type J thermocouples are taken to high temperatures,
especially above 900°C, they will lose the accuracy of their calibration
when they are recycled to lower temperatures. If type J thermocouples
are used in air at temperatures above 760°C, only the largest wire, AWG 8
(3.3 mm) should be used and they should be held at the measured
temperature for 10 to 20 minutes before readings are taken. The
thermoelectric voltage of the type J thermocouples may change by as
much as 40mV (or 0.6°C equivalent) per minute when first brought up to
temperatures near 900°C.
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards
[7] specifies that the initial calibration tolerances for type J commercial
thermocouples be ±2.2°C or ±0.75 percent (whichever is greater)
between 0°C and 750°C. Type J thermocouples can also be supplied to
meet special tolerances, which are equal to approximately one-half the
standard tolerances given above. Tolerances are not specified for type J
thermocouples below 0°C or above 750°C.
The suggested upper temperature limit of 760°C given in the above ASTM
standard [7] for protected type J thermocouples applies to AWG 8 (3.25
mm) wire. For smaller diameter wires the suggested upper temperature
limit decreases to 590°C for AWG 14 (1.63 mm), 480°C for AWG 20
(0.81 mm), 370°C for AWG 24 or 28 (0.51 mm or 0.33 mm), and 320°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 sheathed
thermocouples having compacted mineral oxide insulation.
Type K Type K Type K Type K Type K
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 Alloy Versus Nickel-Aluminum
Alloy thermocouples, called type K thermocouples. This type is more
resistant to oxidation at elevated temperatures than types E, J, or T
thermocouples and, consequently, it finds wide application at temperatures