Allen-Bradley MicroLogix 1200 User Manual page 106

C-6 Thermocouple Descriptions
Publication 1762-UM002A-EN-P - July 2002
emphasized that type JN thermoelements are NOT generally
interchangeable with type TN (or EN) thermoelements, although they
are all referred to as "constantan". In order to provide some
differentiation in nomenclature, type JN is often referred to as SAMA
constantan.
Type J thermocouples are recommended by the ASTM [5] for use in
the temperature range from 0°C to 760°C in vacuum, oxidizing,
reducing, or inert atmospheres. If used for extended times in air above
500°C, heavy gauge wires are recommended because the oxidation
rate is rapid at elevated temperatures. Oxidation normally causes a
gradual decrease in the thermoelectric voltage of the thermocouple
with time. Because iron 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 40µV (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.