Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 118)

118 Rockwell Automation Publication 1769-UM004B-EN-P - March 2010
Appendix C Thermocouple Descriptions
Type J thermocouples are recommended by the ASTM [5] for use in the
temperature range from 0…760 °C (32…1400 °F) in vacuum, oxidizing, reducing,
or inert atmospheres. If used for extended times in air above 500 °C (932 °F),
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 (32 °F). In addition, they should not be used
unprotected in sulfurous atmospheres above 500 °C (932 °F).
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 because
copper is converted to nickel and zinc.
Iron undergoes a magnetic transformation near 769 °C (1416 °F) and an
alpha-gamma crystal transformation near 910 °C (1670 °F) [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 (1400
°F). If type J thermocouples are taken to high temperatures, especially above 900
°C (1652 °F), 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 (1400 °F), only the largest wire, 3.3 mm
2
(8 AWG)
should be used and they should be held at the measured temperature for 10…20
minutes before readings are taken. The thermoelectric voltage of the type J
thermocouples may change by as much as 40 μV (or 0.6 °C (33.08 °F) equivalent)
per minute when first brought up to temperatures near 900 °C (1652 °F).
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 (±35.96 °F) or ±0.75% (whichever is greater) between
0 °C (32 °F) and 750 °C (1382 °F). 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 (32 °F) or above 750 °C (1382 °F).
The suggested upper temperature limit of 760 °C (1400 °F) given in the above
ASTM standard [7] for protected type J thermocouples applies to 3.25 mm
2
(8 AWG) wire. For smaller diameter wires the suggested upper temperature limit
decreases to 590 °C (1094 °F) for 1.63 mm
2
(14 AWG), 480 °C (896 °F) for
0.81 mm
2
(20 AWG), 370 °C (698 °F) for 0.51 or 0.33 mm
2
(24 or 28 AWG),
and 320 °C (608 °F) for 0.25 mm
2
(30 AWG). 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.