Thermocouple Restrictions
B–3
Publication 1794-6.5.7
“Type J thermocouples are recommended by the ASTM [1970] for
use in the temperature range from 0 to 760C in vacuum, oxidizing,
reducing or inert atmospheres. If used for extended times above
500C, heavy gage wires are recommended because the oxidation rate
is rapid at elevated temperatures.”
“They should not be used in sulfurous atmospheres above 500C.
Because of potential rusting and embrittlement, they are not
recommended for subzero temperatures. They should not be cycled
above 760C even for a short time if accurate readings below 760C
are desired at a later time.”
“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.”
“Commercial iron undergoes a magnetic transformation near 769C
and <an alpha – gamma> crystal transformation near 910C. Both of
these transformations, especially the latter, seriously affect the
thermoelectric properties of iron, and therefore, the Type J
thermocouples. If Type J thermocouples are taken to high
temperatures, especially above 900C, they will lose accuracy of their
calibration when they are recycled to lower temperatures.”
“ASTM Standard E230–72 in the Annual Book of ASTM Standards
[1972] specifies that the standard limits of error for Type J
commercial thermocouples be +/–2.2C between 0 and 277C and
+/–3/4 percent between 277 and 760C. Limits of error are not
specified for Type J thermocouples below 0C or above 760C. Type J
thermocouples can also be supplied to meet special limits of error,
which are equal to one half the limits given above. The
recommended upper temperature limit for protected thermocouples,
760C, applies to AWG 8 (3.3mm) wire. For smaller wires the
recommended upper temperature decrease to 593C for AWG 14
(1.6mm), and 371C for AWG 24 or 28 (0.5 or 0.3mm).
* It should be noted that the Constantan element of Type J thermoelements is NOT interchangeable
with the Constantan element of Types T or N due to the different ratio of copper and nickel
in each.
Summary of Contents for 1794-IT8
Page 1: ...User Manual Thermocouple Millivolt Input Module Cat No 1794 IT8 Allen Bradley ...
Page 4: ...soc ii Summary of Changes Publication 1794 6 5 7 April 1997 ...
Page 8: ...Table of Contents iv Publication 1794 6 5 7 ...
Page 14: ...Overview of FLEX I O and your Thermocouple mV Module 1 4 Publication 1794 6 5 7 ...
Page 64: ...Specifications A 10 Publication 1794 6 5 7 March 1997 ...
