Thermocouple Restrictions
B–4
Publication 1794-6.5.7
K (Nickel–Chromium vs Nickel–Aluminum) Type Thermocouple
“This type is more resistant to oxidation at elevated temperatures
than the Types E, J or T thermocouples and consequently it finds
wide application at temperatures above 500C.”
“Type K thermocouples may be used at” liquid hydrogen
“temperatures. However, their Seebeck coefficient (about 4uV/K at
20K) is only about one–half of that of Type E thermocouples.
Furthermore, the thermoelectric homogeneity of KN thermoelements
is generally not quite as good as that of EN thermoelements. Both
the KP and the KN thermoelements do have a relatively low thermal
conductivity and good resistance to corrosion in moist atmospheres
at low temperatures.”
“Type K thermocouples are recommended by the ASTM [1970] for
continuous use at temperatures within the range –250 to 1260C in
oxidizing or inert atmospheres. Both the KP and the KN
thermoelements are subject to oxidation when used in air above
about 850C, but even so, Type K thermocouples may be used at
temperatures up to about 1350C for short periods with only small
changes in calibration.”
“They should not be used in sulfurous, reducing, or alternately
reducing and oxidizing atmospheres unless suitably protected with
protecting tubes. They should not be used in vacuum (at high
temperatures) for extended times because the Chromium in the
positive thermoelement vaporizes out of solution and alters the
calibration. They should also not be used in atmospheres that
promote ”green–rot” corrosion (those with low, but not negligible,
oxygen content).”
“ASTM Standard E230–72 in the Annual Book of ASTM Standards
[1972] specifies that the standard limits of error for Type K
commercial thermocouples be +/–2.2C between 0 and 277C and
+/–3/4 percent between 277 and 1260C. Limits of error are not
specified for the Type K thermocouples below 0C. Type K
thermocouples can also be supplied to meet special limits of error,
which are equal to one half the standard limits of error given above.
The recommended upper temperature limit for protected Type K
thermocouples, 1260C, applies for AWG 8 (3.3mm) wire. For
smaller wires it decreases to 1093C for AWG 14 (1.6mm), 982C for
AWG 20 (0.8mm), and 871C for AWG 24 or 28 (0.5 or 0.3mm).”
Содержание 1794-IT8
Страница 1: ...User Manual Thermocouple Millivolt Input Module Cat No 1794 IT8 Allen Bradley ...
Страница 4: ...soc ii Summary of Changes Publication 1794 6 5 7 April 1997 ...
Страница 8: ...Table of Contents iv Publication 1794 6 5 7 ...
Страница 14: ...Overview of FLEX I O and your Thermocouple mV Module 1 4 Publication 1794 6 5 7 ...
Страница 64: ...Specifications A 10 Publication 1794 6 5 7 March 1997 ...