Rockwell Automation 1746-INT4 Thermocouple/MV Isolated/ User Manual - Manual (Page 99)

Publication 1746-UM614B-EN-P - September 2007
Thermocouple Descriptions 99
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 ±2.2 °C (±35.9 °F) between 0…277 °C
(32…530.6 °F) and ±3/4% between 277…1260 °C (530.6…2300 °F)/
Limits of error are not specified for the Type K thermocouples below
0 °C (32 °F). 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, 1260 °C (2300 °F), applies for
3.3 mm (8 AWG) wire. For smaller wires, it decreases to 1093 °C
(2000 °F) for 1.6 mm (14 AWG), 982 °C (1800 °F) for 0.8 mm
(20 AWG), and 871 °C (1600 °F) for 0.5 or 0.3 mm (24 or 28 AWG).
T Type Thermocouple
(copper versus
copper-nickel
<Constantan
(1)
>)
The homogeneity of most Type TP and TN (or EN) thermoelements is
reasonably good. However, the Seebeck coefficient of Type T
thermocouples is moderately small at subzero temperatures (about
5.6 uV/K at 20 k), being roughly two-thirds that of Type E
thermocouples. This, together with the high thermal conductivity of
Type TP thermoelements, is the major reason why Type T
thermocouples are less suitable for use in the subzero range than
Type E thermocouples.
Type T thermocouples are recommended by the ASTM [1970] for use
in the temperature range -184…371 °C (-300…700 °F) in vacuum or in
oxidizing, reducing or inert atmospheres. The recommended upper
temperature limit for continuous service of protected Type T
thermocouples is set at 371 °C (700 °F) for 1.6mm (14 AWG)
thermoelements, since Type TP thermoelements oxidize rapidly above
this temperature. However, the thermoelectric properties of Type TP
thermoelements are apparently not grossly affected by oxidation since
Roeser and Dahl [1938] observed negligible changes in the
thermoelectric voltage of Nos. 12, 18, and 22 AWG Type TP
thermoelements after heating for 30 hours in air at 500 °C (932 °F). At
this temperature, the Type TN thermoelements have good resistance
to oxidation and exhibit only small changes in thermal emf with long
exposure in air, as shown by the studies of Dahl [1941]. Operation of
Type T thermocouples in hydrogen atmospheres at temperatures
above about 370 °C (698 °F) is not recommended since severe
embrittlement of the Type TP thermoelements may occur.
(1)
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 ration of
copper and nickel in each.