Rockwell Automation 1746-NT8 SLC 500 Thermocouple/mV Analog Input Module User Manual - Manual (Page 93)

1 Publication 1746-UM022B-EN-P - January 2005
Appendix
B
Using Grounded Junction, Ungrounded
Junction, and Exposed Junction
Thermocouples
This appendix describes the types of thermocouples available and
explains the trade-offs in using them with the 1746-NT8 module.
Thermocouple Types There are three (3) types of thermocouple junctions:
•
Grounded Junction - The measuring junction is physically
connected to the protective sheath forming a completely sealed
integral junction. If the sheath is metal (or electrically
conductive), then there is electrical continuity between the
junction and sheath. The junction is protected from corrosive or
erosive conditions. The response time approaches that of the
exposed junction type.
•
Ungrounded Junction - The measuring junction is electrically
isolated from the protective metal sheath. This may also be
referred to as an insulated junction. This type is often used
where noise would affect the reading and for frequent or rapid
temperature cycling. The response time is longer than the
grounded junction.
•
Exposed Junction - The measuring junction does not have a
protective metal sheath, so it is exposed. This junction style
provides the fastest response time but leaves the thermocouple
wires unprotected against corrosive or mechanical damage.
The following illustrations show each of the three (3) thermocouple
types.