Spectrum Controls 1746sc-NI8u - Manual (Page 125)

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 NI8u 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 illustration that follows shows each of the three (3) thermocouple
types.
Grounded Junction
Extension Wire
Metal Sheath
Measuring Junction is
connected to sheath
Appendix C