Spectrum Controls 1769sc-IF8u User Manual

Page 107

Appendix D: Using Thermocouple Junctions

example, ceramic). An alternative is to float the metal sheath with respect
to any path to chassis ground or to another thermocouple metal sheath.
Thus, the metal sheath must be insulated from electrically conductive
process material, and have all connections to chassis ground broken. Note
that a floated sheath can result in a less noise-immune thermocouple
signal.

Using an

U n g r o u n d e d

( I s o l a t e d )

J u n c t i o n

T h e r

h e r

h e r m o c o u p l e

m o c o u p l e

An ungrounded (isolated) junction thermocouple uses a measuring junction
that is electrically isolated from the protective metal sheath. This junction
type is often used in situations when noise will affect readings, as well as
situations using frequent or rapid temperature cycling. For this type of
thermocouple junction, the response time is longer than for the grounded
junction.

Using an

E x p o s e d

J u n c t i o n

T h e r

h e r

h e r m o c o u p l e

m o c o u p l e

An exposed junction thermocouple uses a measuring junction that does not
have a protective metal sheath. A thermocouple with this junction type
provides the fastest response time but leaves thermocouple wires
unprotected against corrosive or mechanical damage.

As shown in the next illustration, using an exposed junction thermocouple
can result in removal of channel-to-channel isolation. Isolation is removed
if multiple exposed thermocouples are in direct contact with electrically
conductive process material.